Problems with MJ-1035

[Jeffhs]

My Zenith MJ-1035 radio was working very well, until I tried to push the volume control knob further onto the shaft than it was (I thought I could set the knob flush with the front panel). I turned the radio back on and heard very weak audio at first, then I lost the sound completely. Looking under the chassis, I found what could well be the problem: a broken connection on one terminal of the concentric volume pot (the inner shaft for the right channel; the other control hasn't worked since I got the radio--in fact, it looks as if it is broken ). I'm wondering where that connection goes. My Sams schematic (set 715, folder 9) is not much help, as it does not show the concentric dual pot my radio has; rather, two separate controls are shown, one for each stereo channel.

Note: I was able to get the volume control working again by temporarily connecting the broken lead to ground, so that may well be where that side of the resistance element connects.

Another problem: Several tubes in my MJ1035 are not lighting. I am at a loss to figure out why, as the MJ-1035 has a filament transformer that supposedly supplies all 11 tubes in the radio. However, I did notice from the schematic that there are two separate series filament strings, wired in parallel. I've never seen anything like this, and I'm stumped. I'm half thinking one of the tubes has an open filament, but I don't have a tube tester so I really cannot tell the condition of the filaments at this point.

This is not the first time my MJ1035 has exhibited this problem (several tubes in the filament string not lighting). The first time this happened, I was able to correct the problem by removing and reseating one tube in its socket, but that dodge didn't work this time around. I'd try replacing the tubes, but the MJ1035 uses several tubes which I've never run across, such as a 19GQ7, 19EA8 and so forth. I don't have spares for these tubes, although I can replace some of the others with tubes swiped from my other vintage radios if need be.

Thanks in advance.

[Jeffhs]

I found the problem with the tube filaments in my MJ1035. It was as I had suspected all along: a tube in one of the two series filament strings was defective. Replaced it and all tubes are now lighting as normal.

I'm still baffled as to why Zenith designed this radio with two series strings in parallel, powered by a filament transformer but still using a voltage doubler for B+; it baffles me because the plate voltages as well as filaments could have been supplied just as well by a dual-voltage transformer, and the radio would be 100 percent isolated from the AC line to boot if such an arrangement were in place. I'm tempted to say this was a cost-cutting move on Zenith's part, but somehow I can't really believe that was the reason for the use of a transformer for just the filaments, and wiring two series strings in parallel. I'd have expected in a radio of this caliber that the power supply would have been designed to use a power transformer for both filaments and B+, with separate windings for each. I don't know what Zenith's design engineers had in mind when they designed the power supply for this receiver, which after all was not cheap--it sold for almost $200 when it was new 46 years ago. For that kind of money, IMHO, they could have afforded to design the set for full transformer power rather than using a voltage doubler; the latter is almost always used in simple AA5 radios, which is why AA5s sold for such low prices by the '60s. But in an MJ1035? Sheeesh!

On second thought, maybe that was the intent with the later (1964-'65) version of the MJ, which was a less-expensive model. Did the original MJ1035 (1961-'62) use a full-transformer power supply?

Found the source of the weak/missing audio as well. One side of the volume control was disconnected (the open connection I mentioned in my last post), so I connected that side of the pot to the case of the control, reassembled the radio, plugged it in, turned it on, and it worked. The audio was weak at first, but I was able to get usable volume, and then some (this radio can produce earsplitting volume on strong stations with its single-ended 50C5 audio output tubes in both stereo channels) by gently pulling the volume control shaft outward, which leads me to believe there is still a loose connection somewhere on the control. But for now, I'm happy to have the radio working again; in fact, I am half afraid to do anything with it at this point, as well as it seems to be working--such as it is.

I was amazed when I looked at the chassis of my MJ1035 for the first time after I got it and found the rear volume control loose on the chassis, not to mention the tone control being just as loose. As well as the radio had been packed when Nolan Woodbury sent it to me, I cannot imagine how these controls could have worked loose as they did, not to mention the rear volume control being broken--especially when Nolan told me, in a private message, that the radio had been working well in stereo before he sent it.

How the dual concentric volume potentiometer broke as it did is a mystery to me as well (the control shaft for the left channel amplifier simply turns 360 degrees and doesn't seem to have a carbon resistance element). Again, as well as the radio was packed, I don't know and cannot imagine for the life of me how this could have happened. Maybe the box with the radio was punted across the floor or even thrown or dropped (!) in a UPS shipping facility before it was put on the truck? This reminds me of the story I read here of one MJ1035 Nolan had gotten on ebay that arrived at his home smashed to bits, no thanks to FedEx. While my MJ arrived at my apartment in one piece and basically worked, however (it still works today, albeit at very low volume due to a poor connection on the right channel volume control as I mentioned above), there is still the question in my mind as to how the control could have broken as it did in the first place. If it was working before Nolan sent it to me, the only thing I can think of that makes any sense to me is what I mentioned above, that the box containing the radio had been manhandled at one or more UPS shipping facilities between Arizona and my home in northeastern Ohio. Hmmm.

