Introduction: Three Transistor Short Wave Radio
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 Three Transistor Shortwave Radio (Catalog #28-110) 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. 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 Three Transistor Short Wave Radio based on the original from 1968.
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Step 1: Obtain the Assembly Manual
The Three Transistor Short Wave Radio 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 and controls that can be obtained from electronics suppliers on Amazon. I've built the updated radio kit described here and believe it works just as well as the original kit did back in 1968. 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 radio, you will need the revised assembly manual available here.
I've kept the original branding and publishing style of the Science Fair Three Transistor Shortwave Radio 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.
Step 2: 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:
- The transistors for the radio can be purchased from Amazon or Radio Shack (assuming they are still in business in your area). I highly recommend the excellent Joe Knows Electronics semiconductor kit. It includes the transistors you need for this radio project and over 150 different types of transistors and diodes for just $22. And it includes a set of documents that are really good reading for the beginner. Check out www.joeknowselectronics.com. You will not be sorry.
- The resistors for the radio can be purchased from Amazon or Radio Shack. Radio Shack has a good selection of 1/4W resistors in a big 500 piece bundle for about $15.00 if you have a store nearby. Joe Knows Electronics also has a nice 800 piece package of 1% resistors for $12.00 if you don't mind ordering online. Joe's is a really good and well organized kit even if 1% resistors are a bit of tolerance overkill for this radio project.
- I strongly recommend ordering NP0 ceramic disk capacitors from 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 every other capacitor at 645 pieces for $13.00. Don't bother with Radio Shack for capacitor kits as they are mostly junk values you will never use.
- I purchased several crystal earphones from AmplifiedParts on Amazon and they work great despite the poor reviews. Whatever quality problem they had in the past seems to have been ironed out. The crystal earphone comes with a 1/8" mono phone plug so I added a 1/8" mono phone jack from Radio Shack. The phone jack is also available from various sellers on Amazon.
- The variable capacitor (and a lot of other rather old and interesting parts) can be found at Uxcell (via Amazon) 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 here that will help you figure out how to wire it in the radio.
- 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. 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.
- The knobs I used are Radio Shack knobs I've had in inventory for decades. Use anything you think is cool that will fit on the pot/varicap shafts.
- You will need to be creative on how you mount the variable capacitor on the vector board. I used a piece of 1/32" sheet metal cut to size with a Dremel tool grinding wheel and then drilled the holes to mount the variable capacitor with a power drill. Then I bent the end of it 90 degrees to form an L shape.
- You will need to be creative on how you mount the tuning knob to the variable capacitor. The shaft on the varicap is only about 1/4" long so you will need something to extend it. I found a plastic cylinder with a hole drilled through it that was about 1" long at my local Ace Hardware store. They have a really nice selection of odd hardware that is very useful.
- The 2-position barrier strips are available from Amazon or from Radio Shack in a pack of four. These are a great value at the price so if your local Radio Shack hasn't yet been turned into a Sprint cell phone shop you should definitely buy all of the packs on the peg. I know I did.
Step 3: Review the Schematic to Become Familiar With the Radio Design
The Three Transistor Short Wave Radio is composed of 4 major building blocks:
- Tuner - Selects a station from the band of frequencies the radio can receive.
- RF Amplifier - Amplifies the audio modulated carrier selected by the Tuner.
- AM Detector - Strips off the RF carrier leaving the original audio signal intact.
- AF Amplifier - Amplifies the audio signal so it can be heard on the earphone.
Variable capacitor C5 and tuning coil L1 comprise the Tuning section. L1 is a fixed inductor wound according to the Assembly Manual for the frequency band of interest. Stations are selected by rotating the variable capacitor.
