A variable power supply is one of the most important pieces of equipment to have on the electronics workbench. It's only a matter of time before the voltage or current required in a circuit isn't practical for battery power.

Bench-top variable power supplies available today are typically transformer-fed linear voltage regulators which are both simple and inexpensive to manufacture. However these supplies are also large, heavy, and inefficient for most of their output voltage range. Many linear designs cannot operate anywhere near their rated output current when large Vo-Vi is required, but for low power applications they provide stable and noise free output.

Switched-mode power supplies are more than 90% efficient through almost the entire output voltage and current range, require much less space for heat sinks and transformer cores (90% less in medium to high current designs), and are as much as 5 times lighter than an equivalent linear power supply. But these advantages come at the expense of ripple, noise, and transient response; the three parameters that linear power supplies excel at.

I was working with some high power LED designs recently that required 2.5V to 9V and forward current between 1 Amp and 2 Amps. My LM317-based lab supply couldn't run more than a few minutes without tripping the thermal overload due to VI and Pmax limitations. This was a pretty hefty supply but it was getting too hot to operate reliably. So I decided to build my own 100W dual variable switched supply that could drive 2 Amps at an output voltage between 1V and 20V.

I wanted the regulation specifications to be competitive with the LM317 but my application did not require an extremely low ripple/noise figure. Current limiting and overload protection were important so independent voltage and current adjustments were required. And it would be nice to include a V/I meter for each supply for convenience.

This project will illustrate how to construct the 100W dual output switched mode variable power supply I have for about $150 using off-the-shelf modules and a prefabricated enclosure available on amazon.com or ebay.com. This power supply is compact, weighs less than 3 lbs, and deliverers professional appearance and performance competitive with commercial switched power supplies.

I've used this supply for a lot of heavy duty circuits including a DC motor controller and a 50W prototype audio amplifier with great results. I would not recommend this supply for precision Op-Amp or Radio Frequency circuits but for just about everything else it has worked really well.

Review the Power Supply Design

The Switched Mode Variable Power supply was designed using off-the-shelf modules that could be wired together using simple tools and basic soldering and wiring techniques. Two modules require modification so that front panel controls can be used instead of the PCB mounted multi-turn potentiometers included with the modules. These modifications are covered in a later step.

Power Supply Specifications

   Input: 120VAC (+/- 15%) 60Hz 1A Full Load

   Output 1: 1.2V - 20V @ 2 Amps

   Output 2: 1.2V - 20V @ 2 Amps

   Load Regulation: 0.5% Full Load

   Line Regulation: .001% Full Input Range

   Noise/Ripple: 20mV RMS, 100mVpp

General Circuit Description

AC power is connected to the supply via an IEC 320-C13 AC input module. AC safety ground is bonded to the power supply case and feed through to switching power supplies 1 and 2. The case of the power supply is grounded to the AC mains circuit. DC output ground is electrically isolated and independent of the AC mains ground.

Switching power supplies 1 and 2 are energized and de-energized through an illuminated DPST power switch. These supplies provide the constant 24V DC needed for DC-DC converters 1 and 2, the cooling fan, and the V/I displays. DC-DC converters 1 and 2 provide controlled output voltage and current to the power supply binding posts.

The output voltage and current set points are determined by two 50K Ohm and two 100K Ohm single-turn potentiometers. The DC positive power connection can be disconnected from the circuit by flipping the output power switch to the off (Down) position.

Two panel meters provide direct readout of the voltage set point and the current being consumed by the circuit attached to each power supply. The panel meters use a shunt type current sensor inline with the DC ground conductor. Power for each meter (< 20mA each) is taken directly from the 24V switching power supplies.

All power supplies are current and thermal overload protected and include last resort short circuit protection via fuses on switched power supply AC input and converter DC output. Cooling for the power supply is forced air via a 27 CFM fan using 24V @ 100mA drawn from switching power supply 2.

Design Tradeoffs

In order to keep total cost around $150, single-turn potentiometers were used instead of precision multi-turn potentiometers. Setting the output voltage is easier with 10-turn precision pots but a well made set would have increased the cost of the supply by $40. I decided to live with the fiddly nature of a general purpose single-turn pot for setting the output voltage. My applications do not require exact voltages. Close enough is good enough.

In order to keep costs low and simplify module wiring, I did not use DC-DC converters with an external voltage sense feature. This results in a slight degradation in load regulation (0.5% instead of 0.1%) due to the current shunt used in the current meter.

The 60mm x 60mm fan I used is overkill for this design and a bit louder than I would have liked. Lower CFM fans from Delta Electronics were non-stocked at Mouser so I decided to accept the overkill. With the supply installed on the instrument shelf I hardly notice the fan noise among all the other fan noise going on in the lab.

The current limit control is usable for only about half of it's range due to the 5A current rating of the DC-DC converter. I could have used two resistors to scale the current control to use the full rotation but did not feel the wiring complexity was worth the effort. I usually start with minimum current on a new circuit and slowly increase the current limit until a stable output voltage is achieved. I might add the scaling resistors at a later time if I think it's needed.

A full size PDF file for all Power Supply diagrams and parts list can be downloaded >>> HERE <<<. 

Review the Parts List

Obtain the parts listed for the power supply project. Everything can be purchased from Amazon, eBay, and Mouser as of August 2015. All prices are current as of August 2015.

I keep an inventory of plastic cable ties and a variety of screws, nuts, and washers. I used a few of these in the finishing of the power supply and didn't list them in the parts list because their single unit cost is extremely low. Have on hand some 4" cable ties and a few pieces of #6 and #10 screws/nuts/washers.

With respect to the IAASR SimCase product there are a number of options available on the iaasr.com website. Be sure to choose the color you want, select fan/power/ac-input, and choose the 24V fan option.

Please Note: I have no business relationship with any of the vendors in my parts list. Nothing of financial value was exchanged for my recommendation. None of the above vendors provided compensation of any kind during the creation of this project. I will not be compensated in any way if you choose to build this project or purchase components from any vendor I recommend. I simply had a good experience with the vendors I recommend and believe you will too.

A full size PDF file for all Power Supply diagrams and parts list can be downloaded >>> HERE <<<. 

A Word About Electronics Enclosures

I love to see a useful project professionally finished in a nice enclosure. A well designed enclosure improves the durability and appearance of a DIY project and enhances the "I made that" pride in craftsmanship a builder earns from the work. However, many builders have the following complaints with the enclosure products available on the market today:

1. Project boxes cost more than the value they add, and sometimes more than the parts they enclose.

2. Poking holes of various shapes and sizes in an enclosure is hard work. If not done properly, the appearance of an expensive enclosure can be destroyed.

3. Designing the front and back panel layout is time consuming and not as much fun as designing and building the circuit.

4. It's difficult to find an enclosure that is the right size and shape for a particular type of project.

When I started looking for an enclosure for the dual switch-mode power supply project, I was shocked at the prices manufacturers were asking for a simple project case. The basic grey on grey cabinet without a single hole cut in it was $100 and up! If I was going to spend that kind of money, I better have a fully equipped machine shop to do the job right. But then the sizes available were either too large or too small, too deep or too tall. I didn't want unpainted aluminum or battleship grey. None of the manufacturers in Mouser or Digikey had anything that fit my design in an affordable, easy to build way.

While searching through Amazon and eBay I happened to discover IAASR (www.iaasr.com) and their line of SimCase and HexCase enclosures. These are purpose-built enclosures with holes already cut and parts already installed for specific use-cases. When I saw the IAASR SimCase I said "That's exactly what I need!". The SimCase product is designed by IAASR to house a DIY power supply. It includes an EMI shielded mild steel enclosure, the AC input module, an illuminated AC power switch, a fan, and ventilation holes validated with thermal analysis software... for $49. That's a layout I didn't have to design, parts I didn't have to research and order, and holes I didn't have to cut which would save me a huge amount of time. IAASR offers their enclosures in 5 standard colors and 15 custom colors which means your project can look cool like you imagined it would instead of like a low-bid government job.

But that's not all. I contacted Shiraz Macuff, CEO, about the front panel design. He says, "Send me a layout and we'll cut the holes before we ship at no extra charge". That's service you can only get when you order quantity 10,000 from any other manufacturer. I ordered quantity one from IAASR. It turns out that IAASR is disrupting the enclosure market with purpose-built products that save time, add value, and can be mass-customized to meet the requirements of the DIY, prototype, and small-medium volume manufacturer. IAASR enclosures can make your DIY project seem more like a professionally designed kit. And you don't have to worry about accidentally mutilating your enclosure with a power drill.

In this article, I am including the design drawings and assembly steps for a generic enclosure. But I strongly encourage you to use the IAASR SimCase product indicated in the parts list instead of trying to make do with the generic cases sold elsewhere. You will enjoy the building experience much more when you can focus on the assembly work and not have to put up with the dull, dirty, and sometimes dangerous fabrication work. Shiraz and his team can save you a lot of time.

Please Note: I have no business relationship with IAASR of any kind. Absolutely nothing of any financial value (money, product, gift cards, work for free, etc.) is exchanged between IAASR and I (or anyone associated with me) if you choose to buy from them. I'm recommending them because I like their product and the support I have received has been excellent. IAASR saved me a lot of time building this project and I think you will be happy with them as well.

Prepare the Enclosure

The power supply project described in this article requires an enclosure with the following minimum dimensions:

   7" Wide x 3.5" High x 6" Deep

Although the enclosure can be constructed of any rigid material (plastic, aluminum, etc.) I recommend using a material that can provide some EMI shielding and AC ground fault protection. In this design I used a painted steel enclosure from IAASR which had the holes cut and AC input, AC switch, and fan already installed. I removed the components for illustration purposes showing the product being fully assembled.

Attached below are the detailed shop drawings needed to fabricate the enclosure front, back, and bottom panels.

The shop drawings are full-scale and can be used as a template for transferring the layouts to the enclosure. When cutting holes, I strongly recommend protecting the panels with two layers of painters tape to prevent accidental scratches and tool marks from marring the finish.

If using a power drill for round holes, be sure to use a thick piece of wood at the back to avoid bending/cracking the panel and to act as a drill stop. Square openings can be cut and smoothed out with a Dremel tool cutoff wheel. Curved openings in steel can be rough cut with a Dremel tool cutoff wheel and finished up with a Tungsten Carbide cutter.

If you purchase the SimCase enclosure from IAASR you can skip this step.

A full size PDF file for all Power Supply diagrams and parts list can be downloaded >>> HERE <<<.

Install the Case Feet

1. Remove the plastic case feet from the package and verify all mounting hardware is present.

2. Install three plastic feet as shown in the illustration above. Do not install the left rear foot (next to the AC input module position) yet.

3. Cut one piece of green #18 AWG wire 2" long, and one piece of green #18 AWG wire 4" long. Strip and tin 1/4" from each wire end.

4. Insert one end of the 2" and 4" green wire into a #8 Ring Terminal and solder the wires to the terminal.

5. Solder a 0.25" Female Quick Disconnect connector to the free end of the 2" green wire.

4. Scrape off the paint inside the case around the foot screw hole so that the ground cable ring terminal makes metal-to-metal contact with the case.

5. Install the last plastic foot as shown in the illustration below making sure that the ground cable ring connector is installed first, then the lock washer, and finally the hex nut.

Install the AC Input Module

Note: If you purchased a case from IAASR the AC Input Module is already installed. Skip to the next step.

1. Insert the AC Input Module oriented as illustrated in the diagram above.

2. Fasten the AC Input Module to the case with two #8 machine screws and hex nuts.

3. Firmly tighten the machine screws but do not over-tighten.

For reference, the AC Input Module datasheet is included <<HERE>>.

Install the AC Power Switch

Note: If you purchased a case from IAASR the AC Power Switch is already installed. Skip to the next step.

1. Insert the AC Power Switch as illustrated in the diagram above.

2. Push the AC Power Switch into the case cutout until the top and bottom retaining clips snap into place.

Install the Cooling Fan

Note: If you purchased a case from IAASR the Cooling Fan is already installed. Skip to the next step.

1. Hold the fan cover against the outside fan opening and thread a single #8x1" machine screw through the cover and into the case.

2. Determine the direction of fan flow from the datasheet and orient the fan so that it's exhaust side is facing the fan cover.

3. While holding the screw in place, slide the fan (wire leads facing up) over the machine screw and thread a #8 hex nut onto the screw until both the fan cover and fan are held loosely against the case.

4. Line up the cover and fan so that each machine screw can be threaded through the cover and fan.

5. Insert the remaining three machine screws through the cover and fan.

6. Thread a #8 hex nut onto each machine screw until all four corners of the cover and fan are held loosely in place against the case.

7. Tight each machine screw until firm. Do not over-tighten.

For reference, the fan datasheet is included >>> HERE <<<.

Wire AC Input, AC Switch, and Ground

1. Connect the ground lead quick disconnect to the center lug of the AC input module as illustrated in the diagram above.

2. Cut one piece of white #18 AWG wire 1.5" long, and one piece of black #18 AWG wire 1.5" long. Strip and tin 1/4" from each wire end.

3. Solder two 0.25" Female Quick Disconnect connectors to the black wire.

4. Solder two 0.25" Female Quick Disconnect connectors to the white wire.

5. Connect one end of the black lead to the left lug of the AC input module. Connect the other end to the bottom left lug of the AC Power switch. Refer to the diagram below to verify that the black lead is connected properly.

6. Connect one end of the white lead to the right lug of the AC input module. Connect the other end to the bottom right lug of the AC Power switch. Refer to the diagram below to verify that the black lead is connected properly.

Install Front Panel Binding Posts

The binding posts from Vktech are ruggedly constructed and include a lot of mounting hardware which makes them a good value for the money. However the center conductors are about 1/2" longer than necessary which can waste a lot of space in the cabinet. To keep the supply compact, the binding posts must be modified as indicated in the above diagram and the following steps:

1. Remove all the hardware from the binding post and pull off the rear plastic insulator.

2. Unscrew the Red and Black post caps several turns and push down firmly from the end each cap to make sure the center conductor is seated all the way down on the front plastic insulator.

3. Using a Sharpie pen and a ruler, measure and place a mark on the metal center conductors 1/2" from the front plastic insulator (refer to diagram above).

4. Using a cutoff wheel and a Dremel tool, cut through the metal center conductors at the marks to remove the top portion of the center conductors (refer to diagram above).

5. Insert the front portion of the binding post into the case (refer to above diagram).

6. Slide on the rear plastic insulator, followed by two flat washers, and a lock washer on each center conductor (refer to above diagram). All hardware is included with the Vktech binding posts.

7. Thread a hex nut onto each binding post and hand tighten while sliding the insulators back and forth until they are seated properly in their holes. Do not tighten fully yet.

