WEAK-AM

It took me a week, but I finally found the time to fire up Chris Cuff’s creation on the test bench. My unit came set for operation on 1500 kHz (at my request). You can change the crystal to operate on other frequencies. Lacking a power supply with the correct connector, I fired it up on batteries (an internal battery holder is included). Eight AA’s will apparently power the unit for 8-10 hours continuously.

I connected a short wire antenna to the Fahnestock clip on the back and grounded it to the wall outlet. Next, I hooked up a stereo audio source to the RCA input connectors and fired it up! Immediately, I heard the program material in stereo on my tuner. I have only one AM stereo radio left; it is a modified Radio Shack tuner. The sound quality was not perfect, but it was in stereo!

I reduced the level a bit until the audio sounded cleaner. I seem to recall that you have to limit the modulation to 100% with AM stereo. In any event, I am not certain that the transmitter is capable of more than 100% peak positive modulation.

Although the transmitter comes with a few explanatory notes on how to set it up, there is no schematic. The output circuit is interesting; the coil appears to be wound on a glass test tube (with great care, apparently), along with a nice vernier tuned air variable capacitor. You adjust the capacitor for the strongest signal, as observed on a field strength meter or nearby radio. I jumped in the car and drove down the street. My preliminary observation is that the range is comparable to my SSTRAN when operated under similar conditions. The output appears to be taken directly from the “hot” side of the parallel tuned tank circuit.

This is an interesting unit, and probably destined to become a collector’s item. Chris told me that he has built 350 of them by hand and that he does not have parts to build any more. Classical music sounds great in AM stereo, as those of you who live in the Kansas City area can probably attest, due to the presence of KXTR.

Update: I have discovered that the carrier frequency is off by quite a bit; probably in excess of several hundred Hz. I’m not sure if I’ll bother fixing it as I intend to purchase the programmable oscillator from Digi-Key (if I can remember the part number)! But for anyone who wants to use it crystal controlled, it should be an easy matter to add a trimmer on the board. The microprocessor crystal Chris sent with the unit is probably not very accurate.