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.
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