Install Front Panel Output Switches

1. Remove the outer hex nut, lock washer, and flat washers from the SPST toggle switch.

2. Hand tighten the inner hex nut until snug against the switch body.

3. Install the large flat washer with the tab facing toward the switch body as illustrated in the diagram above.

4. Insert the SPST toggle switch into the bottom left hole in the front panel.

5. Orient the SPST toggle switch so that the two solder lugs are closest to the bottom of the enclosure as indicated in the diagram above.

6. Install the small flat washer onto the toggle barrel at the front of the enclosure as indicated in the diagram above.

7. Thread the hex nut onto the toggle barrel until hand tight.

8. Hold the switch body in position and firmly tighten the hex nut. The switch body should not move when the toggle switch is operated. If it does, tighten the hex nut until the switch body does not move.

Repeat the above for the SPST toggle switch on the bottom right of the front panel.

For reference, the SPST toggle switch datasheet is included >>> HERE <<<.

Install Potentiometers

1. Thread a hex nut onto two 50K Ohm potentiometers and hand tighten until snug.

2. Insert the 50K Ohm potentiometers into the positions indicated in the above diagram.

3. Install a flat washer onto the 50K Ohm potentiometer shafts.

4. Thread a hex nut onto the 50K Ohm potentiometer shafts until hand tight.

5. Hold the potentiometer body in the position indicated in the above diagram and firmly tighten the hex nut. The potentiometer body should not move when the shaft is turned throughout it's full range of motion. If it does, tighten the hex nut until the potentiometer body does not move.

Repeat the above steps with the 100K Ohm potentiometers.

For reference, the potentiometer datasheet is included <<< HERE >>>.

Install V/I Meters

1. Insert the V/I displays half way into the cutouts provided.

2. Using fingertips or a screwdriver, depress the plastic retaining clips on the display bezel so that they clear the panel cutout.

3. While keeping the plastic retaining clips depressed, press the display into the cutout until the clips snap into place. Use care not to bend the panel while installing the displays.

Note: On some V/I displays the plastic retaining clips are too thick or too rigid to allow the display to be easily installed without bending the front panel. The best solution is to trim some of the plastic from the retaining clips until the display can be installed with reasonable force.

Note: On some displays the plastic retaining clips are too far back from the front bezel which causes the displays to fit loosely in the front panel. The best solution is to hold the display against the front panel while running a small bead of hot glue along the left and right side of the display (inside the case). Use caution to avoid getting glue on the outside front panel.

Install Control Knobs

Insert the Red and Blue control knobs onto the front panel potentiometers as indicated in the above diagram.

The enclosure is now complete with all attachments and controls. The next section will describe how to install and wire the power supplies and converters.

For reference, the knob datasheet is attached >>> HERE <<<.

Prepare the DC-DC Converters

The DC-DC Converters used for this project accept a wide range of input voltages (5V - 32V) and convert that to a variable voltage between 1V and 20V with adjustable current limit between 0.1A and 3A. The DC-DC converters are operated in step-down switch-mode from a 24V DC input. The DROK converters are compact, easy to use, and >95% efficient for most of their range.

Adjustment of the output voltage and current limit is accomplished with two multi-turn trimmer resistors. In order to bring these adjustments to a potentiometer on the front panel, it is first necessary to remove the trimmer resistors. The quickest way to do that is to carefully cut them off of the board with a small pair of wire cutters. This might seem extreme but the PCB is very thick and the trimmer resistors are soft and easy to cut through. When the trimmer resistor body is removed, there will be three small component leads sticking up that can be easily desoldered. I believe this method is faster and results in less chance that pads and traces will be damaged from excess heat. I used the cut and release method on both converters with no problems. If you have a vacuum powered desoldering station by all means give that a try. Start with DC-DC Converter #1:

1. Notice that on each board the parameter that the trimmer resistor adjusts is indicated in white letters. On a blank piece of paper, draw an outline of the board and make a note of which trimmer is the voltage adjustment (CV) and which trimmer is the current adjustment (CC). On the DC-DC converters used for this project, the voltage trimmer was on the outside of the board and the current trimmer was next to the voltage trimmer.

2. Starting with the outside trimmer, use a small pair of wire cutters to cut a small groove into the outside corner of the trimmer body. Use one hand to hold onto the trimmer resistor while cutting with the other hand. Use only the force necessary to keep the wire cutter blades in contact with the trimmer body. Let the scissor action of the wire cutters do the work. The goal is to cut through the plastic body of the trimmer. Don't try to cut too much at one time.

3. When the corner gap is deep enough, begin cutting a groove into the adjacent corner next to the inside trimmer. When the second groove is deep enough, cut through the side of the trimmer body. There may be a crunching sound as the wire cutters reach internal ceramic components. Do not be concerned. Only the trimmer is being damaged.

4. Starting with the opposite outside corner of the trimmer, begin cutting a groove into the trimmer body.

5. When the groove is deep enough, cut through the short side of the trimmer body. Don't attempt to cut through the metal adjustment screw.

6. At this point, the trimmer body will crack and separate in half. Remove the ceramic disk and brass adjustment hardware.

7. Using needle nose pliers, straighten the three wires sticking up from the remains of the trimmer body.

8. Carefully cut away the remaining bottom of the trimmer body leaving only the three wires sticking up from the PCB. Do not cut these wires as the remaining length will aid in removing the wires from the PCB.

9. Repeat steps 2 through 8 for the remaining trimmer resistor.

10. Desolder the trimmer wires and clear as much as solder as possible from the pad holes.

By working carefully and slowly, it is easy to remove the trimmer resistors without harming nearby components or the PCB.

11. Remove the 10A output fuse and replace with a 3A fuse.

12. Cut 4" lengths of #28 or smaller stranded hookup wire. Choose wire with different color jackets to make identification easier when the wires are soldered to the potentiometers in a later step.

13. Strip 1/4" insulation from both ends of each wire and tin the ends with solder.

14. Solder each wire to the DC-DC converter solder pads as illustrated above.

Repeat the above steps for DC-DC Converter #2.

Note: In the illustration above the potentiometer pads are labeled 1, 2, and 3. These numbers (and wire colors) will be referred to when soldering the converter leads to the potentiometers. The jacket color chosen in this step is arbitrary. Any color can be used as long as the builder remembers to match the instructable colors with the actual colors so that the potentiometer leads are soldered to the correct pin.

Attach Enclosure Standoffs Using Screws and Insulating Washers

The IAASR SimCase product comes with 9 standoffs and 18 machine screws that work perfectly with the DROK DC-DC converters. Remove these standoffs from the case for use with mounting the converters.

If using a different enclosure, the converters will require eight #8 hex standoffs and 16 #8 machine screws.

Insulating washers are used ensure no exposed traces on the DC-DC converters come in contact with the enclosure and AC safety ground. They also add some height to the standoffs so that there is enough clearance for the PCB heat sink.

1. Insert one insulating washer on a #8 machine screw.

Note: The washer may be a tight fit on some screws. If the washer cannot be easily installed on the machine screw, increase the diameter of the washer slightly with a push drill.

2. Insert the machine screw and washer into a mounting hole on DC-DC converter #1.

3. Insert one insulating washer onto the screw where it exits the opposite side of the PCB.

4. Thread the hex standoff onto the screw until hand tight.

5. While holding the standoff with a pair of pliers or adjustable wrench, firmly tighten the screw.

6. Repeat steps 1 through 5 for the remaining PCB mounting holes.

Repeat the above for the DC-DC Converter #2.

Install DC-DC Converters

1. Line up the holes in the front of the enclosure with the hex standoffs of DC-DC Converter #1.

2. Insert a #8 flat washer onto a #8 0.5" machine screw.

3. Tread the machine screw through the bottom of the enclosure and into the hex standoff.

4. Hand tighten the machine screw.

5. Repeat steps 2 through 4 for each remaining hex standoff.

6. When all 4 machine screws and flat washers are installed, firmly tighten each screw.

7. Repeat steps 1 through 6 for DC-DC converter #2.

Build Front Panel Cables

1. Cut a piece of 22AWG stranded red wire 4" long. Strip 1/4" insulation from each end and tin with solder.

2. Solder a #10 Ring Connector on one end of the 4" wire wire.

3. Cut a piece of 22AWG stranded red wire 3" long. Strip 1/4" insulation from each end and tin with solder.

4. Solder a #10 Ring Connector on one end of the 3" wire.

5. Cut a piece of 22AWG stranded red wire 5" long. Strip 1/4" insulation from each end and tin with solder.

6. Cut a piece of 22AWG stranded red wire 6" long. Strip 1/4" insulation from each end and tin with solder.

7. Strip 1/4" insulation from each lead attached to the 3-pin display connector and tin with solder.

8. Solder a #10 Ring connector to both Yellow leads attached to the 3-pin display connector.

9. Strip 1/4" insulation from each lead attached to the 2-pin display connector and tin with solder.

Wire the Front Panel Power Connections

Use the diagram above in parallel with the following instructions to complete the output power connections on the front panel.

1. Remove the hex bolt and lock washer from the positive (Red) Power Supply 1 binding post.

2. Install the Ring Terminal of Cable A followed by the lock washer onto the positive (Red) Power Supply 1 binding post.

3. Thread the hex bolt onto the positive (Red) Power Supply 1 binding post and tighten firmly. Do not over-tighten the hex bolt.

4. Solder the free end of Cable A onto the lower pin of the Power Supply 1 SPST power switch.

5. Remove the hex bolt and lock washer from the positive (Red) Power Supply 2 binding post.

6. Install the Ring Terminal of Cable B followed by the lock washer onto the positive (Red) Power Supply 2 binding post.

7. Thread the hex bolt onto the positive (Red) Power Supply 2 binding post and tighten firmly. Do not over-tighten the hex bolt.

8. Solder the free end of Cable B onto the lower pin of the Power Supply 2 SPST power switch.

9. Solder one end of Cable C to the upper pin of the Power Supply 1 SPST power switch. Route the other end of Cable C to the output connector of DC-DC Converter 1 but do not attach yet.

10. Solder one end of Cable D to the upper pin of the Power Supply 2 SPST power switch. Route the other end of Cable D to the output connector of DC-DC Converter 2 but do not attach yet.

11. Remove the hex bolt and lock washer from the negative (Black) Power Supply 1 binding post.

12. Install the Ring Terminal of Cable E followed by the lock washer onto the negative (Black) Power Supply 1 binding post.

13. Thread the hex bolt onto the negative (Black) Power Supply 1 binding post and tighten firmly. Do not over-tighten the hex bolt.

14. Remove the hex bolt and lock washer from the negative (Black) Power Supply 2 binding post.

15. Install the Ring Terminal of Cable F followed by the lock washer onto the negative (Black) Power Supply 2 binding post.

16. Thread the hex bolt onto the negative (Black) Power Supply 2 binding post and tighten firmly. Do not over-tighten the hex bolt.

17. Plug the 3-pin connector of Cable E onto Power Supply 1 display module.

18. Plug the 3-pin connector of Cable F onto Power Supply 2 display module.

19. Insert the Red wire from Cable E and the Red wire Cable C into the DC-DC Converter 1 output terminal block positive (+) position and firmly tighten the terminal block screw. Do not over-tighten the terminal block screw.

20. Insert the Black wire from Cable E into the DC-DC Converter 1 output terminal block negative (-) position and firmly tighten the terminal block screw. Do not over-tighten the terminal block screw.

21. Insert the Red wire from Cable F and the Red wire Cable D into the DC-DC Converter 2 output terminal block positive (+) position and firmly tighten the terminal block screw. Do not over-tighten the terminal block screw.

22. Insert the Black wire from Cable F into the DC-DC Converter 2 output terminal block negative (-) position and firmly tighten the terminal block screw. Do not over-tighten the terminal block screw.

Wire the Front Panel Potentiometers

Use the diagrams below in parallel with the following instructions to complete the potentiometer connections on the front panel.

1. Solder the CV leads from DC-DC Converter 1 to the 50K voltage adjust potentiometer. Solder pins 1 through 3 on the DC-DC Converter PCB to pins 1 through 3 on the potentiometer as illustrated below.

2. Solder the CC leads from DC-DC Converter 1 to the 100K current adjust potentiometer. Solder pins 1 through 3 on the DC-DC Converter PCB to pins 1 through 3 on the potentiometer as illustrated below.

3. Solder the CV leads from DC-DC Converter 2 to the 50K voltage adjust potentiometer. Solder pins 1 through 3 on the DC-DC Converter PCB to pins 1 through 3 on the potentiometer as illustrated below.

4. Solder the CC leads from DC-DC Converter 2 to the 100K current adjust potentiometer. Solder pins 1 through 3 on the DC-DC Converter PCB to pins 1 through 3 on the potentiometer as illustrated below.

Dress Front Panel Wiring With Plastic Cable Ties

Double-check front panel wiring. Using small plastic cable ties, dress the front panel cabling for appearance.

Wire the 24V Power Supplies

1. Remove one screw above the barrier strip from each 24V power supply as indicated in the above illustration.

2. Install a plastic P-Clip using the screw just removed oriented as indicated in the above diagram.

3. Build cables A, B, C, and D as indicated in the diagram above. Strip 1/4" insulation from both ends of each wire. Solder all connectors and tin all bare wire ends.

4. Attach cables A and C to 24V Power Supply 1 as indicated in the above illustration. The Black wire of cable A connects to the barrier screw marked 'L'. The White wire of cable A connects to the barrier screw marked 'N'. The Black wire of cable C connects to the barrier screw marked '-V'. The Red wire of cable C connects to the barrier screw marked '+V'.

5. Attach cable D to 24V Power Supply 2 as indicated in the above illustration. The Black wire of cable D connects to the barrier screw marked '-V'. The Red wire of cable D connects to the barrier screw marked '+V'.

6. Place 24V Power Supply 1 and 2 back to back as illustrated in the diagram above.

7. Connect cable B between 24V Power Supply 1 and 2 by threading the wire through the P-Clips. The Black wire of cable B connects to the barrier screw marked 'L' on both supplies. The White wire of cable B connects to the barrier screw marked 'N' on both supplies. The Green wire of cable B connects to the barrier screw marked 'G' on both supplies.

Install the 24V Power Supplies

Lift both 24V Power Supplies and place them in the enclosure as indicated in the diagram above. Verify that the power supply modules align with the enclosure mounting holes but do not fasten the power supplies at this time.

