Chris:
Usually the only thin seperate from the AM is the local oscillator, they use a double tuned IF strip which passes 455 kHz and 10.7 mHz, so if the AM is working, the IF, audio, and power supply are OK. In my years of working on airplanes, I have learned not to say "this will always happen, or this will never happen, or this can't happen" or stuff like that, but I can say with confidance that if the local oscillator is not working the radio is not working, either, period, paragraph, end of chapter.
Lewis
:
If you prefer to collect/repair a specific brand, as many people do, you may have uncovered a design weakness specific to that make of radio. For example, many Zenith models from the mid-to-late 1930s seem to have undersized power transformers which tend to run hot. However, this problem is specific to Zenith- most radio makers didn't share this problem with them.
I think that some of the AM-FM combos made right after WWII were somewhat poorly executed design-wise when it came to the FM circuits- so if you concentrate on repairing sets of that era, you may be seeing a disproportionate number of failures.
It would be interesting if you could jot down the repairs that the FM sections of your sets have needed, just to see if there is any "common denominator" to the failures.
Best regards,
Bill