Zenith MJ1035 - no sound

[Jeffhs]

I was working on my Zenith MJ1035 stereo FM radio tonight, trying to get more volume from it (the volume control is intermittent and is now to the point where it will not vary the volume level at all, although it did work well enough that the radio was playing at reasonable volume before the problems began). I looked under the chassis and found the ground connection from the volume control housing had come loose. I resoldered it, connected the other end of the lead to the chassis, and tried the radio again. This time, nothing but hum from the speaker. I worked on it for several hours tonight trying to find out where I am losing the signal, to no avail. Any ideas?

Thanks in advance.

[Hemingray]

Check to be sure you didn't nudge any components into any other connections. I had a similar paroblem with an old Zenith 7G04's tube string, turned out that I had inadvertently pushed the leads of a nearby component into a pin on the string, which caused that end to ground out.

[Chad Hauris]

I have had several cases where volume controls went open and I had to replace them....sounds like maybe the carbon track or wiper is open inside the control. First thing I would do is put the ohmmeter on the control (disconnected from the circuit) and check from wiper to each end terminal and across the end terminals.
It will probably be harder to replace the control on a radio like this as opposed to on, say a 1940's console radio as the shaft may be insulated to avoid a hot chassis shock hazard and also it may be a half-moon type to fit the knob.

[Jeffhs]

Originally Posted by Chad Hauris

I have had several cases where volume controls went open and I had to replace them....sounds like maybe the carbon track or wiper is open inside the control. First thing I would do is put the ohmmeter on the control (disconnected from the circuit) and check from wiper to each end terminal and across the end terminals.
It will probably be harder to replace the control on a radio like this as opposed to on, say a 1940's console radio as the shaft may be insulated to avoid a hot chassis shock hazard and also it may be a half-moon type to fit the knob.

I had a feeling the volume control was going to fail eventually, since it had been extremely intermittent almost from the time I got the radio last year.

The volume control in the MJ1035 is a 2-megohm dual pot. I don't know if these are available anymore, after 43 years. If they can still be found, where can I find a replacement? If not, is there a suitable substitute for it, even a single pot for just one stereo channel? I'm not really interested in restoring the stereo function, as I already have a reasonably good stereo system; if I can get the MJ1035 working in mono, that will be good enough for me, as I use it for background music only at this time.

BTW, I am not aware of any special precautions Zenith took with the MJ1035 to insulate the control shafts from the chassis, but the control does have a half-moon shaft on the inner control for right channel volume. (The left channel pot has a shaft that fits over the inner shaft.) If necessary, I could replace the knob with a setscrew type knob from my junk box.

Thanks in advance.

[Jeffhs]

...but very weakly. I worked on the set for about an hour yesterday (11.13.08) and decided to test it, just to see if the signal circuits were working. When I was tuning across the FM band to check the multiplex circuits (was looking for the indicator to come on when the radio was tuned to a stereo station), I put my ear to the speaker and heard very faint audio from a Cleveland station, in addition to the stereo indicator illuminating. This tells me the volume control must be working well enough to complete the audio output circuit, but there are still issues with the wiper contact as the volume level will not change when I turn the shaft. I'm thinking perhaps the wiper isn't making contact with the resistance element (as old1625 mentioned in a reply to one of my previous posts a few months ago regarding this issue). I have not yet run the ohmmeter test on the control that Chad mentioned in his reply to my last post; perhaps I'd better make that my next step in trying to find this problem. However, I think there is also a defective electrolytic filter cap in the radio's power supply, as there is a 60-Hz AC hum that is much louder than the audio output. If I could get rid of that hum I'd probably find the audio is a bit louder than it seems right now. The MJ1035 has a can-type electrolytic, with three sections. Are replacements for this cap still available after 43 years (my MJ1035 was made in 1965), and if not, is it possible to use one or more caps connected in parallel to replace it? There seems to be plenty of room under the chassis.

[Old1625]

As for the volume pot you may want to explore for an old radio/TV parts shop that may have carried the old Clarostat line. Such product came in bits that could be assembled to make multi-section pots of all types.

As for the 'lytic your best bet is to install a terminal strip underneath and use individual capacitors of appropriate value to subsitute. Be sure to exclude the old lytic--even if it is left in place, and be sure the negatives of the new caps return to the exact same ground point.

Good luck!