Complete Wiring of the 24V Power Supplies

1. Attach the 24V Power Supply AC input and ground leads as indicated in the diagram above above.

2. Plug the 2-Pin connectors G and H into the V/I displays.

3. Connect cable C from 24V Power Supply 1 to DC-DC Converter 1. The Red wire connects to the 'IN+' screw terminal of the DC-DC Converter as shown in the diagram above. The Black wire connects to the 'IN-' screw terminal of the DC-DC Converter as shown in the diagram above.

4. Connect 2-Pin cable G to DC-DC Converter 1. The Red wire connects to the 'IN+' screw terminal of the DC-DC Converter as shown in the diagram above. The Black wire connects to the 'IN-' screw terminal of the DC-DC Converter as shown in the diagram above.

5. Connect cable D from 24V Power Supply 2 to DC-DC Converter 2. The Red wire connects to the 'IN+' screw terminal of the DC-DC Converter as shown in the diagram above. The Black wire connects to the 'IN-' screw terminal of the DC-DC Converter as shown in the diagram above.

6. Connect 2-Pin cable H to DC-DC Converter 2. The Red wire connects to the 'IN+' screw terminal of the DC-DC Converter as shown in the diagram above. The Black wire connects to the 'IN-' screw terminal of the DC-DC Converter as shown in the diagram above.

7. Connect the Cooling Fan cable to 24V Power Supply 2 as shown in the diagram above. The Red wire connects to the barrier screw marked '+V' on 24V Power Supply 2. The Black wire connects to the '-V' barrier screw on 24V Power Supply 2. The Cooling Fan Blue tachometer wire is not used.

8. Secure the 24V power supplies to the enclosure with four #8 machine screws, washers, and lock nuts.

9. Dress all wires and secure with cable ties.

Testing Before Power-On

Before plugging in and powering on the completed power supply for the first time, perform the following checks:

1. Using a digital VOM set to Ohms, measure the resistance between the 'L' and 'N' binding screws on 24V Power Supply 1. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

2. Using a digital VOM set to OHMS, measure the resistance between the 'L' and 'G' binding screws on 24V Power Supply 1. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

3. Using a digital VOM set to OHMS, measure the resistance between the 'N' and 'G' binding screws on 24V Power Supply 1. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

If any of the above measurements are not correct and all wiring has been verified, do not proceed and do not connect the power supply to an AC outlet. Contact the 24V AC Power Supply representative for further instructions.

4. Make sure the power supply output SPST switches are in the OFF (Down) position.

5. Using a digital VOM set to OHMS, measure the resistance between the Positive (Red) output binding post and the Negative (Black) output binding post of power supply 1 (Left Side). The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all DC-DC Converter output wiring is correct using the power supply schematic diagram. Check that the output binding post is properly seated in the front panel and that there are no bits of wire or solder touching the power supply enclosure or other circuit connections. Do not proceed until the resistance is within the indicated range (>10K Ohms).

6. Using a digital VOM set to OHMS, measure the resistance between the Positive (Red) output binding post and the Negative (Black) output binding post of power supply 2 (Right Side) . The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all DC-DC Converter output wiring is correct using the power supply schematic diagram. Check that the output binding post is properly seated in the front panel and that there are no bits of wire or solder touching the power supply enclosure or other circuit connections. Do not proceed until the resistance is within the indicated range (>10K Ohms).

7. Put both power supply output SPST switches in the ON (Up) position.

8. Using a digital VOM set to OHMS, measure the resistance between the Positive (Red) output binding post and the Negative (Black) output binding post of power supply 1 (Left Side). The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all DC-DC Converter output wiring is correct using the power supply schematic diagram. If after verifying that circuit wiring is correct, do not proceed. Contact the DC-DC Converter representative for further instructions.

9. Using a digital VOM set to OHMS, measure the resistance between the Positive (Red) output binding post and the Negative (Black) output binding post of power supply 2 (Right Side). The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all DC-DC Converter output wiring is correct using the power supply schematic diagram. If after verifying that circuit wiring is correct, do not proceed. Contact the DC-DC Converter representative for further instructions.

10. Attach a power cord to the power supply AC Input Module. DO NOT PLUG THE POWER CORD INTO AN AC OUTLET.

11. Turn on the AC Power Switch.

12. Using a digital VOM set to OHMS, measure the resistance between the power cord hot and neutral conductors. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring between the AC Input Module and the AC Power Switch is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

13. Using a digital VOM set to OHMS, measure the resistance between the power cord hot and ground conductors. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring between the AC Input Module and the AC Power Switch is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

14. Using a digital VOM set to OHMS, measure the resistance between the power cord neutral and ground conductors. The VOM should read very high (>10K Ohms) or infinite resistance. If the VOM reads low resistance or a short circuit, verify that all AC wiring between the AC Input Module and the AC Power Switch is correct using the power supply schematic diagram. DO NOT CONNECT THE POWER SUPPLY TO AN AC OUTLET UNTIL THE MEASUREMENT IS CORRECT (VERY HIGH OR INFINITE RESISTANCE).

Attach the Top Cover

Place the enclosure top over the power supply chassis and line up the holes in the top cover with the holes in the base. Secure the top to the base with the screws supplied by the enclosure vendor.

Power-On Testing

1. Verify that the back panel AC Power Switch is in the Off position.

2. Verify that the front panel Output SPST switches are in the Off (Down) position.

3. Verify that the front panel Voltage and Current controls are rotated fully counter clockwise.

3. Plug the power supply power cord into an AC outlet.

4. Turn on the AC Power Switch

The AC Power Switch will illuminate. If it does not illuminate, verify that the AC Outlet is energized and that the power cord is fully plugged into the outlets at both ends. IF THE AC POWER SWITCH DOES NOT ILLUMINATE, REMOVE THE POWER CORD FROM THE AC OUTLET.

5. The Cooling Fan will turn on and the front panel V/I displays will illuminate. If the cooling fan or V/I displays do not come on, turn off the AC power switch and disconnect the power cord from the AC outlet. Remove the top cover and verify that the fan or display wiring is correct. If the fan or display wiring is correct, contact the fan or display representative for further instructions.

6. The front panel V/I displays should indicate approximately 1.00V and 1.50V for the output voltage and 0.00A for the output current. If the V/I display indicates approximately 20V with the voltage controls fully counter-clockwise, turn off the AC power switch and disconnect the power cord from the AC outlet. Remove the top cover and verify that the voltage potentiometer wiring is correct. If the potentiometer wiring is correct, contact the DC-DC Converter representative for further instructions.

7. Rotate the Power Supply 1 (Left Side) voltage control clockwise. The V/I display should show the output voltage increasing as the control is turned clockwise and decreasing when the control is turned counter-clockwise. If the V/I display does not change in value as the voltage control is rotated, turn off the AC power switch and disconnect the power cord from the AC outlet. Remove the top cover and verify that the voltage potentiometer wiring is correct. If the potentiometer wiring is correct, contact the DC-DC Converter representative for further instructions.

8. Rotate the Power Supply 2 (Right Side) voltage control clockwise. The V/I display should show the output voltage increasing as the control is turned clockwise and decreasing when the control is turned counter-clockwise. If the V/I display does not change in value as the voltage control is rotated, turn off the AC power switch and disconnect the power cord from the AC outlet. Remove the top cover and verify that the voltage potentiometer wiring is correct. If the potentiometer wiring is correct, contact the DC-DC Converter representative for further instructions.

Optional Load Testing

9. Rotate all front panel voltage and current controls fully counter clockwise.

10. Verify that both output SPST switches are in the Off (Down) position.

10. Attach a 10 Ohm, 20W resistor between the Positive (Red) and Negative (Black) output binding posts of Power Supply 1 (Left Side).

11. Flip the Power Supply 1 output SPST switch to the ON position.

12. The V/I display should show an output current of approximately 0.10A

13. Flip the Power Supply 1 output SPST switch to the OFF position.

14. Rotate the voltage control until the V/I display reads 10V.

15. Flip the Power Supply 1 output SPST switch to the ON position.

16. The V/I display should indicate a reduced output voltage and between 0.10A and 0.20A.

17. Slowly rotate the current control until the V/I display reads 10V and approximately 1.00A.

18. Flip the Power Supply 1 output SPST power switch to the OFF position.

19. Remove the 10 Ohm, 20W resistor from the output binding post of Power Supply 1.

20. Attach a 10 Ohm, 20W resistor between the Positive (Red) and Negative (Black) output binding posts of Power Supply 2 (Right Side).

21. Repeat steps 11 through 19 for Power Supply 2.

22. Turn off the AC Power Switch and disconnect the AC Power Cord from the AC wall outlet.

The Dual Switched-Mode Power Supply is ready for use.

I've been fascinated with the early history of radio and television between 1870 through 1929 since reading a very old copy of "The Boys Second Book of Radio and Electronics" by Alfred Morgan, found in the dusty science shelves of my highschool library (I'm a "geek" and that book made me happy).  This period is dominated by some very interesting and bizarre personalities like Thomas Edison, Guglielmo Marconi, John Fleming, Nikola Tesla, Reginald Fessenden, Lee de Forest, and Edwin Armstrong. These people are credited with the fundamental inventions commercialized by RCA, GE, AT&T, and Westinghouse; companies which flourished in the golden age of radio and still exist today!

They were mostly experimenters and tinkerers at a time when academic research was often unconcerned with the practical application of a scientific discovery. As a result the industry did not have a good understanding of how the electronic devices they were building actually worked. At times the money to be made from broadcast radio, the competitive litigation that inevitably follows the profits, and the government regulation intended to control on overheated emerging industry became more significant than the technology. Kind of like it does today, right? These are the major reasons why there isn't a Wright Brothers airplane company around today.

The development of radio even struggled through two world wars and the Great Depression. But consumer desire for a well-deserved break from daily life through entertainment created demand for innovation in radio that sustained the industry beyond hard times. Just like it does today. This is what I find inspiring from all that I have learned from engineering: Imperfect people with mostly good intentions and sometimes bad behavior, equipped with what the future will refer to as "stone knives and bearskins", can still accomplish something good and useful for society. When I see that I think, "There's hope for us all".

The 60's and 70's were the golden age of electronic kit building. The novice had a great many educational kits to choose from that taught basic skills and electronics theory while assembling a device that could do something useful or entertaining. An advanced builder could use "sweat equity" to inexpensively obtain popular electronic products like HiFi stereo systems, color televisions, CB radios, and automotive test equipment.

Today there are still quite a few electronics kits available for education purposes, but rapidly evolving features and shrinking components ended the consumer electronics kit business. I would have loved to have assembled some of the products that were popular in the 70's, but unfortunately most of the companies that made all those wonderful kits were gone when I began my career. In honor of a great set of educational Radio Shack electronics project kits available during the 60's and 70's, I've redesigned the original Science Fair brand One Tube AM Radio (Catalog #28-100) using modern components still available from electronics component re-sellers in the US. All of the components in the updated kit can be found on Amazon.

There were many things Radio Shack did extremely well during its prime. It's too bad the company is "history" along with Lafayette Radio, Allied Radio, Heathkit, Popular Electronics, Byte, and many others who were here for a time and are here no more. Their humor, their creativity, hand-drawn artwork... Experimenters young and old, especially in rural areas, are less served by their demise.

For the nostalgic or the enthusiast who would like to build an updated version of this classic radio, I've included schematics, parts information, assembly documentation, and pictures of a completed and tested One Tube AM Radio based on the original from 1968, which is itself derived from a design attributed to Lee de Forest in 1908. If like me you enjoy walking in the footsteps of the great inventors of the 20th century, you'll love this project.

Just a Song Before I Go...

An original Radio Shack One Tube AM Radio kit was given to me by a mentor of mine when I was still learning how to solder. But I didn't really have the proper tools for electronics assembly work. While putting the kit together, I accidentally broke off one of the tube socket pins. I tried as hard as I could to solder a wire to the little stub of metal sticking out but couldn't. When I went to my local Radio Shack, the One Tube Am Radio kit had been long discontinued. "A 7-pin tube socket? Ha! Good luck kid." So I shelved the kit and moved on. But I never forgot that early failure. So for me this project was a redemption, so to speak.

After redesigning the tuning circuit with available parts and building the radio, I was disappointed in its performance. After checking with a few people online that could remember building the original Radio Shack kit back when it was available, I found the consensus to be that the radio never really worked well. Slightly better than a crystal radio I believe was the usual outcome. I didn't want a nice looking tube radio that didn't work, so I decided to rewire it using some old amateur radio articles published in the 1920's for inspiration. I was very happy to find that the new design presented in this article worked much better than the original. I can easily receive all three local AM stations in my city with a 10ft piece of wire for an antenna and an earth ground connection.

Although I feel my results made the project costs, time, and effort worthwhile I think it wise to present to the reader a few observations and opinions:

1. I recommend this project for the intermediate to expert builder. Moderate soldering skills are required and some metal fabrication must be done for the variable capacitor and tuning coil brackets. Connecting and soldering wires to the mounted tube socket can be a challenge for the inexperienced. Several of the components are fragile and relatively expensive. If you have successfully completed a few projects from scratch using breadboard construction techniques, then you should be good to go for the One Tube AM Radio project.

2. If you decide to build this project, you should have no problem obtaining the parts. I deliberately avoided using rare or hard to find vintage parts in order to save those for the folks that are trying to restore a vintage radio. Instead, I provide component names and part numbers from distributors of new or new-old-stock in volume that are compatible with the original look of the radio as much as possible.

3. The 22.5 V battery I used is the most expensive component of the lot at $25. I chose that battery because I wanted to keep to the original look and feel as much as possible, which required me to spend more money than I would otherwise recommend. By all means feel free to substitute three 9V batteries in series for the 22.5V battery.

Should you decide to use the 22.5V battery I did, you can obtain it from Excell via Amazon (ASIN B009Z1ERAE). This battery works extremely well in the radio, and will run for 4 months of continuous use or more than a year of occasional use. I suspect what Excell has done is connect fifteen 180mAh alkaline button cells in series and secure them to a plastic carrier that is the same size as the old NEDA 215. So perhaps there is some hacking that can be done instead of buying a new one.

4. Current consumption on the 22.5V B Battery is 40uA so it can run quite a long time. However, A Battery consumption on the 1T4 tube is 50mA, in line with the manufacturers specifications. Which means you can expect between one to two days of continuous use and perhaps several weeks of occasional use. Keep a pack of AA's handy.

5. The most significant deviation from the original Radio Shack kit is the use of a tuning capacitor. After many days spent searching through the web for a volume source for the antenna coil, I concluded that it is not possible to obtain one similar to the original without resorting to a rare vintage supplier. So I decided to use an adjustable Miller-style antenna coil from AmplifiedParts.com and tuning capacitor from uxcell.com. The adjustable ferrite slug in the antenna coil I used allows for alignment of the tuning range only. Main tuning is provided by the variable capacitor. But the tuning coil does look enough like the original part to make me happy, and the redesign allows the radio to tune into the Extended AM Broadcast band between 1610 kHz and 1710 kHz, something the original kit wasn't designed to do.

