Zenith MJ-1035 mystery--weak sound with open volume pot?

[Jeffhs]

I have a 1965 Zenith MJ-1035 AM/FM/stereo FM radio that brings in stations like crazy, using just a 6' length of wire on the FM antenna terminal, but the audio output is almost nonexistent with the volume control full on. The stereo indicator operates as it should when any of our local FM stations are tuned in, so I know the set is OK up to the audio stages, but I have to put my ear right up to the speaker to hear anything, with the volume control set anywhere within its range; in fact, the setting has no effect whatsoever on the radio's audio output.

Here's the mystery. I have been told by several folks here and on AntiqueRadios.com that the cause of my problem is an open volume control. That makes sense, but what I can't figure out for the life of me is why I am hearing any audio at all if the control is open; after all, an open control would stop the audio signal right there (at the defective pot), wouldn't it?

Oh yes, one more thing. The foregoing occurs when I first turn the radio on from a cold start. It will play weakly for perhaps a minute, then the audio disappears. Turn the radio off, let it set a li'l while, then turn it back on again, and it plays -- again weakly for about a minute, then the sound disappears once more, being replaced by loud 60-Hz AC hum. I know this is caused by a defective filter cap, but to get at the connections to it I'd have to work my way through a couple of layers of underchassis wiring, directly in front of the cap's terminals--why Zenith built the radio this way is waaaaaay beyond me; maybe they figured the main three-section filter cap would last forever, so they hard-wired the thing into the set in such a way as to make it darn nearly impossible to replace?


I can cause this behavior (the weak sound being replaced by hum) to repeat indefinitely, so I'm thinking there is either a problem with an intermittent somewhere in the audio stages or within the volume pot itself.

I'm baffled as all get out as to what is going on with this thing. I'd like to get this radio working again as it once did (these were close to the Cadillac of high-fidelity sets in their day), but this problem has stopped me dead in my tracks as far as finding the trouble goes.

Thanks in advance.

[Phlogiston]

If I was to bet on this, I would not bet on a bad vol pot. A test would be easy. Inject a signal into the center pin of the pot. Not too much signal - could be very loud. I would test the tubes first, watch for shorts and grid emissions. Then replace all of the paper caps and filter caps.

[DaveWM]

open filter cap will not bypass the audio, getting around the preamp tube directly into the power amp tube. So an open decouplier and a defective preamp or VC would maybe do what you describe.

the vc is easy just jumper the middle to the hot side. does it hum loudly when touching the vc middle?

Check for an open coupling cap on the audio tube grid

Agree with previous poster sounds like a flter cap, one or more section prob open.

once that is done, check voltage on tubes look for plate and screen voltages, check high watt load resistors.

[sprman55]

Hi 1st off check all tube voltages and resistance readings for each tbe socket.Check any electrolitic caps in the set.Check all hi watt resistors and all resistors in the audio section. Check and clean bandswitch.Check speaker too.Let us know what you find.Sprman55