[mikethedruid]

If your FM IF frequency is about 910 khz, look at the difference between what you were hearing and where your sig gen was set. Looks like the sig gen was acting as a local oscillator. This may well mean that the local oscillator in the set is not working on FM. I'd look into that situation. If you have an O scope, check the local oscillator output, and then the wiring from the local oscillator to where it mixes with the received RF in the set. If it is all done in one tube, and things are working on the AM bands, then look at the wiring that pertains to the FM band, including the band switch which may well have a bad contact in that position, or the wiring to that switch may have a problem.