6. This project requires "alignment" in order to set the tuning range between 540 kHz and 1710 kHz. Which means the builder will need a hex alignment tool for the tuning coil. I used a 0.100" hex alignment tool from Aven Tools (Amazon ASIN B001Q4YGQS) which is a nice sturdy kit that has worked well for me. Alignment is best accomplished by setting the coil slug and variable capacitor trimmers as indicated in the "Before You Begin" section. An LCR meter is suggested for the most accurate alignment but is not required.

If you are still with me after all of the above, then let's get started.

Please Note: I have no business relationship with any vendors mentioned in this article. Nothing of financial value was exchanged for any recommendation I make. None of the vendors mentioned in this article provided compensation of any kind during the creation of this project. I will not be compensated in any way if you choose to build this project or purchase components from any vendor I recommend. I simply had a good experience with the vendors I recommend and believe you will too.

Obtain Components Listed in the Assembly Manual Parts List

Review the parts list and obtain the components indicated. Everything but a few pieces of hardware are available on Amazon or can be obtained directly from the suppliers indicated at the bottom of the parts list.

Below are a few notes regarding the parts used for the radio:

1. Amplified Parts (www.amplifiedparts.com) is my go-to resource for tons of components used in vacuum tube amplifier and radio circuits. Tubes, knobs, jacks, coils, earphones, resistors, capacitors.. it's all there. They have tons of NOS components in regular inventory, online ordering is easy, and shipping is extremely fast. The critical components of the One Tube AM Radio were all obtained here. I purchased several 1T4 tubes, 7-pin sockets, antenna coils, and crystal earphones from AmplifiedParts and they all work great. The crystal earphone comes with a 1/8" mono phone plug so I added a 1/8" mono phone jack from Amazon. If you like to build, repair, or restore vintage equipment you have to check out Amplified Parts.

2. For bulk resistors used in project building and experimentation, I highly recommend the excellent Joe Knows Electronics (www.joeknowselectronics.com) resistor kit in 1/4W and 1/2W sizes. These kits include the most popular values you will be likely to use with 1% tolerance (which is overkill for basic circuits but nice to have).

3. I strongly recommend ordering NP0 ceramic disk capacitors from Allied, Mouser or Digikey as they will far outperform most anything you can get on Amazon. The Joe Knows Electronics capacitor kit is an extremely good buy for general purpose capacitors at 645 pieces for $13.00.

4. The variable capacitor (and a lot of other rather old and interesting electronic parts) can be found at Uxcell (www.uxcell.com) which seems an unlikely domain for radio stuff, but they do have a lot of radio stuff that's interesting. I've created a diagram of the variable capacitor below that will help you figure out how to wire it into radios circuits of your own design.

5. The case for the radio I built is a Hammond 1591GSBK ABS Project Box from Amazon.com with a piece of vector breadboard cut to fit on the top and spray painted with high temperature automotive flat red and finished with a semi-gloss clearcoat. I like the look of red on black, and the red color of the breadboard matched the red color of the original pbox kit. It's completely up to you how you want to house and color the kit you build.

6. The knob I used is a Radio Shack product I've had in inventory for decades. Use anything you think is cool that will fit on the varicap shaft. The shaft on the varicap is only about 1/4" long so you will need something to extend it. I used a nylon hex standoff and shaved it down to fit. Use anything you have that gives the knob a confident solid feel.

7. For those curious about the 1T4 vacuum tube, I've included the datasheet I used with the 1T4 from Amplified Parts. The evaluation I did on filament current, plate and grid current/voltage, and Mu parameters for the tube sold to me matches very closely with the parameters in the datasheet. Very useful when doing your own designs.

8. Also below is the datasheet for the antenna coil from Amplified Parts. My measurements show the following:

Primary (Pins 3/4) - 2120uH to 3372uH variable when ferrite is positioned on the primary

Secondary (Pins 1/2) - 163uH to 333uH variable when ferrite is positioned on the primary

Unloaded resonant Q is around 250.

These are the parameters I used to redesign the tuning section of the radio and they were dead on as built.

The datasheet for the 1T4 Tube is >>> HERE <<<.

The datasheet for the Tuning Capacitor is >>> HERE <<<.

The datasheet for the Tuning Coil is >>> HERE <<<.

Before You Begin - Variable Capacitor Alignment

Before you start the build process, the variable capacitor needs to be set to its minimum value by adjusting the position of the trimmer capacitors on the back. If you look closely at the back of the variable capacitor, you will notice 4 small adjustment screws.

Illustration 1 - Variable Capacitor Trimmer Adjustments

These adjustment screws are provided to fine-tune the range of each section of the variable capacitor. The variable capacitor you will receive may have its trimmers set to minimum, maximum, or some value in between (See Illustration 1). What you need to do is adjust each trimmer to it's minimum value as shown in Illustration 6:

Caution: Avoid applying heavy pressure into the trimmer while turning. If the trimmer is hard to turn in one direction, try to rotate it in the opposite direction. Let the tool do the work.

1. Hold the variable capacitor with the trimmer screws facing you. Using a small precision screwdriver or the steel slotted alignment tool from the Aven kit described earlier, turn the T1 trimmer screw clock-wise or counter-clockwise as shown in Illustration 2 until it is in the position indicated in Illustration 3.

2. Do the same for trimmer T2 (see Illustration 3) until it is in the position indicated in Illustration 4.

3. Do the same for trimmer T3 (see Illustration 4) until it is in the position indicated in Illustration 5.

4. Do the same for trimmer T4 (see Illustration 5) and then verify that all trimmers are in their minimum value position as indicated in Illustration 6.

The variable capacitor is now ready for use.

If you have an LCR meter, the variable capacitor should provide the following range:

   Low Value: 18pf to 24pF

   High Value: 298pF to 304pF

A document describing the variable capacitor connections is included >>> HERE <<<.

Before You Begin - Antenna Coil Alignment

The P-C70-A antenna coil provides an adjustable ferrite core for fine tuning the inductance of the secondary winding so that the tuning range of the variable capacitor fits comfortably within the AM broadcast band (540 kHz to 1710 kHz). The antenna coil you will receive has not been aligned and requires adjustment before installation. The alignment can be fine tuned after installation. To align the antenna coil, follow the steps below:

Caution: Use only a 0.100" plastic hex alignment tool. A tool of the wrong size will damage the ferrite slug making it impossible to align the antenna coil. Use only the minimum force necessary to rotate the slug. Do not push against the slug while turning. Let the alignment tool do the work. Check the slug position after the first few turns to make sure it is moving in the correct direction (toward the front of the coil). If it isn't, reverse the direction of alignment tool rotation.

1. Measure 1/2" along the barrel of a 0.100" Hex Alignment Tool starting from the shoulder as shown in the illustration above.

2. Insert the alignment tool into the front of the antenna coil as shown until the alignment tool is fully seated into the ferrite core.

3. Rotate the alignment tool in a counter clockwise direction making sure that the alignment tool is moving out of the front of the coil instead of inward toward the back of the coil.

4. Stop rotation and remove the alignment tool when the top of the ferrite core is 1/2" from the front of the antenna coil.

The antenna coil is ready for use.

If you have an LCR meter, the coil inductance should be between 300uH and 330uH.

A datasheet for the antenna coil is included >>> HERE <<<.

Before You Begin - Fabricate Mounting Brackets

 I've included templates for fabricating the mounting brackets used with the variable capacitor and the antenna coil. I used two PC card slot blanks I had laying around and simply cut them to size, drilled holes, and used a Dremel tool to grind off the flash. You can use any soft metal you have in your junk box. Or even a piece of plastic that is shaped like an 'L' bracket will work. Imagination will pay off in this step.

Caution: Always wear eye protection when using power tools. This project is more fun to share with your friends than your eye doctor.

A 1:1 scale printable version of the bracket template is included >>> HERE <<<.

Some Findings on Earphones From Amplified Parts

There is distinct difference between the earphones originally supplied in Radio Shack kits and the ones available today. The original earphones were Rochelle Salt (potassium sodium tartrate tetrahydrate) which had a typical capacitance of around 300pF and an impedance of around 500K Ohms at 1000 Hz. The salt crystals were cut square and bonded to conductive foil on one side, then attached to a conductive conical aluminum diaphragm and center pushrod on the other side. When energized, the salt crystal bends and moves the aluminum diaphragm up and down via the center pushrod.

Rochelle Salt Piezoelectric Earphone

The earphones available today are made from a thin ceramic disk composed of barium titanate. A conductive pad is bonded to the back of the disk for the positive lead attachment. A conductive flat aluminum diaphragm is bonded to the front side of the disk and the negative lead glued to the diaphragm. When energized, the ceramic disk expands perpendicular to the diaphragm moving it up and down. These earphones have a typical capacitance of 2500pF and an impedance of around 6K Ohms at 1000 Hz.

Ceramic Disk Piezoelectric Earphone

There is a huge difference between the impedance of each type of earphone so I thought I'd compare the performance of each to determine what the differences would be. I found and purchased one of the original Radio Shack earphones in good condition, and purchased several new earphones from Amplified Parts. My first test was an impedance check with various source impedances to determine the roll-off for old and new earphone.

As expected, the original Radio Shack earphone capacitance is around 300pF. Impedance falls where expected; around 500K Ohms at 1000 Hz, around 100K Ohms at 5000 Hz, 10K Ohms at 50 kHz, and 1K Ohms at 500 kHz. Impedance at 1MHz was approximately 1K Ohms.

The new ceramic earphone from Amplified Parts came in at 24nF with impedance of 500K Ohms at 10 Hz, 100K Ohms at 60 Hz, 10K Ohms at 600 Hz, and 1K Ohms at 6000 Hz. Impedance at 1MHz was approximately 7 Ohms. The new earphone has an odd resonance (see photo below) between 3000 and 4000 Hz but rolls off relatively smoothly.

Ceramic Earphone Frequency Response

At first I thought the new earphone might not work well with a 1N34 crystal radio or the One Tube radio described in this article. However after constructing both and trying each headset in a blind test, I couldn't tell that there was a difference in sound volume.

So I decided to compare the sound pressure level from each earphone with a microphone, sweep generator, and spectrum analyzer set as flat as possible between 10 Hz and 10 kHz. As seen in the two images below, the new Amplified Parts earphone is more efficient at converting the sweep generator signal into sound. So I figured I was onto something.

In the next test, I compared sound levels in a radio test circuit with 100K Ohms output impedance to see if there was a significant difference between the original Radio Shack earphone and the new Amplified Parts earphone.

The Amplified Parts earphone loaded down my radio test circuit far more than the original Radio Shack earphone did, but appears to have almost made up for the impedance losses with conversion efficiency, which I think is why I'm not able to subjectively tell the difference between the two when doing a blind listening test.

ONE THING TO KEEP IN MIND, THOUGH:

I've bought several of these and had no issues up until just recently when one failed on me after a few days of radio listening. The two that are still working I've been using for over a year now. So I think there are some occasional quality issues with these that warrant getting more than one when building a project that uses them. I recently placed an order for three just to have extras on hand. You might want to do the same.

Review the Schematic to Become Familiar With the Radio Design

The original Radio Shack design used a variable inductance tuning coil and a fixed capacitor to form the LC parallel resonant tuning circuit. However that variable tuning coil is no longer available from a specialty electronic component retailer. However I was able to find a Miller-style P-C70-A antenna coil (sold by Amplified Parts) with sufficient inductance to permit a variable capacitor to set the LC resonant frequency in the AM broadcast band. The P-C70-A antenna coil has a similar appearance to the original Radio Shack part so I thought it would be fun to redesign the radio using available parts.

Tuning Section

L1 (primary) of the antenna coil is used to lightly couple the long wire antenna to the LC tuning circuit composed of L2 (secondary) and the parallel combination of variable capacitor sections C1, C2, and C3 (see the Varicap datasheet for more information). The ferrite alignment slug is adjusted so that the inductance of L2 is approximately 300uH. The trimmer capacitors of C1-C3 are adjusted to their minimum value so that the combination of C1-C3 provides a variable capacitance between 25pf and 300pF. Using the standard formula for parallel LC resonant frequency and plugging in the values for L and C results in the following tuning range:

Fc = 1 / [ 2 * pi * sqrt ( L * C ) ]

where L = .0003 Henrys and C varies between .000000000025 Farads and .0000000003 Farads.

Fc1 = 1 / [ 2 * pi * sqrt ( .0003 * .0000000003 ) ] = 530 kHz

Fc2 = 1 / [ 2 * pi * sqrt ( .0003 * .000000000025 ) ] = 1837 kHz

Since the medium wave broadcast band in ITU Region 2 (Americas) is 540 kHz and 1710 kHz with 10Khz spacing, my choice of inductor and capacitor worked out well for the tuner section (see block diagram above).

Detector Section

A popular AF detector in the 1920's was the grid leak detector composed of the 1T4 vacuum tube and the parallel combination of R1/C4. The Grid (Pin 6) and Cathode (Pin 1) of 1T4 form a diode that is forward biased by the B battery connected to L2. The large size of R1 (10 Meg Ohm) ensures that the Grid voltage with respect to the Cathode is held at approximately 0V with no signal. This causes the tube Cathode and Plate (Pin 2) to act almost like a closed switch resulting in a fixed voltage across resistor R1 of around 10V. When a station is tuned in, the alternating carrier wave received is directly coupled to the tube Grid (Pin 6) through capacitor C1. On the positive half cycles of the carrier wave the tube Grid is forced positive, but since the tube is already saturated almost no change in current occurs at the Plate output resulting in almost no voltage change across resistor R2. However, on the negative half cycles of the carrier wave, the tube Grid is forced negative which reduces the current across the Cathode/Plate resulting in a voltage change across resistor R2. The top half of the carrier is removed leaving the bottom half of the carrier intact with a copy of the audio signal modulated on it. The internal capacitance of the earphone (2500 pF) filters out the RF carrier wave leaving only the original audio signal. With the earphone connected, the audio signal voltage is converted into sound waves in the earphone that can be heard by the radio listener.

AF Amplifier Section

A basic operating characteristic of a vacuum tube is that small changes in Grid voltage result in large changes in Cathode/Plate current. When the Plate is connected to a load resistor like R2, large changes in Plate current cause large changes in load resistor voltage. Thus a small change in input voltage on the Grid results in a large change in output voltage on the Plate, which in electronics is called Amplification. The 1T4 vacuum tube works as an amplifying diode detector with a gain of around 100. This helps the radio perform better than a simple crystal radio.

