AM/FM radio troubleshooting

Well we all knew that you were off your rails Phil, but now we know just what it was that drove you nuts. An intermittent fuse. No TWO intermittent fuses. Possibly if not the most, then one of the most simple components known to electronics techs. I sincerely hope they weren't "chemical" fuses. I guess you must have slept through that class huh? Maybe that can happen to "2AG" fuses, dunno.....Lenny

So meatplow's proffered schematic is even more spectacularly inappropriate to the OP's situation -- trouble shooting a nine- transistor AM-FM radio from 1970. Which, as I recall, likely used a variable capacitor for tuning.

Googling shows a schematic of the Transistor 305 is available for purchase on radiomuseum.org

Tune as in select a frequency? Maybe by mind control? It's just a crude example, not meant to be a service diagram.

I actually have the book with this radio. It has a schematic but comes up somewhat short when it comes to voltages and waveforms. And the intermittent nature of the problem makes it so hard to troubleshoot. The other night it played on my bench for over ten hours. It never failed and sounded fine. I finally went over and tuned the FM to a few other stations and they seemed to sound OK. I then switched to AM and although I did hear some stations, it being late evening I would have expected to hear much more. But I wasn't sure about this as the shop is in the basement. I tuned the AM between 550KHZ and 1600KHZ at which point the radio suddenly quit. The tuning capacitor is not the issue. And the switches don't appear to be noisy either. The scenario about tuning the AM and the set quitting seems a bit of a stretch to be a coincidence but then I just don't know. Anyone have any theories on if this might be a small clue here? Thanks for following this thread, Lenny

Put a scope on the output of the mixer and keep it there until the radio dies, and see if that first stage still is working. Do this one stage at a time until you find which stage is the problem. Have you tried heating the radio to induce the failure, you don't mention trying that?