The FM volume on my radio has dropped to the point where I have to turn it all the way up to hear it. If I then change to AM, it blasts me out of the room. Is this likely a tube problem? I have spares for all (5?) tubes and could systematically swap them out.

Mark

Could be a tube problem....could be a resistor or capacitor problem (drifted resistor or leaky capacitor). It could also be a silver mica problem. If the IF transformers use fixed value silver mica capacitors, and they have corroded, the IF transformers will no longer peak, and won't work efficiently. Wiggling the transformers and noting a change in volume is one way of testing them. Another is to open them up and have a peek. If the silver coated mica pieces are open to the air and are visible, and if they have turned dark, most likely they are not making good contact anymore with their spring contacts. Also, if they are corroded badly enough, the silver area might not be as large as it used to be, which will reduce the capacitance.

Verify that your trouble isn't due to something else first, because re-capping IF transformers and removing all of the old silvered mica material can be a challenge.

T.

If the weak volume problem only occurs on FM, then the audio section must be good as well as the RF/IF section of the AM. If the radio plays, even though faintly, then the FM oscillator, mixer, and demodulator ckts must be working. So that leaves the RF/IF FM amplifier sections. Could be an IF alignment problem?

I doubt that the problem is a tube, but it's conceivable. You'll find out if you swap out tubes like you suggest.
Doug