Advantages of this radio design:

1. Relatively simple with few parts but better performance than the even simpler crystal radio.

2. Receives all local stations with a short 10' antenna and a ground connection.

Disadvantages of this radio design:

1. AC impedance of the grid leak detector loads the LC tuner which reduces selectivity of the radio.

2. Gain is insufficient for non-local stations. Regenerative radios achieve gains greater than 10,000 but add complexity to the circuit and station tuning process.

3. A grid-leak detector is easily overloaded by strong local stations resulting in noticeable distortion in the output. Making the antenna only as long as needed is important.

Review the Circuit Board Layout

The Assembly Manual provides a step-by-step checklist for installing and soldering each component to the perfboard. I've used point-to-point wiring with 20 AWG solid hookup wire. Some of the connections can be made with just the component leads. Power, ground, and signal bus leads are best done with lengths of hookup wire. Parasitic capacitance isn't much of an issue for the frequencies this radio will operate at and I've already compensated for most of these in the design. When built, the radio is free of unwanted oscillation or noise.

When it comes to wiring, try to be as neat as I've indicated in the assembly manual. You don't have to be the world's best soldering artist but there's no good reason to do the work half-way. Go all out and make your radio look as good as you can.

Follow the Steps in the Assembly Manual to Complete the Radio

To make the assembled radio look more like the original Radio Shack PBOX kit, I purchased some flat red and satin clear spray paint. I've applied two coats of red and two light coats of clear which I think gives the perfboard a natural texture. Almost like it was made that way.

Use any color you wish but I recommend not skipping the clear coat if you paint the perfboard with flat colors. Flat paint tends to show marks and texture changes at the slightest touch. The natural vector board finish also looks nice so you don't have to use paint of you don't want to.

The perfboard specified in the parts list is too large to fit onto the Hammond case so some trimming is required as indicated in the images below. Follow the steps below to prepare the perfboard before installing components:

1. Using a sharp edge, carefully score the perfboard along the TOP ROW and BOTTOM ROW row of holes all the way to the end of the board. Run the sharp edge over the score line several times until it penetrates 1/4 to 1/2 way through the perfboard.

2. Using a sharp edge, carefully score the perfboard along the RIGHT COLUMN and LEFT COLUMN of holes all the way to the end of the board. Run the sharp edge over the score line several times until it penetrates 1/4 to 1/2 way through the perfboard.

Score edges of perfboard along dashed lines

3. Using a small pair of pliers, carefully bend the TOP ROW section back away from the score line. Work with one end of the TOP ROW, moving to the center, and then the other end. The TOP ROW section will eventually break away from the perfboard.

4. Using a small pair of pliers, carefully bend the BOTTOM ROW section back away from the score line. Work with one end of the BOTTOM ROW, moving to the center, and then the other end. The BOTTOM ROW section will eventually break away from the perfboard.

5. Using a small pair of pliers, carefully bend the RIGHT COLUMN section back away from the score line. Work with one end of the RIGHT COLUMN, moving to the center, and then the other end. The RIGHT COLUMN section will eventually break away from the perfboard.

6. Using a small pair of pliers, carefully bend the LEFT COLUMN section back away from the score line. Work with one end of the LEFT COLUMN, moving to the center, and then the other end. The COLUMN section will eventually break away from the perfboard.

Carefully break apart perfboard edges along score lines

7. Using an Exacto knife, match the trimmed perfboard with the Hammond case and enlarge holes at the corner to match with the corner mounting holes in the case. Work slowly and carefully with minimal pressure to avoid breaking the corner piece.

8. Place the potentiometer mounting brackets onto the perfboard and enlarge holes in the perfboard to match the holes in the brackets. Work slowly and carefully with minimal pressure to avoid cracking the perfboard.

Cut holes for case and bracket screws

9. Spray paint the perfboard with the color of your choice or leave it natural. It's your choice.

Paint with desired color

I typically use flat red followed up with 2 to 3 coats of gloss clear, but feel free to use any color that you like best.

When the perfboard is finished, follow the step-by-step instructions in the assembly manual <<< HERE >>>.

Installation of the Battery Holders

The above is what your radio should look like after the battery holders have been mounted. The battery holders have four screw holes in them, but you only need to use two screws to mount the holders to the perfboard: One in the upper left and the other in the lower right. I just placed the holders in position and marked the perfboard holes with a Sharpie pen. Next I used an Exacto knife to enlarge the holes. Use caution, work slowly, and watch the sharps!

Tube Socket Prepared and Mounted

Before you mount the tube socket, very carefully bend the lugs out a little so that it is easier to attach wires to them and solder the connections. Don't do like I did my first time and break one off (man that was soul crushing). Just a little bend about 45 degrees is all that's needed.

Variable Capacitor Bracket Installed

The above photo shows the variable capacitor bracket installed. Mine has the funny offset shape at the bottom because it's an old PC card slot blank. Yours will likely be different from mine depending on the materials you use. Just make sure it's tall enough so that the variable capacitor body will miss the perfboard mounting screws. And that the bracket is mounted close enough to the edge that the tuning knob can be tightened on the variable capacitor shaft without binding against the case. I've included a 1:1 scale drawing below to help you make brackets similar to mine.

Variable Capacitor Mounted on the Bracket

It's very important to use the proper screw length for the variable capacitor mounting holes. Screws that are too long will damage the variable capacitor. Always carefully test fit the screws to make sure they do not intrude into the case. Add some washers if you feel the screws may be too long based on the thickness of the material you use for the bracket.

I used an old nylon hex standoff from my junkbox for the variable capacitor shaft extension. The standoff was too thick to fit into the knob shaft so I whittled it down to size with an X-Acto knife, using extreme caution not to whittle down my thumb in the process.

Antenna Coil Bracket Mounted

I test-fit the antenna coil in the bracket mounting hole before attaching the bracket to the perfboard. On mine I needed to expand the hole just a tiny bit and clean up the flash with my Dremel tool to get a nice tight fit. Don't push the antenna coil all the way in until the bracket is mounted or you might have trouble reaching the mounting screws. It's not easy getting the coil off the bracket once it's on.

Antenna Coil Mounted on the Bracket

Now it's starting to look like the radio I remember. Be sure that the red dot on the antenna coil is facing directly toward the tube socket.

Soldering in the Grid Leak Detector Components

Just twist the capacitor leads around the resistor and solder. Then solder one end to tube socket pin 6. This is one of the hardest soldering tasks in this project. Once you have this and the rest of the tube socket pins soldered it's pretty easy to do the rest of the project.

A word from the soldering iron police: When you've finished using the soldering iron after turning it on, always remember to turn it back off again. If it's already off... then just walk away!

Solder Variable Capacitor Sections C1 Through C3 Together

I've included a detail photo of this step as it might be a little more clear what's required and why. The variable capacitor for this project is actually 4 variable capacitor sections in one package. Two of the sections (C1 and C2) vary between 5pF and 135pF each. The remaining two sections (C3 and C4) vary between 5pF and 25pF. By combining these sections individually, in series, or in parallel you can make a variable capacitor with 10 different ranges (several of which are illustrated in the variable capacitor document included in an earlier step). These ranges can be fine tuned with 4 trimmer capacitors included on the back. This is a very versatile component, but it requires a little bit of wiring to get all the sections connected together the right way.

In this radio design we need to connect sections C1, C2, and C3 in parallel. If you observe the photo above, you can see that I have soldered a wire from the pin at the bottom left (C1) to the pin at the top left (C2). And I've soldered another wire from the pin at the top left (C2) to the pin at the top right (C3). The capacitor sections are all tied together internally at the three COM pins in the middle of the variable capacitor body. So by soldering these pins together, I have effectively placed capacitor sections C1, C2, and C3 in parallel. Since the values of capacitors in parallel add together, I've built a single variable capacitor with a range of 15pF to 295pF plus the value of the trimmer capacitors on the back which vary between 1pF and 10pF. In a previous step, I indicate how to set the trimmer capacitors to their minimum value, which makes the final capacitance range between 18pF and 298pF. The actual range will vary slightly due to manufacturing tolerances, but so will stray capacitance in the circuit you build. When doing the design calculations for tuning range, always allow for some room to adjust one way or the other.

The Finished Radio

The assembly instruction document will walk you through each wiring step until your radio is completed and ready to power up. All you need to do when finished is double check your work to be certain everything is connected in the right place and well soldered.

After that, connect the antenna and ground connection, plug in the earphone, install the batteries, and tune in the stations.

I really like the look of the finished radio and am happy with it's performance. It's nowhere near as good as a modern superheterodyne, and not quite as good as a regenerative radio, but that's not the point. The point was to build a working radio of the type common in the early 1920's and get a sense for what it was like to use and listen to one; to walk in the footsteps of the people that invented radio. And, enjoy a useful device that looks vintage but is made by hand with off-the-shelf parts. The same way many radio amateurs did 100 years ago.

I hope you will enjoy this project as much as I have.

Some Comments About the AM Broadcast Band

Finding stations nearest you

If you are wondering what AM radio stations are nearest you, what frequencies they are broadcasting on, what content is broadcast, and what the operating daytime and nighttime power levels are then you might find the following FCC resource of interest:

https://www.fcc.gov/media/radio/am-query

By entering your city and state in the form, you can select a simple or detailed listing of the stations in your area. For example, in my city of Austin, TX there are three stations nearby that broadcast 24 hours a day on 590 kHz, 1300 kHz, and 1490 kHz. I was able to receive these three stations during the day, but two of them were stronger. After reviewing the list the reason was obvious: Two stations (KJBJ and KVET) are licensed for 5KW. The third station (KTAE) is licensed for only 1KW.

You can look up the station service area map to see how strong the radio signal is in your area (see image above). And you can find out more information about the station such as the current owner, number and location of antennas, how long the station has been licensed, how station ownership has changed over time, and probably a lot more than you want to know about a radio station.

I use the FCC data to determine what to expect from the radio's I build and whether or not my tuning is on target.

AM Broadcasts during the Day

During the day, ultraviolet radiation from the sun causes heavy ionization of three layers within the upper atmosphere (the ionosphere). The lowest layer, called the D layer, easily absorbs the energy of AM radio waves and confines AM broadcasts closely to the surface of the earth. These AM "ground waves" are attenuated as they pass over the surface of the earth depending on soil conductivity (sea is best, cornfield is good, city is worst), and are gradually refracted into the earth where they are eventually absorbed. The result is that useful daytime AM radio service is limited to a radius of no more than about 100 miles. If you were wondering why your homemade AM radio can't pick up anything but nearby stations, it's not your radio. It's the way AM works. During the day.

AM Broadcasts at Night

At night when ultraviolet radiation from the sun is no longer present, the D and E layers of the ionosphere almost disappear leaving only the F layer, which due to height and composition make it an excellent reflector of radio waves below 10 MHz. At night, AM radio waves can travel high into the atmosphere and be reflected back to earth hundreds of miles away. These AM "sky waves" could cause massive interference if every station were to transmit at full daytime power. To prevent this, the FCC requires most stations to reduce transmit power at night. For example, in my area there are two stations licensed for 5KW during the day, but only 1KW at night. The exception to this rule are Clear Channel stations (not to be confused with the former Clear Channel Communications company now known as iHeartMedia Inc). Clear Channel stations were licensed under FCC intent to provide radio service to rural areas and create a cross-country communications system for use in the event of a national emergency. In the US, all Clear Channel stations are limited to a maximum power of 50KW. However, in Mexico many Clear Channel stations operate at 100KW and higher, which is why you have a good chance of receiving music and news from Mexico City at night.

Troubleshooting Your Radio

If you build the radio and don't hear anything, immediately remove the batteries and try the following checklist to find the problem:

1. Test the earphone. With a multimeter set to Ohms and the earphone loosely in your ear, measure the resistance across the earphone jack conductors. If you hear a slight clicking sound touching the multimeter leads to the earphone jack conductors, the headphone is most likely working.

2. Verify that the batteries were inserted correctly. The (+) end of each battery points at the top of the radio when the tuning knob is facing you. Insert the batteries in the direction indicated and test station reception again. If you don't hear a station, immediately remove the batteries.

3. Insert the batteries in the battery holder and use a multimeter set to DC Volts to check the voltage on each battery by measuring from the battery holder lugs. If you do not observe approximately 1.5V across the A Battery and approximately 22.5V across the B Battery, immediately remove the batteries and check for loose connections or solder at the battery holders. If there are no wiring errors, one or both batteries may be dead or damaged. Insert fresh replacement batteries and repeat Step 3.

4. With a multimeter set to Ohms and the A and B batteries removed, check the resistance across the A Battery holder terminals. If you observe a high resistance greater than 40 Ohms, double check the A Battery wiring with the assembly manual. Look for miswiring, cold solder joints, missing solder, broken wire, or other wiring related problem. If you observe 0 Ohms on the multimeter, remove the 1T4 tube from its socket and take the measurement again. If you still observe 0 Ohms on the multimeter, double-check all wiring for errors and look for any solder bridges, wire clippings, or unclipped wires under the tube socket. If the multimeter only measures 0 Ohms with the 1T4 tube inserted, the 1T4 tube may be bad. Insert a replacement 1T4 tube and repeat Step 4.

5. With a multimeter set to Ohms and the A and B batteries removed, check the resistance across the B Battery holder terminals. If you observe anything other than a very high reading (1 Meg Ohm or higher), double-check all wiring for errors and look for any solder bridges, wire clippings, or unclipped wires under the tube socket.

6. Verify the 1T4 tube is inserted fully in it's socket and then Insert the A and B Batteries into the battery holder. Wait 5 seconds and then with a multimeter set to DC Volts, measure the voltage across 270K resistor R2. Remove the A and B Batteries. The measured voltage should have been between 9V and 12V. If the measured voltage was not within this range, check the color codes on R1 and R2 to verify they are the correct value. Double-check all wiring and verify no bad solder joints, missing solder, loose connections, or broken wires are found.

7. Using a multimeter set to Ohms, check the resistance across tuning coil L1. The resistance of L1 should be approximately 42 Ohms. If the multimeter measures much less than 42 Ohms or 0 ohms, double-check all wiring at the antenna coil. Look for hard to see solder bridges, wiring errors, wire clipping, or unclipped wires touching. If all wiring is correct, unsolder one of the wires to L1 and measure the resistance of L1 again. If the measured resistance is still much less than 42 Ohms then the tuning coil is likely bad. Replace the tuning coil, insert batteries and the earphone, and try to receive a station. If the measured resistance of L1 is very high (> 100 Ohms), then the antenna coil is likely bad. Replace the tuning coil, insert batteries and the earphone, and try to receive a station.

