Those of you not into technical stuff might find this boring, but I share it here for my fellow tech/engineer types who might enjoy reading of the troubleshooting and repair of a transmitter.
I recently picked up a couple of AM-30 transmitters on eBay. One was in near-mint condition, super clean inside and out. It was as if it had never been used except there was a lead connected to the input terminals. Perhaps it was operated once to test, and then reserved as a backup/spare? Anyhow, I connected it to a dummy load and an audio source and powered it up. It put out its rated power and sounds good. SCORE!
The other AM-30 was a different story. I considered that it might have a problem from the photos as the AC line fuse was missing, but I took a chance anyway.
Right off the bat it looked like someone had replaced the voltage regulator transistor. Upon closer investigation, I saw that the insulating washer (which isolates the transistor tab, which is connected to the collector and is at power supply potential, from the chassis) had been omitted. Perhaps it would be that simple? Nope. Further inspection revealed that the wires for the emitter and base had been crossed. The previous 'technician' (who didn't know how to properly mount the device) apparently was trying to install it as shown in the schematic, which, BTW, is incorrect. In fact, having three leads, there are exactly three ways to connect a transistor. There is the correct way, which is not too difficult for someone familiar with basic power supply circuits to determine. There is the incorrect way shown in the schematic. And this one was connected in the remaining, also incorrect way.
Surprisingly, the transistor survived the incorrect wiring unscathed, but there remained a dead short in the power supply. That turned out to be a shorted bridge rectifier. I just happened to have one around.
With the bridge replaced, it no longer blew the fuse. Power supply voltages seemed reasonable, and it made RF. As I slowly increased the power, though, the power supply voltage began to sag and I began to notice quite a bit of hum, which got worse as I increased the power. At about half-power the audio began to get severely distorted.
Have you guessed yet? It was the main electrolytic filter capacitor for the 30-volt supply. Possibly just dry from age, or maybe stressed from the shorted bridge (the safety vent looks like it may have let some of the Magic Smoke escape at least once).
In any case, I sumped it with a good one and now the voltage holds steady and teh signal is clean -- free of hum and distortion, even a full power. YAY!
Still has a minor niggle or two - the on-board meter works for power output, but not for audio level. That needs looked into, but even if unrepairable is only a minor inconvenience.
I'd have probably been disappointed had I paid the asking price, but for what I actually paid via "Make Offer" I guess I did OK, and it's always rewarding to successfully repair something and return it to useful service.
Tomorrow I'll connect up one of the 'new' (to me) transmitters and TCU-30 coupler to see how the range and performance compare to my Radio Systems TR20/CP15