Zenith chassis 7Y02 unknown model AM-FM radio

[electra225]

This radio was given to me by a friend and is in really nice condition. It has traditionally been a decent performer. This particular set has the optional tone control mounted on the side of the cabinet. Its turn came up in the rotation, and it ran about three hours when the audio became distorted, the radio personalities sounded like they had mush in their mouths. Ah, ha! says I. It has a gassy 35C5, no question. The chassis must be removed to replace the audio output tube, due to its location. Now would I have a 35C5 laying around in easy reach? Nope. I had a 50C5, tried it with no difference in performance. I went upstairs, dug thru my disorganized tube stash, found three 35EH5's and two 35C5's, so I brought them down where I could test and evaluate them. I tried all five with no difference in performance. The distorted audio starts with the radio cold, so that eliminates the "gassy tube" theory. I don't have a schematic, so that is my next step. I did notice that this set is running its original selenium rectifier. I can't really say reception is weak, but it's not the strongest, either. I think my first step will be to measure B+ at the rectifier and see if it seems low. This doesn't sound like a serious issue, but let's remember my luck working on Zenith radios. I like Zenith TV, but I detest working on Zenith radios. "Obscure problems go in before the name goes on" is my philosophy on Zenith. I don't have enough information yet to blame SMD, which is a Zenith trademark, in my experience. I need to find a schematic, measure B+ before I get too concerned......

[TC Chris]

Doesn't SMD typically cause intermittent "crashing" noise? And is the problem behavior the same on AM and FM?

Chris Campbell

[electra225]

It's the same on AM and on FM, worse at higher volume, not too bad at low volume. I subbed the 19T8 first audio tube as well, no change. One of the 12BA6 original Zenith branded, GE supplied IF tubes has a really scratchy and intermittent tube socket. I don't remember ever doing anything to this radio since I have had it, probably close to 30 years, I reckon. The 35C5 that was in it was branded Truetone, RCA sourced. I think it's still okay. I saw a Sylvania 6BJ6 in it, so it has had work sometime in its past.

[William]

Impressive radio having 7 tubes. It should be a good performer once you get it sorted.

Bill

[electra225]

It has always been a good radio. It sets on the top shelf, so it hasn't been played probably since I set it up there three years ago. (Have I had the shop THREE years already?) I have been dusting and cleaning, so I have been playing all the top shelf radios. The chief suspect right now is that selenium rectifier.

[electra225]

The model number on this set is a letter, followed by "723Z". Maybe a "Y" then the "723Z", but don't hold me to that.

This radio has, unfortunately, been the victim of hack repairs, "happy hands at home" to quote Art Wright. This set has the optional tone control, which is rather crudely installed in a hole drilled into the bakelite cabinet. Some of these sets had them, some didn't. Someone has replaced the audio output transformer. The one there is smaller than the original, since they only used one screw. The bracket is not as wide as the original was. I used the signal tracer on this set and found clean audio clear thru the plate of the output tube, but distorted at the speaker. I had the scope warmed up, so I used it and found essentially the same thing. It is kinda cool to watch a radio run on a scope. I believe, at this point, that the combination of the replacement output transformer and the tone control is not compatible. I need to find a better output transformer. BTW, the B+ is 126 VDC, certainly adequate. This clears the selenium rectifier of suspicion. This set uses ceramic caps, so there are no paper caps to replace. With some tinkering and cleaning tube sockets and controls, performance is generally better. A better, more well-matched output transformer should improve matters as well.

[electra225]

A forum member was kind enough to send the schematic for this set. This will give me a bit more information on details on this chassis. I'm going to pull the chassis back out and go over it well. This thing is a Zenith, but I suspect the issue may be minor. I have another output transformer I'd like to try.

[electra225]

I've taken some time reading the schematic for this set, trying to familiarize myself with the circuits. One thing I noticed is that this chassis uses a 6BJ6 FM RF amplifier. This is correct, per the sticker pasted on the bottom of the cabinet. The tube in the chassis is a Sylvania. The schematic gives a 12BA6 tube for that position. I don't see any notations on the schematic that would indicate the possibility of an alternate tube in this position. I'm not sure what, if anything, that changes. I think I'll experiment a bit. I'll compare the specs on a 12BA6 to those of a 6BJ6. Maybe a 12BA6 would work better in that spot.

[Dr. Radio]

Here's a helpful hint for those out there in InternetLand. The old saying "leave those disc caps alone" does NOT apply to the Zenith in-house made units. Get rid of 'em. These are the disc caps that are "waxy" looking and have vertically repeated numbers printed on them like "01 01 01".

You see them in the "garbage" pile with the other discarded caps in this article:

https://www.nutsvolts.com/?/magazine/ar ... 014_koebel

[electra225]

It's good to see you on the forum, Doc! Please don't be a stranger......

Thanks for the tip about Zenith disc caps. What does one replace those with? Another disc, or can you use the little yellow caps everybody uses. As you already know, I am not a fan of Zenith radios. This one definitely needs some love to make it right again.

[Dr. Radio]

Yeah, Greg, finally cleared the cache to login....but that doesn't help with the ever-growing project list around the house before winter....that's why I'm not on here much.

I don't see why you can't just replace those with ceramic disc caps. The only thing you'd want to watch out for is (and I doubt Zenith actually did it) was if the cap was in a tuned circuit, you'd want something very stable like mica instead of a ceramic disc cap. You'd want to figure the voltage rating too, seems like the schematics I've seen, lousy Zenith did not always list the voltage ratings for their in-house stuff. Here's a related post:

https://antiqueradios.com/forums/viewtopic.php?t=309923