8. Using a multimeter set to Ohms, check the resistance across tuning coil L2. The resistance of L2 should be approximately 10 Ohms. If the multimeter measures much less than 10 Ohms or 0 ohms, double-check all wiring at the antenna coil. Look for hard to see solder bridges, wiring errors, wire clipping, or unclipped wires touching. If all wiring is correct, unsolder the wire from the Variable Capacitor COM pin to Pin 1 of L2 and measure the resistance of L2 again. If the measured resistance is still much less than 10 Ohms then the tuning coil is likely bad. Replace the tuning coil, insert batteries and the earphone, and try to receive a station. If the measured resistance of L2 is very high (> 100 Ohms), then the antenna coil is likely bad. Replace the tuning coil, insert batteries and the earphone, and try to receive a station. If the measured resistance of L2 with Pin 1 unsoldered from the variable capacitor is approximately 10 Ohms, verify all wiring at the variable capacitor. If all wiring is normal, the variable capacitor may be shorted. Remove the variable capacitor and verify that the mounting screws are not too long. Check the resistance of each capacitor section between C1, C2, C3, C4, and COM. If any capacitor section measures 0 Ohms, replace the variable capacitor.

Occasionally I encounter a circuit that is schematically simple but challenging in theory, yet has no practical purpose other than to look cool. Every electronics lab needs a black box with blinking lights hanging around in a corner somewhere. Just about every science fiction movie or television show ever made has blinking lights of one type or another. The Neon Goofy Lite project described in this article is easy to build but contains enough interesting parts, theory, and history to start conversations and invite questions from expert and novice alike. The version covered here has been running non-stop making that vintage orange neon glow for over two weeks on four AA batteries.

Between 1968 and 1970, Radio Shack offered 28 project kits designed to introduce customers to breadboard construction techniques and electronics theory. By 1971 there were 26 P-Box kits to choose from and many pages of electronic components, tools, and educational project labs. Unfortunately, after 1974 Radio Shack's preoccupation with Arthur Fiedler, stereo electronics, and CB radio gradually reduced the size of the electronic components and kits section of the catalog. In 1979 there were only six remaining P-Box kits offered and by 1980 they were dropped from the catalog entirely, replaced by printed circuit "kits" that were not as interesting and more than 3 times the cost of the most expensive P-Box project.

In honor of a great set of educational Radio Shack electronics project kits available during the 60's and 70's, I've redesigned the original Science Fair brand Neon Goofy Lite (Catalog #28-130) using modern components still available from electronics component re-sellers in the US. All of the components in the updated kit can be found at Mouser or Digikey. There were many things Radio Shack did extremely well during its prime. For the nostalgic or the enthusiast who would like to build an updated version of this classic "blinking light" kit, I've included schematics, parts information, assembly documentation, and pictures of a completed and tested Neon Goofy Lite based on the original from 1970.

Obtain The Assembly Manual

The Neon Goofy Lite project described here is based on the Radio Shack pbox kit of the same name, but it has been updated with silicon transistors and passive components that can be obtained from electronics suppliers like Mouser and Digikey. I've built the updated Neon Goofy Lite kit described here and believe it works just as well as the original kit did back in 1970. To make it easy to replicate my work, I've provided illustrations and step-by-step assembly documentation based on the original assembly manual from Radio Shack.

To build the kit, you will need the revised assembly manual available >>> HERE <<< .

I've kept the original branding and publishing style of the Science Fair Neon Goofy Lite in order to preserve the original look and feel of the documentation set for the builder. But every page has been updated to reflect the changes I've made in order to incorporate modern and available parts.

Obtain Components On The Parts List

Review the parts list and obtain the components indicated. All components are available from Mouser or Digikey, or can be obtained from other suppliers that may be more convenient to your geography. Total cost is just less than $27. To put that cost in perspective, the Goofy Lite kit was introduced in 1970 at a retail price of $6. The economic value of $6 in 1970 is equivalent to $44 today. The project described in this article can be built for about half the cost of the project kit offered by Radio Shack in 1970. But keep in mind that Radio Shack needed to make a profit from the sale of their kit which explains where the other half of the cost went.

Below are a few notes regarding the parts used for the Goofy Lite project:

1.  The resistors for the project can be purchased from Mouser or Radio Shack (assuming they are still in business in your area). But I highly recommend the excellent Joe Knows Electronics resistor kit. It includes the resistors you need for this project and over 860 different values that can be used for other projects, all labeled in individual plastic packages for just $17. For this project you can use 1/4W or 1/2W resistors. It's your choice. Check out www.joeknowselectronics.com. You will not be sorry.

2.  The case for the Goofy Lite project I built is a Hammond 1591GSBK ABS Project Box from Mouser. I used a piece of vector breadboard cut to fit on the top and spray painted with high temperature automotive flat red. I like the look of red on black, and the red color of the breadboard matched the red color of the original pbox kit. It's completely up to you how you want to house and color the kit you build.

3.  The original pbox kits used tin-plated spring clips for attaching the battery holder to the perf-board enclosure. These clips were a pain to solder to when new and they tarnished like crazy after they were installed resulting in intermittent connections. Fortunately for everyone they are no longer available. I am continuing to experiment with different connector types that are small and inexpensive. For this project I found some small screw connectors from Mouser (part number 534-8730). They aren't perfect but they are very small and make consistently good connections. Feel free to use the battery connector of your choice and let me know if you find something that works really well for you.

4.  The original pbox kit used an audio coupling transformer for T1. This coupling transformer is no longer available from the original manufacturer or any alternative source I could find. Fortunately it was quite easy to find a suitable replacement transformer of equivalent turns ratio from Hammond Power Solutions (Mouser Part 546-161C10). This transformer is roughly twice the size of the original Radio Shack transformer but performs much better in my opinion. The only disadvantage is that the Hammond transformer core doesn't whine like the old audio transformer used to. It's very effective but completely silent.

5.  Pay particular attention to the voltage rating of the capacitors. This project generates up to 130V DC on the T1 secondary in order to ionize the neon gas in the lamps. Don't use capacitors less than 200V DC for C1 through C5.

CAUTION: Although the current generated by T1 is extremely low and considered harmless, don't go out of your way to touch components on the high voltage side of the circuit when it is running. Don't take this circuit with you to the tub to see if it will float while you are taking a bath. Don't try to shock your friends with it. Treat all electronic circuits with the respect they deserve.

Please Note: I have no business relationship with any of the above vendors. Nothing of financial value was exchanged for my recommendation. None of the above vendors provided compensation of any kind during the creation of this project. I will not be compensated in any way if you choose to build this project or purchase components from any vendor I recommend. I simply had a good experience with the vendors I recommend and believe you will too.

Review the Schematic to Become Familiar With the Goofy Lite Design

You have a decision to make, which is actually pretty cool since most projects can only be constructed one way and can only do one thing. But the Neon Goofy Lite project is more flexible than that. It is possible to configure the project to flash the 5 neon lamps in a pseudo-random pattern or a sequential pattern. I built both circuit types and liked the pseudo-random pattern the best (see the video on the summary page) but your preference may be different from mine. The assembly manual covers both types in detail so it's entirely up to you which circuit you build.

The Goofy Lite circuit looks simple but contains enough theory to keep a second year engineering student busy. The core of the circuit is a negative resistance device: The NE2 neon lamp (also known as the A1A lamp). There aren't many devices in electronics that exhibit negative resistance; a few microwave diodes, SCRs, the uni-junction transistor, and gas discharge tubes like the neon lamp. The great thing about a negative resistance device is that it can be used to build a simple oscillator with only a couple of passive components. No amplifiers or feedback networks required.

Negative Resistance

To get an idea of what is meant by "negative resistance", review the two V/I charts below. The first represents a V/I graph of a diode and a resistor, both positive resistance devices you are probably already familiar with. The second represents the V/I graph of a neon lamp, a negative resistance device. Notice in the positive resistance graph that the slope of the V/I curve is always positive (up and to the right). Increasing current through the device always results in an increasing voltage drop across the device. However, notice that the neon lamp has a portion of its V/I curve with a negative slope at the lamp breakdown voltage (90 volts). In the negative slope region, increasing current through the lamp results in a decreasing voltage across the lamp. This characteristic is what makes the blinking neon lamp in the Goofy Lite project possible.

In the 50's and 60's, neon lamps were used to build all sorts of interesting circuits including oscillators, timers, binary counters, binary dividers, lamp dimmers, and light detectors. The only disadvantage with neon lamps today is their high voltage requirement (60V to 150V). But in the past, these voltages were common for electron tube (valve) circuits making it very easy to use a neon lamp. Review the two neon lamp circuit diagrams below. One is a Relaxation Oscillator circuit and the other is a Multivibrator circuit. The Relaxation Oscillator is used in the Random version of the Goofy Lite project. The Multivibrator is used in the Sequential version of the Goofy Lite project. Below is a description of how each circuit works:

Neon Lamp Relaxation Oscillator

In order to control the flash rate of the neon lamp, we need a time delay. A simple time delay circuit can be constructed with a resistor and a capacitor in series, often referred to as an RC circuit. When energized, the capacitor gradually charges to near the power supply voltage. How quickly the capacitor charges is determined by the value of the resistor and the value of the capacitor according to the following formula:

   Trc = R * C

Refer to the Relaxation Oscillator Schematic below. When power as applied to the series circuit C1/R1, capacitor C1 will begin to charge. When C1 voltage reaches the neon lamp firing voltage (90V), the neon lamp will light and C1 will discharge into the neon lamp. Resistor R1 will essentially be isolated from the circuit due to the relatively low resistance of the neon lamp. When C1 voltage falls below the neon lamp holding voltage (50V), the neon lamp will extinguish and C1 will begin charging again through R1. This cycle will repeat for as long as power is applied to the circuit.

Review the chart underneath the Relaxation Oscillator Schematic. The chart contains the capacitor voltage over time. Notice that the capacitor does not charge in a straight line but in an exponential curve function. The equation for this curve is:

   Vcapacitor = Vpower * [ 1 - e^(-t/RC)]

You might recognize this equation if you've had a class in statistics, physics, or calculus. This equation is very easy to use with a modern calculator so don't let it intimidate you. Solving the equation for the resistor and capacitor values RC will render it a little more useful when designing circuits like the Goofy Lite:

   RC = -t / ln (( Vf - Vh ) / Vf )

In the above equation, ln is the natural log function on a calculator, Vf is the neon lamp firing voltage, Vh is the neon lamp holding voltage, t is the desired lamp flash rate, and RC is the product of the capacitor and resistor needed to produce a flash rate of time t.

For example, I wanted a flash rate around 1 second for each lamp so I used the following:

   Lamp Firing Voltage = 90 VDC

   Lamp Hold Voltage = 50 VDC

   RC = -1 / ln (( 90 - 50 ) / 90 ) = 1.23

I wanted to keep the .22uF capacitor values used by the original P-Box kit, so all I needed to do was divide the RC value by .00000022 Farads in order to obtain the value for the resistor:

   R = 1.23 / .00000022 = 5.5 Megohms

The nearest standard resistor value I had in inventory was 4.7 Megohms so I used that for the resistor values in the random circuit.

The reason this circuit is referred to as "Random" is that the flash rate for each neon lamp will vary depending on the tolerance of the capacitors, resistors, and lamps. Many capacitors have a tolerance of 10% and 20%. The resistors I used were 5% tolerance. The neon lamps seem to have firing and hold voltages that varies as much as 20%. All of these tolerances combined will vary the actual neon lamp flash rate between .8 seconds and 1.2 seconds. Sometimes more. This results in what appears to the eye to be a random flash pattern among the 5 neon lamps. If you focus on one lamp you will see that the flash rate is actually fixed for that lamp. The other lamps are firing at different rates which creates the illusion of a random flash pattern.

Neon Lamp Sequential Multivibrator

All of the principles described above apply to the sequential circuit. The only difference is where the capacitors are attached. The sequential version of the Goofy Lite is composed of several Multivibrator circuits distributed among the 5 neon lamps. Review the sequential circuit diagram below and the chart attached to it.

When power is applied, one of the neon lamps will immediately fire first due to manufacturing tolerances. In this description, lets assume that neon lamp NE2-1 fires first. When NE2-1 fires, it creates a conductive path for capacitor C1 to charge. When the voltage on C1 reaches the firing voltage of NE2-2, that lamp will fire which causes the voltage on C1 to extinguish neon lamp NE2-1. C1 will then charge through the conductive path provided by NE2-2 until it approaches the firing voltage of NE2-1. NE2-1 will then fire causing the voltage on C1 to extinguish neon lamp NE2-2. This cycle will repeat for as long as power is applied to the circuit.

By carefully distributing three copies of the Multivibrator circuit among the 5 neon lamps, the sequential circuit can be made to flash each neon lamp one after the other.

DC-DC Step-Up Converter

If all the above theory wasn't enough, we still need some way of producing 150V DC from a set of AA batteries. The original P-Box kit used an audio transformer with a 24:1 turns ratio to step up the 6 VDC battery voltage to around 150 VDC for the neon lamps. However that audio transformer is no longer available from any commercial source (I've tried them all). Occasionally someone on eBay will offer one for sale from an estate auction, but you can't buy these in volume from any commercial company. An acceptable solution to this problem turns out to be very simple. Any step-down power transformer can be used as a step-up power transformer when the primary and secondary windings are transposed. All I needed to do was find the smallest step-down power transformer commercially available that provided a turns ratio of around 24:1. It turns out that Hammond Power Solutions makes a small transformer with a 115V primary and dual 5V secondaries which is just perfect for the Goofy Lite project. The Hammond transformer is about twice the size of the original Radio Shack audio transformer but is still quite small and performs extremely well as a substitute.

The DC-DC Step-Up Converter is actually a classic Blocking oscillator which uses a tapped primary inductor for the energizing and feedback circuits. All I needed to do was connect the Hammond transformer secondary coils together so that they appeared to Q1 as a tapped inductor. R6 is a current limit for the base of Q1 while C7 ensures that the base of Q1 gets a "kick" when power is first applied to ensure that the DC-DC converter circuit starts oscillating. The second secondary winding of T1 provides the positive feedback signal to Q1 which keeps the DC-DC converter circuit oscillating. C6 isolates the feedback winding of T1 from the 6V battery so that only the feedback signal controls the operation of Q1.

When power is first applied, the base of Q1 will be tied briefly to V+ which immediately saturates Q1 and starts current flowing through the first secondary winding of T1. Feedback from the second secondary winding will begin to reverse bias Q1 resulting in less current through the first secondary winding. When Q1 becomes turned off, the combination of R6/C7 will begin turning Q1 back on again and the entire cycle will repeat as long as power is applied.  The frequency of oscillation for the converter is around 135Hz.

