Zenith MJ1035 volume pot question

[Jeffhs]

I pulled the chassis of my Zenith MJ-1035 radio the other evening, with the intention of getting the volume control working as it should. (The radio works, and well; gets lots of stations in my area just using a piece of wire on the antenna terminals, as I can tell by the stereo indicator, but I can barely hear it with the volume control at maximum.) No luck so far, so I think I'm going to substitute a 1K pot from my junk box for the 2-megohm dual pot already in the set. Will I get any audio at all, or am I risking damage to the audio output transformer or tubes (two 50EH5s with a 12AX7 preamp) by using a much smaller pot than the schematic calls for? I am not interested in stereo sound from this radio at this time; I would simply like to get it working monophonically. Once it does work well in mono, then and only then will I tackle the problems with restoring the other stereo channel (the speaker for which plugs into an RCA jack marked "left channel speaker socket" at the rear of the chassis).

If this is a duplicate post, I apologize, but it seems that my last post got lost when all threads were moved to the new Videokarma.org sub-site of AK. I think my original thread was lost because, when I logged on to Videokarma.org, I was greeted with the message: Welcome, Jeffhs. You last visited: 08-19-2009 at xx:xx pm. (I don't recall the time of that particular post offhand.)

Thanks in advance.

[electroking]

The 1K pot will essentially short out the signal from the previous stage, so it will
not work. Better get the correct part.

And yes, everything posted is these forums from August 15 to sometime earlier
today is gone forever, but some people think it is a good move...

[Jeffhs]

Originally Posted by electroking

The 1K pot will essentially short out the signal from the previous stage, so it will not work. Better get the correct part.

Hmmm. I agree with you as to using the correct value of potentiometer to replace the one in my set (even though I don't see how a 1K pot would "short out" anything--it's a resistor, not a short circuit--maybe the value is so low it might as well be a dead short across the output stages?), but I'm not sure I could find a 2-megohm dual pot anymore, as my MJ1035 was made in the mid-1960s.

It is nearly impossible to find parts for anything older than about the 1970s, and most repair shops won't touch anything that old or older these days with a ten-foot pole. (I once tried to get my Zenith H511 radio, made in 1951, serviced by a local TV/radio repair shop, but was told over the phone, once I mentioned the year the radio was manufactured, that they wouldn't have a thing to do with it, even if I brought the radio to them personally.)

If you or someone else here on A/VK could advise me as to where I could find a 2M potentiometer, I would appreciate it greatly. This radio has been sitting in my bedroom, unused, for months--I'd like to get it working again, as I mentioned in my last post, in mono if not stereo.

Thanks in advance.

[electroking]

If a circuit has a source impedance such that it wants to see a 2,000,000 ohm load,
and you load with anything below 200,000, you have essenially shorted the source,
meaning that the output voltage is now about 10% of what it should be. With 1000
ohms, it would be much worse. A short-circuit need not be exactly 0 ohms, it
just needs to be much smaller than the optimum load impedance.

I will look for a correct pot for you, and as you said if anyone has ideas, please
come forward with them. Regards.

[Ampico-kid]

I guess my first question would be....what is it about the volume control itself that would lead you to believe it's defective?

The fact that the volume is low with the control turned up all the way would most likely have nothing to do with the control. And...the fact that you've got a dual control makes the probability of BOTH pots being defective pretty unlikely.

I think you might be barking up the wrong tree.

Try connecting the wiper (center terminal of the volume control) to the signal source (the third leg of the control that you're rotating the wiper toward to increase the volume) and see if you get your full volume. If so...then the control probably is defective (as unlikely as that would seem). If the volume is still low, the control is fine and you've got trouble somewhere else.

Bob

[electroking]

Hello again,

You were right, 2-Megohm units are hard to come by, except as microscopic
multiturn trimmers. Also, for a volume control, you need what is known as a
log taper (or audio taper) pot, meaning that the resistance varies faster
at the high volume end, so that the volume variation is perceived as fairly
uniform along the range. Anyone knows of a good source for this kind of
part?

[Jeffhs]

Originally Posted by Ampico-kid

I guess my first question would be....what is it about the volume control itself that would lead you to believe it's defective?

The fact that the volume is low with the control turned up all the way would most likely have nothing to do with the control. And...the fact that you've got a dual control makes the probability of BOTH pots being defective pretty unlikely.

I think you might be barking up the wrong tree.

Try connecting the wiper (center terminal of the volume control) to the signal source (the third leg of the control that you're rotating the wiper toward to increase the volume) and see if you get your full volume. If so...then the control probably is defective (as unlikely as that would seem). If the volume is still low, the control is fine and you've got trouble somewhere else.

Bob

I suspect the volume control is bad because it, and the tone control, were extremely loose when I got the radio from an AK member in Arizona well over a year ago. When I first looked at the control, the rear half of the unit was separated from the front; in fact, the carbon track for the rear pot had separated from the wiper and was (still is) hanging loose under the chassis. At first I was able to get full volume by gently pulling outward on the outer shaft of the control, but even that doesn't work now. I am, however, still able to hear some audio from FM stations, but as I mentioned in my previous post, the sound is so weak I can barely hear it over a hum that is perhaps 50 times louder than the stations I'm trying to hear. I realize the hum is due to a bad filter capacitor, but that's another problem I don't quite know how to deal with; that is, I could replace a defective cap, even a multi-section can-type electrolytic such as is in this radio, but the problem is that there is a bunch of wiring and components directly in front of the capacitor's terminals. Seems to me I would have to disconnect every one of those wires, components and such before I could even begin to think about replacing the cap. I can't for the life of me imagine why Zenith put the filter in this way, with so much in front of the terminals; there is plenty of room under the chassis, so those other parts could have been spread out a lot more than they are. Hmmm.

[Ampico-kid]

Take a look at the "Play Things of the Past" website.

http://www.oldradioparts.com/

Gary has a ton of potentiometers both NOS and old parts. I spotted at least one that sounded good for your purposes:

1487-621 CLARO AD47-Z 2 MEG DUAL POT,1 SHAFT, AUDIO 1 $8.00

Once on the site if you choose to have the pots listed by NUMERICAL RESISTANCE VALUE you'll see a long list of 2 meg pots. Depending on the shaft configuration you might find something there.

As for the filter can access...I'm afraid you're on your own with that one. It's a problem every repair technician and restorer has grappled with from day one. There is no easy solution. Sometimes just moving the wires and components out of the way is sufficient to gain access. Sometimes as you say, the parts and pieces in the way have to be removed. There is no dead set rule of thumb to apply here. Common sense, enginuity and patience are the key to a successful job.

Good luck.
Bob

[Findm-Keepm]

What's the Zenith part number? It'll start with "63-"

I and a few others here on VK have deep Zenith parts stashes. Doug (drh4683) also has some Zenith stuff.

Cheers,