Problem with shifting volume: Zenith K-731

[RetroHacker]

If all the resistors are OK, it could be a cap. Start checking around the output tubes, and work out from there. A leaking cap can cause similar problems - as the set heats up the cap starts leaking and the tube draws more current and then the cap leaks more and then the tube draws even more current... ad infinium until the tube goes into cutoff.

Both possibilites (resistor or capacitor) are pretty likely. I've just seen more resistors cause this problem than caps in later radios, a lot of the smaller table sets (AM/FM) used ceramic disc caps.

-Ian

[Chad Hauris]

Sounds like it's time for the freeze spray! use isolation transformer when checking...power line is connected to chassis.
Spray the freeze spray on resistors and ceramic caps and see if you can get the audio to come back once it's heated up. Do not spray it on the hot tubes. YOu may have to use a heat gun or hair dryer to help heat up the area to coax out the failure mode.
Probably not electrolytics as there is no hum...only other electrolytic in this set is in the FM ratio detector (if it used that system).