The number of wire turns in the primary coil of T1 are 23 times higher than the number of turns in the secondary coils. This multiplies the voltage applied to the secondary coils attached to Q1 by 23 resulting in a voltage to the neon lamps of approximately:

   Vsecondary * Turns_Ratio = Vprimary

   6 VDC * 23 = 138 VDC

This is more than enough to run both the random and sequential versions of the project. Average current consumption is 18mA so a set of four AA batteries should run at least two weeks non-stop.

Hopefully this section has helped you understand how the Goofy Lite circuit was designed and how it operates.

Review the Circuit Board Layout

The Assembly Manual provides a step-by-step checklist for installing and soldering each component to the vector board. As you can see from the opposite side illustration of the vector board, I've used point-to-point wiring with 22 AWG solid hookup wire. Most of the connections can be made with just the component leads. But power, ground, and signal bus leads are best done with lengths of hookup wire. There are two versions of the Goofy Lite project. I've built the Random version and illustrated the opposite side of the circuit board. The sequential version will be very similar.

When it comes to wiring, try to be as neat as I've indicated in the assembly manual. You don't have to be the world's best soldering artist but there's no good reason to do the work half-way. Go all out and make your project look as good as you can.

Follow the Steps in the Assembly Manual to Complete the Goofy Lite Project

Before you begin:

The perfboard specified in the parts list is too large to fit onto the Hammond case so some trimming is required as indicated in the images below. Follow the steps below to prepare the perfboard before installing components:

1. Using a sharp edge, carefully score the perfboard along the TOP ROW and BOTTOM ROW row of holes all the way to the end of the board. Run the sharp edge over the score line several times until it penetrates 1/4 to 1/2 way through the perfboard.

2. Using a sharp edge, carefully score the perfboard along the RIGHT COLUMN and LEFT COLUMN of holes all the way to the end of the board. Run the sharp edge over the score line several times until it penetrates 1/4 to 1/2 way through the perfboard.

Score edges of perfboard along dashed lines

3. Using a small pair of pliers, carefully bend the TOP ROW section back away from the score line. Work with one end of the TOP ROW, moving to the center, and then the other end. The TOP ROW section will eventually break away from the perfboard.

4. Using a small pair of pliers, carefully bend the BOTTOM ROW section back away from the score line. Work with one end of the BOTTOM ROW, moving to the center, and then the other end. The BOTTOM ROW section will eventually break away from the perfboard.

5. Using a small pair of pliers, carefully bend the RIGHT COLUMN section back away from the score line. Work with one end of the RIGHT COLUMN, moving to the center, and then the other end. The RIGHT COLUMN section will eventually break away from the perfboard.

6. Using a small pair of pliers, carefully bend the LEFT COLUMN section back away from the score line. Work with one end of the LEFT COLUMN, moving to the center, and then the other end. The COLUMN section will eventually break away from the perfboard.

Carefully break apart perfboard edges along score lines

7. Using an Exacto knife, match the trimmed perfboard with the Hammond case and enlarge holes at the corner to match with the corner mounting holes in the case. Work slowly and carefully with minimal pressure to avoid breaking the corner piece.

8. Place the potentiometer mounting brackets onto the perfboard and enlarge holes in the perfboard to match the holes in the brackets. Work slowly and carefully with minimal pressure to avoid cracking the perfboard.

Cut holes for case and bracket screws

9. Spray paint the perfboard with the color of your choice or leave it natural. It's your choice.

Paint with desired color

I typically use flat red followed up with 2 to 3 coats of gloss clear, but feel free to use any color that you like best.

Assembly Photos - DC/DC Step-Up Converter

After Step 12 in the Assembly Document, the Goofy Lite project should look something like the photos above.

Assembly Photos - Neon Lamp Section

After Step 28 in the Assembly Document, the Goofy Lite project should look something like the photos above.

Note: Capacitors C1 through C7 will be in a different location depending on the version of the project that you build. What is shown is the Random layout.

Assembly Photos - the Completed Goofy Lite Project

When completed and powered up, the Neon Goofy Lite project will operate non-stop for weeks at a time. It's a great conversation starter when you have visitors in the lab. And it's an easy to build weekend project that you can do with a family member. Young people are amazed at the voltages that can be generated and used with a few 1.5 volt batteries, and the flash of orange glow from small neon tubes is almost hypnotic.

This revival of a vintage tech project has worked very well for me. Those that have wanted to build a Radio Shack Goofy Lite project but were thwarted due to the lack of a suitable transformer can now proceed with confidence. This was a fun project for me. I hope it is also fun and interesting for you.

Introduction 

While doing some research on the transition to solid state devices I encountered a really interesting article on the worlds first Transistorized Hi-Fi System. This article provides a nice back-stage view on how electronic design is carried out. Often, success depends on convincing your boss that a certain idea or process is worthwhile. Transistors in the 50s were hard to manufacture, noisy and unreliable, and when they worked it was only at low frequency and very low power. Many engineers were struggling to figure out what the transistor was good for outside of miniature hearing aids or signal conditioning in diode logic circuits. Sure the vacuum tube was large and inefficient, but it could do just about anything that needed doing at the time. Except when you needed thousands of them for a computer, then they weren't so great.

 IBM 700 Series Logic Module

By the 1960s, the transistor had finally found it's niche in portable and mobile AM radios, Hi-Fi stereo systems, computers, and telephone repeaters while vacuum tubes continued to carry the day in high frequency radios, RF power amplifiers, and high voltage switches. The transistor had to pick it's battles against the vacuum tube while new materials and manufacturing processes were being developed. Far from an explosion, the shift to transistors and the obsolescence of the vacuum tube required decades. The CRT display, a giant glass vacuum bottle filled with tin, copper, zinc, cesium, silver, and lead didn't leave the scene until the first decade of the 21st century, replaced by a flat glass screen full of transistors. But we still have vacuum tubes in our microwave ovens. Last stand, I guess.

Science Fair PBox Kits

In 1968, Radio Shack introduced it's first P-Box kits marketed to the electronics experimenter. These were great kits containing all the electronic components, wire, and perfboard needed to build a useful electronic device. The first P-Box kits released in 1968 looked suspiciously like those sold by Eico under the Eicocraft brand but quickly expanded (Radio Shack sold experimenter kits from Eico, Knight, and Allied Radio until 1973). In 1969, an 8 Transistor Stereo Amplifier kit was released but withdrawn in 1972. The reasons are unclear, but I suspect the germanium "matched pair" transistors were expensive and hard to source. In addition, the amplifier kit was one of the most complex in the catalog and probably didn't work very well which would have been disappointing given it's cost.

After reading the transistorized Hi-Fi article referred to earlier and reviewing the available documentation on the 8 Transistor Stereo Amplifier kit, I wanted to build one. Most cheap amplifiers in 1969 used at least two coupling transformers. These transformers simplified the amplifier circuit but they added weight and limited low and high frequency response. The Radio Shack kit used a push-pull direct coupled design, found only in "serious" Hi-Fi equipment at the time which eliminated the transformers. All I needed to do was redesign the kit for silicon transistors, improve the push-pull bias circuit, and limit high frequency response to something reasonable. The result of that work is the subject of this article. I was very pleased to have ended up with a nice little "vintage" amplifier that produces room filling sound when connected to an efficient speaker system. As you can tell from the video demonstration at the beginning of this article, you don't need 1000W at .00000001% THD to fill a room with good tunes.

The specifications I obtained from the amplifier I built were:

THD: <1% at 500mW per channel

Input Impedance: 700K Ohms minimum

Sensitivity: -10dbV (0.316 Vrms) see note below

Frequency Response: 35Hz to 20Khz (+/- 1db)

Cutoff Frequencies: 27 Hz and 65 Khz

Input Power: 9V @ 500mA max, or 12V @ 700mA max. 1000mA recommended.

Output Power @ 9V Input Power: 1W into 8 Ohms (0.5W per channel) or 2W into 4 Ohms (1W per channel)

Output Power @ 12V Input Power: 2W into 8 Ohms (1W per channel) or 4W into 4 Ohms (2W per channel)

Note: The input sensitivity of the amplifier is scaled to consumer line level input voltages of about 0.8Vpp (0.316Vrms) for maximum output power. Although this is perfect for a Laptop, MP3 player, mixer, CD player, AM/FM tuner, or vintage tape deck, it may not be sufficient for many acoustic/electric instrument pickups or dynamic microphones without a pre-amplifier or "stomp-box".

ALSO...

Be sure to check out the assembly manual further down the page because it includes several suggestions for improving the amplifier by:

• Adding an LED Power Indicator
• Doubling the output power to 2 Watts
• Adding an AC power supply for both home and portable operation
• And more...

How It Works

Stereo Amplifier Schematic 

The right and left channel amplifiers are identical so I will only describe the left channel in this section.

There are three stages in the amplifier: A differential amplifier (Q1), a Common Emitter driver (Q2), and a Push-Pull Output (Q3 and Q4).

Amplifier Stages 

Differential Amplifier Stage

The Differential Amplifier stage provides three essential functions for the remaining stages:

1. A bootstrap bias circuit consisting of C2 and R2-R5 that increases amplifier input resistance to approximately 2M Ohms.

2. A DC feedback loop composed of R7 and R8 for controlling the overall amplifier gain, reducing distortion, and keeping the push-pull output voltage centered at 1/2 power supply voltage.

3. A first order low pass filter composed of C3 in combination with R7/R8 that limits high frequency response to 65Khz.

Resistors R2 and R3 form a voltage divider bias circuit for Q1 that sets the collector voltage around 8.2V and collector current at 500uA. Voltage gain for the Differential Amplifier is -21db (loss) and the signal phase shift is 180 degrees so another amplifier stage is needed to correct this before sending the signal to the output stage.

Don't be concerned about the first stage looking a little unusual.  It's not the canonical differential amplifier covered in circuits class that uses two transistors. The 8 transistor stereo amplifier project needed to run on only 4 transistors per channel and so the design did away with one of the differential amplifier transistors and made use of the emitter circuit as an input. It has some limitations, but it does work.

Standard Differential Amplifier

For those that would like to experiment on their own with this portion of the circuit to see how it really works, I have provided a quick design similar to the one used in the amplifier project and included the schematic below. It's only a few components and can be wired up on a solderless breadboard.

 Single Transistor Differential Amplifier

Attach a signal generator to the IN+ terminal, ground the IN- terminal, and then attach Channel 1 of a dual trace oscilloscope to the OUT terminal and Channel 2 to the IN+ terminal. Set the signal generator to 1Khz with a 1Vpp output. The oscilloscope trace should look like the first trace below. Notice that the Differential Amp input and output are in-phase and the amplifier gain (Vpp OUT / Vpp IN) is approximately equal to one.

Non-Inverting Input (Yellow) and Differential Amplifier Output (Blue) - In Phase

Remove the signal generator and oscilloscope. Attach the signal generator to the IN- terminal, ground the IN+ terminal, and then attach Channel 1 of a dual trace oscilloscope to the OUT terminal and Channel 2 to the IN- terminal. The oscilloscope trace should look like the second trace below. Notice that the Differential Amp input and output are 180 degrees out of phase and the amplifier gain (Vpp OUT / Vpp IN) is approximately equal to one.

Inverting Input (Yellow) and Differential Amplifier Output (Blue) - 180 Degree Phase Shift

This is the behavior expected from a differential amplifier with a gain of 1. You would see similar behavior from an Operational Amplifier IC or a canonical 2 transistor Differential Amplifier.

One critical measure of differential amplifier performance is Common Mode Rejection Ratio (CMMR). If you apply a signal of the exact same amplitude and phase to the IN+ and IN- terminals of a differential amplifier, the output should be zero. If it isn't, then the amplifier has introduced some error in the output. Practical diffamps aren't perfect so CMMR is often used as a measure of quality. Commercial Operational Amplifier ICs typically achieve CMMRs between 70db and 100db depending on signal frequency. The simple one-transistor differential amplifier won't be that good, but after all it's only one transistor.

To determine the CMMR for the diffamp circuit shown above, replace R6 with a 2K Ohm trimmer resistor (Pin 1 to Q1 Emitter, Pin 2 to C2+). Attach both the IN+ and IN- terminals to the signal generator. Turn off Channel 2 of the oscilloscope and adjust Channel 1 until a signal is displayed. Adjust the trimmer resistor until the the diffamp output voltage is as low as it will go. The output should be around 10mv peak-to-peak as shown in the trace below.

Non-Inverting and Inverting Inputs Tied Together Showing Common Mode Output

The following calculation provides the CMMR value of this differential amplifier:

Differential Mode Gain = OUT / (IN+ - IN-) = 1Vpp / (1Vpp - 0Vpp) = 1

Common Mode Gain = OUT / IN = 0.01Vpp / 1Vpp = 0.01

CMMR = Differential Mode Gain / Common Mode Gain = 1 / 0.01 = 100

CMMRdb = 20 * log (CMMR) = 40db

So this differential amplifier circuit is very simple, uses only one transistor, and can be scaled for various differential mode gains.

But...

It's CMMR isn't as good as a canonical diffamp or an OpAmp, and the input impedance on the non-inverting input is pretty low (around 1500 Ohms).

But if you take these into consideration it is easy to design a small amplifier that performs really well with just a few transistors.

Common Emitter Driver Stage

This is the voltage gain stage of the amplifier. Q2 amplifies the output of Q1 and provides +45db of voltage gain which is used to drive the push-pull output stage. To maximize gain and output voltage swing, Q2 doesn't use emitter degeneration so it's collector output will be fairly non-linear and temperature dependent. This is corrected with the DC feedback loop R7 and R8.

Collector current for Q2 is set at 5mA and flows through the speaker, R11, D1, and D2. This small current through the speaker isn't enough to generate measurable power or audible noise, but it provides a small feedback signal in the collector circuit of Q2 that corrects crossover distortion in Q3/Q4.

Crossover Distortion without Push-Pull Bias

The original amplifier circuit used a resistor to set the quiescent current for Q3/Q4. The voltage drop on that resistor was proportional to the collector current in Q2 and just slightly turned on Q3/Q4 so the amplifier would operate in a Class A mode for small signals. Unfortunately as the temperature changed in Q3/Q4, their base current would increase raising the voltage drop on the resistor. As the voltage drop on the resistor increased, the Q3/Q4 quiescent current would increase raising their temperature. This cycle would repeat until the temperatures of Q3/Q4 became so high they self destruct.