[Old1625]

The back pot will be the center shaft, which extends out further than the sleeve of the front pot, making it vulnerable. The thrust is probably on the back of the back pot for the center shaft. A blow (likely in shipping) may have pushed on the rear pot until the tabs that fasten it piggyback to the other section folded out and rendered the rear pot loose. Ditto the tone control. If the rear pot's wiper assembly pushes back far enough then the wiper will fail to make proper contact with the center (wiper) terminal or the carbon track, causing your signal loss trouble. You need to try to push things back into marriage, and bend the retaining tabs back where they belong.

As for the filament wiring bear in mind that the All American Five have filaments that add up to total the line voltage. Your set may require a bit more creative engineering, involving multiple series strings.

Dunno about the transformer thing.....

[Jeffhs]

The volume control pot (a dual unit, one potentiometer attached to the back of the other) in my Zenith MJ1035 is extremely loose on the chassis, making it nearly impossible to attach the knobs to the shaft without pushing the entire control back into the chassis. The control is mounted under the chassis, so I cannot hold it in place (with the chassis in the cabinet) while installing the knobs. How is or was the dual volume pot originally attached to the chassis? Is there any way, short of super-gluing the pot to the chassis (which I may end up doing as a last resort), to mount the control?

Another problem: The volume control is very intermittent. When it does work, I can barely hear the radio (with the control at maximum). The wiring to the control looks as if it is a mess, with the left-channel volume pot actually broken (the carbon track is split roughly in half and has parted company with the rest of the control). The tone control, on which is mounted the push-pull on-off switch, is quite loose as well. I don't know how this happened, unless the damage occurred during shipping (I got this radio from an AK member in Arizona; I live in Ohio, so the set was in transit over some 2,000 miles). The radio was shipped to me via UPS, which leads me to believe the box in which it was packed may have been kicked across a floor, thrown around, or otherwise manhandled before it was put on the truck. The person from whom I got the radio (AK member Nolan Woodbury) tells me the unit was working in stereo before he shipped it; moreover, the set was extremely well packed for shipping (took me the better part of an hour to unpack it when it arrived here).

The radio still basically works, bringing in all Cleveland FM and AM stations very well (in stereo; at least the red stereo indicator lights when the radio is tuned to a station broadcasting in that mode, although I have yet to hear anything from the left-channel extension speaker), except for the low volume problem. I am amazed that the set didn't have a bunch of broken tubes, etc. if it were manhandled as severely as I think it was.

Speaking of broken tubes, I was working on my MJ1035 last night and had to replace one of the tubes when, to my amazement, a pin broke off the tube in question. No kidding; when I removed the tube from the socket, I found a piece of metal in one socket hole. Looked at the tube I had just removed and found a vacant spot where the pin once was. The "piece of metal" I found in the socket was that pin.

I'll be darned. This is the first time in over 40 years of electronics experimenting that I have ever seen a pin actually break off a miniature vacuum tube. I've seen it happen on occasion with octal Bakelite-base tubes, but never, before now, with a miniature glass tube. (I'm actually amazed as well the tube was still under vacuum when the pin came off; at least I think it was, as there was no evidence of air in the glass envelope). Oh well...

How on earth the controls (volume and tone) could have worked loose (the tone control was loose when I got the radio), let alone how the left-channel volume control broke up as it did, is a mystery to me unless, as I mentioned, the radio's shipping box was subjected to gross manhandling before it was ever loaded onto the truck.

It's things like this that will make me think at least twice before I will ever use UPS for shipment of an important and/or fragile package. I read every once in a while in AK's forums of radios, TVs and such that were shipped via UPS only to arrive at their destinations smashed or mutilated, but this is the first time I personally have ever experienced such a thing myself, if in fact the loose tone control/balance-volume controls were the result of rough handling of the package in transit.

What's this world coming to?

[Old1625]

If you managed to go as long as you have without encountering a broken-off pin on a miniature tube then you are one very lucky sort!

And your being so astounded by such betrayal by UPS.... ?....!....

Hey, I can appreciate your frustration, but all the stuff you are observing as wrong with your newly-acquired set can be corrected. I've encountered the problems you are having with your volume controls many a time. This is all fixable; you need to simply analyse the assembly calmly, and you will find out what tabs have unfolded due to stress, and will probably be able to figure out how to reassemble that control assembly so that all will be within tolerance, and work properly; I've made such repairs many a time--trust me. Bear in mind that it was assembled to begin with, so this is not impossible work. I did it many times over my career--repair of concentric controls and other types that had been "kicked in" by the hapless owners. (Mostly it was car radios in the earlier days. If I took a facetious angle, and asked the male owner how he managed to step on his own radio knobs and ruin these assemblies when they are supposedly safe on the dashboard where no feet should go he'd usually give me either a sheepish, or perhaps a "are you from Earth?" look!)

Look, man! This is all fixable. Just shelve the unit for a spell, take a couple of deep breaths, and then look at it later with a renewed attitude.

Gosh dang me if I didn't have to do this very same thing when confronted with such frustration as you are having right now.

Just remember: You can probably fix this--it's likely not that big a problem. If an old doofus like me has done this same repair hundreds of times on radio gear I'm sure a bright sort such as yourself can probably figure it out... I'm not saying that the control assembly is not necessarily FUBAR, but I strongly doubt it.