Zenith MJ-1035 audio problem - Videokarma.org TV - Video

Jeffhs · #1 10-12-2010, 07:23 PM

My Zenith MJ-1035 AM/FM stereo radio will bring in stations like crazy in my area 35 miles east of Cleveland, but I can barely hear the sound. (I know the radio's RF signal circuits are working, because the neon stereo FM indicator lamp on the front of the set lights on every station the set receives.) A few weeks ago, I tried removing the 2-megohm volume control from the circuit, expecting to be blasted out of my apartment when the radio warmed up; no luck. The sound is almost gone now, replaced by a loud 60Hz AC hum. For awhile I was able to get a tiny bit of sound out of the speakers, barely (and I do mean barely) audible over the hum, but not anymore.

I'm stumped, and wondering if something serious is wrong, such as bad output transformers, or whether it's something as ridiculously simple as a bad preamp or output tube. I would be satisfied just to get this radio working in mono; I don't care about the stereo mode, as I have a much newer and better stereo system in the living room. I'd like to get the MJ1035 working just for the sake of having it working again. I have the Sams Photofact for it, so I can reinstall the volume pot when the time comes to do so.

Thanks much in advance.

jr_tech · #2 10-12-2010, 10:36 PM

Could you post a scan of the schematic? It is not clear how you "removed " The volume control. Did you substitute a fixed resistor, or is the grid of the next audio tube now "floating" ?
jr

Jeffhs · #3 10-13-2010, 10:54 AM

Quote:

Originally Posted by jr_tech

Could you post a scan of the schematic? It is not clear how you "removed " The volume control. Did you substitute a fixed resistor, or is the grid of the next audio tube now "floating" ?
jr

I don't have a scanner. I could take a picture of the schematic and post it, but the resolution of my cheap Radio Shack Flatfoto digital camera is so poor (1.3 megapixels) that the diagram may not be legible, even if supersized.

I took the pot completely out of the circuit; didn't replace it with anything. There was only one lead connected to the control when I removed it -- a white wire going to one pin of the 12AX7 preamplifier tube.

You mentioned substituting a fixed resistor for the 2meg volume pot. I haven't tried that yet. Would it work well enough to get some signal through to the preamp? I'm not interested in getting a lot of volume, just enough to prove to myself that the trouble is not being caused by an open output transformer or some other serious problem. This radio was made 45 years ago, so I'm not sure I could get parts for it today; even the volume control potentiometer itself looks like a very special part that would be difficult (if not downright impossible) to get at this late date, unless I were to salvage one from a junked MJ1035 -- and I don't have the room to store another of those radios here.

jr_tech · #4 10-13-2010, 05:30 PM

Quote:

Originally Posted by Jeffhs

I took the pot completely out of the circuit; didn't replace it with anything. There was only one lead connected to the control when I removed it -- a white wire going to one pin of the 12AX7 preamplifier tube.
You mentioned substituting a fixed resistor for the 2meg volume pot. I haven't tried that yet. Would it work well enough to get some signal through to the preamp?

So where is the white wire connected now? ... Is there a capacitor from the plate of the previous stage? Is there any resistor to ground from the grid of the 12AX7? Something seems to be missing here!
jr

AUdubon5425 · #5 10-13-2010, 03:06 PM

There had to be more than one lead to the volume control - that's why you aren't picking anything up.

It should have either 11J01 or 12J01 stamped on the chassis - seems there was a slight change along the way. It's not in my Beitmans but 12J01 was in Sams 647 and 11J01 was in Sams 751.

dieseljeep · #6 10-13-2010, 07:46 PM

Hey Jeff. Wasn't this the radio that was damaged in shipment when you bought it several months ago. If I recall the volume control was bashed in and damaged. Is the control a dual unit with a tone compensating tap on it. There would have to be at least 5 to 8 wires on it originally. Hopefully someone can post a schematic on it so we can advise. If your get a hum from both channels the output transformers should be good. Best of luck. Dave.