The RF Amplifier/AM Detector section is actually a Colpitts Oscillator with an added variable resistor R2 that serves as the regeneration control. Resistors R1 and R4 provide base voltage to Q1 so that it's collector is fixed at approximately 3V. This collector voltage was chosen so that the RF Amplifier/AM Detector will continue to operate properly as the 9V battery reaches the end of it's life. The large values of R1 and R4, and the bootstrap bias configuration they are connected in, were chosen so that the RF Amplifier/AM Detector will have a high input impedance which improves the selectivity and sensitivity of the radio. Capacitors C1 and C6 were included to bypass RF around resistors R1 and R5 respectively which improves the gain of the RF Amplifier circuit. The collector output of Q1 is fed back to the emitter of Q1 through capacitors C2 and C3. Normally this positive feedback would cause the RF Amplifier/AM Detector to continuously oscillate. However the regeneration control provides an adjustable amount of negative feedback at the emitter of Q1 that counteracts the positive feedback. By carefully adjusting the amount of negative feedback on the emitter of Q1, the circuit can be made to provide extremely high gain just before oscillation occurs, and at the same time remove most of the RF Carrier signal and the unwanted image of the audio signal. This behavior is the reason the regenerative radio works so well.
The AF Amplifier is a simple two-stage direct coupled Common Emitter amplifier for driving the crystal earphone. Transistor Q2 provides a gain of approximately 5 and together with R10 and C9 performs additional filtering of the carrier signal. Transistor Q3 provides a gain of approximately 100 (the transistor current gain at .5mA). Capacitor C11 is provided to bypass audio frequencies around resistor R12 and improve the gain of Q3. Together, Q2 and Q3 provide an additional gain of approximately 500 after the RF amplifier. The value of R12 was chosen so that the collector voltage of Q3 would be set at approximately 1/2 battery voltage which ensures that the Detector and AF Amplifier will continue to operate properly as the 9V battery reaches the end of it's life. Capacitor C7 and resistor R6 are wired together as a simple RC Low Pass Filter to prevent RF noise at the RF amplifier from bleeding into the AF amplifier via the battery connections.
Step 4: 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 20 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. 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 redesign. 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.
Step 5: Follow the Steps in the Assembly Manual to Complete the Radio
Before you begin:
Cut the vector board to closely fit inside the top of the Hammond project box. Measure the lengths needed and try to line up the edges along a row of holes if possible. With an Xacto knife, score a line into the vector board deep enough that it can easily be felt with a fingernail. Then carefully break the board apart along the score line.
Using an Xacto knife or low speed power drill, cut the holes for the Hammond mounting screws to line up with the mounting holes in the Hammond project box.
Using an Xacto knife or low speed power drill, cut the holes for the variable capacitor mounting bracket.
Using an Xacto knife or low speed power drill with a reaming bit, cut the holes for the potentiometers R2 and R7, and the hole for the phone jack.
If desired, spray paint the top of the vector board in flat red. Or any other color you wish. The natural vector board finish also looks nice.
After the paint has cured, follow the step-by-step instructions in the assembly manual.
Step 6: Assembly Photos - Mounting the Terminal Strips
After Assembly Step 3, the radio should look something like the photo above.
Step 7: Assembly Photos - Mounting the Radio Controls
After assembly steps 4-6 the radio should look something like the photos above.
Step 8: Assembly Photos - AF Section Completed
After assembly step 35 the radio will look something like the photo above.
Step 9: Assembly Photos - RF Section Completed
After assembly step 49 the radio will look something like the photos above.
Step 10: Assembly Photos - the Completed Three Transistor Short Wave Radio
The L1 coil winding chart explains how many turns of 20AWG insulated wire to use for the frequency band of interest. The terminal strips make it easy to swap the coils as needed during a listening session.
Any radio is only as good as it's antenna and the environment it is in. In my area, everyone has a wireless router, several cell phones, LCD TV's, and many other devices that make lots of electronic noise. A good antenna as described in the Assembly Manual is essential for getting the best performance from this radio project.
After building the radio I've successfully received WWV on 5, 10, 15, and 20 Mhz with good copy. I've picked up all the well-known short wave KW transmitters from around the globe. And I've picked up SSB on 7Mhz and 14Mhz. SSB can be received but the detector is not designed to clearly demodulate it. But if you thought the Probe Droid from Star Wars sounded cool, definitely tune into SSB with this radio.
I'm extremely happy with how the 3 Transistor Short Wave Radio looks and how well it pulls in distant stations
I hope you have as much fun working on this project as I did. Good luck and good listening!