To avoid this, I've replaced the original resistor bias with D1 and D2. The combination of these two diodes provide a 1.4V bias for Q3/Q4 that is almost independent of the base current from Q3/Q4. Not exactly independent but much closer than a resistor. D1 and D2 also have a negative temperature coefficient with respect to junction voltage. So as the ambient temperature increases which tends to increase current in Q3/Q4, the junction voltage of D1/D2 decreases which reduces current in Q3/Q4. Ideally D1 and D2 should be physically close to Q3/Q4 (mounted on the heat sink if possible) but the diode stabilization circuit used here has worked extremely well in my temperature and output power tests. Capacitor C4 prevents ringing during Q3/Q4 crossover.

No Crossover Distortion with Push-Pull Diode Bias

Push-Pull Output Stage

The Push-Pull output stage provides the current gain needed in combination with the voltage gain of the CE Driver to produce the output power that drives the speaker. Q3 and Q4 work independently for large signals (Class B operation) but in tandem for small signals (Class A operation). For large signals, Q3 will conduct for one half of a cycle while Q4 will conduct for the other half. For small signals, Q3 and Q4 will contribute to both halves of the cycle.

The DC output voltage on Q3/Q4 is 1/2 the power supply voltage in order to provide for the maximum AC voltage swing without clipping distortion. For a 9V battery, the Push-Pull DC output voltage is 4.5V. We don't want this DC voltage to appear on the speaker as that will waste a lot of power heating up the speaker coil and generating no sound in the process. We only want the AC voltage from the CE Driver stage to appear at the speaker. C5 decouples the DC output voltage on the Push-Pull stage from the speaker and allows only the AC voltage to appear. The trade-off with coupling capacitor C5 is that at low frequencies, the impedance of C5 will reduce the output voltage to the speaker which limits the lowest frequency from the music source that can be amplified which in this case is around 27 Hz.

The voltage gain of the Push-Pull output stage is -6db (loss) but the current gain for the Push-Pull stage is +35db which allows a tiny current in Q2 to produce a large current in Q3/Q4. The overall voltage gain of the amplifier is the sum of all the stage gains:

Diff Amp Gain + CE Driver Gain + Push-Pull Gain = (-21db) + 45db + (-6db) = +18db

Operating Parameters

After building the amplifier, the following operating parameters were measured with a power supply of 9V:

DC Quiescent Supply Current = 11mA per channel (22mA total)

Max Voltage Swing = 6Vpp

Power Supply Rejection = -20dB

Voltage Gain = +18db

Output Power at 1% THD = 0.525 W per channel

Input Impedance = 700 K Ohms

Frequency Response +/- 1db = 35 Hz to 20 KHz

Low Cutoff = 27 Hz

High Frequency Cutoff = 65 Khz

How to build the 8 Transistor Stereo Amplifier

The 8 Transistor Stereo Amplifier project described here is based on the Radio Shack pbox kit of the same name, but it has been updated with silicon transistors and passive components that can be easily obtained from electronics suppliers like Mouser and Digikey. I've built the redesigned amplifier kit described here and believe it works better than the original kit did back in 1969. To make it easy to replicate my work, I've provided illustrations and step-by-step assembly documentation based on the publication style used for the original product. But every page was created with original content specifically for the redesigned amplifier.

Download the Assembly Manual >>> HERE <<<.

I built the kit in two evenings, taking my time, and double-checking my progress while following the manual. If you are familiar with breadboard construction techniques, you can probably complete the project in a single evening.

Important Note:

Be sure to read through the "Tweaks and Hacks" section before you order parts and start building. You might want to include some of the suggested modifications during construction or dream up your own before you start.

Obtaining parts for the Stereo Amplifier project

Review the parts list and obtain the components indicated. All components are available from Mouser or Digikey, or can be obtained from other suppliers that may be more convenient to your geography. Total cost for all new parts in small quantities is around $40 not including tax and shipping. To put that cost in perspective, the 8 Transistor Stereo Amplifier kit was introduced in 1969 at a retail price of $8.95. The economic value of $9 in 1969 is equivalent to around $61 today. If you deduct the cost of the volume knobs I've specified (knobs weren't included in the original kit) the amplifier project can be built for about half the adjusted cost of the project kit offered by Radio Shack in 1969. But keep in mind that Radio Shack needed to make a profit from the sale and support of their kit which explains the catalog price.

Below are a few notes regarding the parts used for the 8 Transistor Stereo Amplifier project:

1. The resistors for the project can be purchased from Mouser, Digikey, Newark or other electronics component retailer. But I highly recommend the excellent Joe Knows Electronics resistor kit. It includes most (but not all) of the resistors you need for this project and over 860 different values that can be used for other projects, all labeled in individual plastic packages for $20. For this project I used all 1/4W resistors to save space. You can also find good deals on resistor kits from Amazon by searching for "resistor kit" and looking for a 1% tolerance kit that includes the most values and parts count for the best price. Some really good resistor kits can be found for less than $20. I've done that several times and have always been happy with the parts I received regardless of supplier.

2. The capacitors for the project can be purchased from Mouser, Digikey, Newark or other electronics component retailer. I used capacitors from the Joe Knows capacitor kit and an electrolytic assortment I found on Amazon for $10.

3. The transistors for the project are common 2N3904/2N3906 transistors that can be found just about anywhere by searching on the transistor number. All of the semiconductors I used came from the Joe Knows semiconductor kit. This is a great component assortment that comes with three booklets explaining how the components work and provides some example circuits to help illustrate how to hook them up in a circuit.

4. The case for the Stereo Amplifier project I built is a Hammond 1591GSBK ABS Project Box from Mouser. I used a piece of vector breadboard cut to fit on the top and spray painted with high temperature automotive flat red and finished with clear coat. I like the look of red on black, and the red color of the breadboard matched the red color of the original pbox kit. It's completely up to you how you want to house and color the kit you build.

5. The original pbox kit used tin-plated spring clips for attaching the battery, speakers, and input connections to the perf-board enclosure. These clips were a pain to solder to when new and they tarnished like crazy after they were installed resulting in intermittent connections. Fortunately for everyone they are no longer available. For this project I found that two position terminal strips and a 1/8" (3.5mm) stereo jack were a better solution.

Please Note: I have no business relationship with any of the above vendors. Nothing of financial value was exchanged for my recommendation. None of the above vendors provided compensation of any kind during the creation of this project. I will not be compensated in any way if you choose to build this project or purchase components from any vendor I recommend. I simply had a good experience with the vendors I recommend and believe you will too.

A bit of history associated with this project

The original circuit design for the Stereo Amplifier from Radio Shack used 8 germanium transistors arranged in three stages: Differential Amplifier, CE Driver, and Push-Pull Output. These stages were common in high-end commercial amplifier design so the 8 Transistor Stereo Amplifier project should have earned a lot of fans back then. However, after only 3 years the kit was withdrawn which suggests that after the initial catalog marketing interval the product was only around for a year. When the kit was produced in 1969, Silicon junction transistors had replaced Germanium in new commercial amplifier designs. However, GE and ETCO continued selling Germanium transistors to hobbyists until about 1979 through Radio Shack, Lafayette, Poly Paks, and others. So, why did the kit not have a successful run?

Well, there are some good reasons:

Germanium was a relatively easy element to work with during the early days of the transistor but it's weaknesses made Silicon a more attractive element after manufacturing challenges were overcome. Germanium has a lower energy gap between the valence and conduction band which results in higher leakage current proportional to temperature. Germanium has lower thermal conductivity than silicon which makes it harder for the transistor get rid of internally generated heat. As internal temperature rises, more leakage current occurs which generates more heat... and so on. These two properties combined can result in thermal runaway ending in device self destruction. The original amplifier design did not provide a method for preventing thermal runaway in a push-pull configuration which made the amplifier unreliable.

By 1959 no manufacturer had discovered a way to build an oxide on Germanium. This allowed Silicon oxide planar transistors to achieve manufacturing dominance in terms of yield, cost, and reliability. As the cost of Silicon transistors fell, Germanium could not keep up and eventually the transistors used in the kit became too expensive. To minimize the potential for thermal runaway, Radio Shack sold matched pair semiconductors that were sorted by hand which further increased the cost of the transistors.

At $9.00 ($61 in 2017) the 8 Transistor Stereo Amplifier was the most expensive kit in the Pbox line and contained 56 parts. By comparison the 3 Transistor Short Wave Radio kit at $8 had only 39 parts and the FM Wireless Microphone kit at $7 had only 23. I suspect there were very few young customers able to afford a product this complex, and there were more factory rework and refund returns than anticipated.

So...

To overcome the limitations of the original kit, I've included a few modifications to improve performance and reliability including:

1. Redesign the amplifier to use readily available Silicon transistors.

2. Improved bias control for push-pull stage.

3. Increase output coupling capacitance to improve low frequency response.

4. Add compensation capacitor to increase high frequency roll-off above audio frequencies.

5. Replace tin-plated spring terminals with terminal strips and stereo jack to improve connection reliability.

6. Re-scale amplifier gain for maximum power at modern line level voltages.

I haven't attempted to simplify the project. My redesign requires 69 parts and the assembly document is 9 pages long. So I do not recommend this project for a novice. But if you like working with discrete analog components on perfboard and you have good soldering skills, this project is just the right challenge.

I think this kit redesign is far and away the most fun I've had working on these Pbox redesigns. The 8 Transistor Stereo Amplifier project I built can easily fill a room with great sound when attached to a set of good speakers. It's impressive what can be achieved with a few transistors and a standard 9V battery (even if that battery won't last long at full power).

Review the circuit board layout

 Circuit Board Layout (Top)

The Assembly Manual provides a step-by-step checklist for installing and soldering each component to the pefboard. As you can see from the opposite side illustration, I've used point-to-point wiring with 24 AWG solid hookup wire. Most of the connections can be made with just the component leads. But power, ground, and signal bus leads are best done with lengths of hookup wire.

 Circuit Board Layout (Bottom)

When it comes to wiring, try to be as neat as I've indicated in the assembly manual. You don't have to be the world's best soldering artist but there's no good reason to do the work half-way. Go all out and make your project look as good as you can.

Preparing the perfboard before mounting components

The perfboard specified in the parts list is too large to fit onto the Hammond case so some trimming is required as indicated in the images below. Follow the steps below to prepare the perfboard before installing components:

Warning: When I write, "sharp edge" I mean Xacto knife or similar.  Those products are extremely sharp (think surgical scalpel) and can do a lot of damage if you aren't extremely careful.  Use extreme care when operating any cutting tool.

1. Using a sharp edge, carefully score the perfboard along the TOP ROW and BOTTOM ROW row of holes all the way to the end of the board. Run the sharp edge over the score line several times until it penetrates 1/4 to 1/2 way through the perfboard.

2. Using a sharp edge, carefully score the perfboard along the RIGHT COLUMN and LEFT COLUMN of holes all the way to the end of the board. Run the sharp edge over the score line several times until it penetrates 1/4 to 1/2 way through the perfboard.

Score edges of perfboard along dashed lines

3. Using a small pair of pliers, carefully bend the TOP ROW section back away from the score line. Work with one end of the TOP ROW, moving to the center, and then the other end. The TOP ROW section will eventually break away from the perfboard.

4. Using a small pair of pliers, carefully bend the BOTTOM ROW section back away from the score line. Work with one end of the BOTTOM ROW, moving to the center, and then the other end. The BOTTOM ROW section will eventually break away from the perfboard.

5. Using a small pair of pliers, carefully bend the RIGHT COLUMN section back away from the score line. Work with one end of the RIGHT COLUMN, moving to the center, and then the other end. The RIGHT COLUMN section will eventually break away from the perfboard.

6. Using a small pair of pliers, carefully bend the LEFT COLUMN section back away from the score line. Work with one end of the LEFT COLUMN, moving to the center, and then the other end. The COLUMN section will eventually break away from the perfboard.

Carefully break apart perfboard edges along score lines

7. Using an Exacto knife, match the trimmed perfboard with the Hammond case and enlarge holes at the corner to match with the corner mounting holes in the case. Work slowly and carefully with minimal pressure to avoid breaking the corner piece.

8. Place the potentiometer mounting brackets onto the perfboard and enlarge holes in the perfboard to match the holes in the brackets. Work slowly and carefully with minimal pressure to avoid cracking the perfboard.

Cut holes for case and bracket screws

9. Spray paint the perfboard with the color of your choice or leave it natural. It's your choice.

Paint with desired color

I typically use flat red followed up with 2 to 3 coats of gloss clear.

Fabricate and mount the volume control brackets

The volume control brackets I made were from an old license plate that I cut with tin snips. But you can use any thin metal you have available. I've used PCI slot covers from an old desktop computer to good effect as well. I spray painted the brackets with a dark grey automotive paint but you can use any color that looks cool to you.  There is no try.  Do, or do not.

Follow the first few steps in the assembly manual to install the volume control mounting brackets. When completed, the project should look something like the images below.

Mount the Volume Controls

The potentiometers specified in the parts list come with hex nuts and flat washers. The assembly manual shows the order in which to install this hardware. So that the potentiometer shaft does not extend out too far from the bracket, I installed the inside hex nut and flat washer so that the outside hex nut and washer would sit just inside the first couple of threads on the shaft.

Important Note:  If you use a different vendor for the potentiometers than I've specified, make sure they ship with mounting hardware or you will need to purchase your own.

The potentiometers and their mounting hardware

Position of mounting hardware on the bracket

Potentiometers and hardware installed

Install input, speaker, and power connectors

The 1/8" (3.5mm) stereo jack specified in the parts list comes with pins pre-formed for automatic insertion into a PCB board. I straightened the pins out with pliers, lined up the pins on the perfboard, and firmly pressed down on the connector to seat it flush. You may need to slightly enlarge the perfboard holes if the connector will not seat with moderate force. Do not apply excessive pressure.

The three 2-position terminal strips will sit flush on the perfboard with very little force. Bend the pins slightly outward to keep them in place.

The assembly manual covers these steps, but I wanted you to be able to see what the board should look like when all connectors are installed.

Stereo Jack Installed

Speaker and Power Connectors Installed

Assembly Photos: Completed Left Channel

When completed, the left channel amplifier should look something like the images below. I used colored hookup wire for the potentiometer connections so it would be easier to identify each potentiometer lug (A, B, or C).

Assembly Photos: Completed Right Channel

When completed, the right channel amplifier should look something like the images below. I used colored hookup wire for the potentiometer connections so it would be easier to identify each potentiometer lug (A, B, or C).

Assembly Photos: Completed Stereo Channel

When completed, the 8 Transistor Stereo Amplifier should look something like the images below. You are now ready to attach the battery and speakers, plug in a sound source, and listen to some tunes on an amp you built yourself.

The assembly manual covers the setup and operation of the amplifier along with some modifications you can make to double the output power if desired.  Although for me, 1W was impressive enough.

If hope you enjoyed this article.  Thanks for reading!

NetZener