The "Radio Shack Special"

My Experience Building a One-Transistor FM Receiver with Saturn Dial

copyright 2006 Larry J Solomon

The Radio Shack Special is a one transistor super-regenerative FM receiver (re-)designed (or "undesigned") by Patrick Cambre from an original design by Charles Kitchen. Patrick streamlined the circuit from 24 to 12 components. You can  find a description of his circuit with complete building instructions at  http://braincambre500.freeservers.com/rss(1)(1)(1).htm.

Although I had some doubts that I could do this, I decided to construct this little receiver, since I am interested in new and simplest FM designs. I didn't expect much, but was astounded with the result. Using no more than a 7-inch antenna and no ground, this little dynamo pulls in almost every FM station in my area with astonishing clarity and loudness.

The project calls for a printed circuit board, and this was my initial hurdle. I remember trying to do PC Boards some decades ago without success, so I didn't relish starting by making my own. The Radio Shack Dry Transfer stuff mentioned on Patrick's website is no longer available, and the instructions he generously supplies are somewhat daunting in spite of his reassuring remarks. I found another venue through FAR Circuits in Illinois; see http://www.farcircuits.net/. I was able to send them a copy of the circuit board diagram, and they made the board for me for a nominal fee.

Most of the other components are still available from Radio Shack, but the variable capacitor I ordered from Electronix Express (via web). Once I received the board and all the pieces, I stuffed the PCB and soldered the components in place. The most difficult part was mounting the variable capacitor, making a knob and dial for it, and the chassis mounting. Some drilling is necessary. The coils are easy to make and mount.

I wanted to have an audio output for an amplifier, because I hoped to be able to play the receiver through my stereo system. To my surprise, no modification was necessary. Simply plug the phone output into an input of your amplifier, and walla!

Patrick's instructions are generally excellent, but I did discover some errors in the section titled "The Ordered Way of Putting all the Items onto the PCB". In Step 2, replace "Item #17" with "hole #17", and Item #23 with "hole #22". In Step 9, "Item #26" should be Item #25. In Step 12, "Item #27" should be Item #26. In Step 16, "Item #8 is a 1K resistor" should be a 10K resistor.

When I finished my "first draft", I turned on the set and got a loud squeal with and no radio signals. It was oscillating wildly. I tried adjusting the pots and variable cap, but that didn't fix the problem. After making some changes as in the previous paragraph, I also discovered a cold solder joint, which I mended. On the third "turn-on" the squeal was gone and it was working a lot better. By adjusting the pots, the performance was amazing. However, I still have problems with hand and body capacitance near the unit. Patrick tells me that this can be improved by mounting the unit in a metal box. However, I want the set to be visible, because I like to make radios that are visually like art.

With this in mind, I mounted the unit on a wood base I got from Michaels Arts and Crafts, and I designed and made the Saturn Special knob and dial from a "doll's head", also purchased at Michaels. I had to drill and chisel the knob joint (rectangular) with an Xacto knife to fit the tuning capacitor. I then drilled a hole through the center of the rectangular hole to the top of the knob and fitted it with a recessed screw to the tuning capacitor. The hole is topped with a trimmed brass paper fastener for appearance.  The plastic circle around the knob is used to dial stations. I fashioned this from acrylic and cemented it to the knob.  I made the dial by drawing it in Paint Shop after getting positions of the stations.

Nylon spacers of 0.75"  can be found at local hardware stores. I also attached a 7 inch brass rod (1/8" dia) from the hardware store for the antenna.

1 transistor fm



rss from above

rear view

dial & set


home
  Home