Zenith 5D611 radio

[electra225]

Doc, just for the record, I actually DID own one of those ultra-miserable Zenith inverted bakelite chassis sets. It was a cute little console that I bought off "Tradio" our local radio station buy-sell program. I had it twenty years and never got it working even close to right. Besides the miles of rubber wiring, all of which I eventually TRIED to replace, and the odd tubes it had, it was infested with the typical Zenith obscure faults. It had a 50Z7 or some such weird rectifier. It was a six tube set. The chassis was about a quarter of an inch deep and was nearly impossible to work in. The oscillator didn't want to run dependably, even though all the components checked good and I got nice negative voltage on the oscillator grid. I suspected the oscillator portion of the tuner being shorted somehow, but was never able to clear it. I would work on the chassis until I got aggravated (which didn't take long...) then on to the end of the bench it went. Spent most of twenty years there, unfortunately. I sold the set at our auction for $165. A fitting end to the most miserable device I ever had to mess with, guitar amps and two-cycle engines included.

I robbed the 12SA7 from a working radio, a tube known to be good, which made no difference. I subbed the rectifier and audio output tubes also, with no appreciable difference. The Zenith plays pretty nice with the exception of the upper end of the dial dead, until if fades until only the strongest local stations are heard. Selectivity and sensitivity seem to be fairly good for not being aligned properly. I can even get the Tucson station, so I'm on the right track. I'm beginning to wonder if oscillator alignment being off might be where I need to start next. The dial seems to track fairly accurately, until you get to the upper end of the dial. I should hear the oscillator in another radio at 1395 KHZ, using the 465 KHZ IF. It comes in at around 1460 give or take. I haven't checked the dial accuracy on the lower end of the dial, since the radio plays at the lower end of the dial, perhaps a mistake? There is a trimmer that is supposed to be in the loop antenna that has been cut out and wired around. I'm sure that is used in the alignment, probably antenna strength, and, knowing Zenith, it probably affects one end of the dial. I'm really close to success here. I just need to figure out why the upper end is dead. I have a feeling that will solve the fade issue.

[electra225]

Is it possible to have TWO 50L6's with grid emissions on the same project?

The audio starts to distort after the set has run for about 20 minutes now with the replacement tube. The odds that both the original tube and its replacement both having grid emissions is not very good. What else can give the same symptoms in the output tube stage? My next step is to check the grid resistor to see if there is appreciable voltage across it. I would think if the coupling cap was leaky, the audio would be distorted all the time, not just when the tubes got hot.

I found out how to "repair" the dead upper end of the dial. I ran an external antenna wire directly to the oscillator section of the tuning cap. So the trimmer in the antenna that was bypassed and removed is a wave trimmer of some kind. This lends credence to the theory that oscillator alignment is off. I replaced the 12SQ7 first audio tube with a metal one. This seems to have cured the fading. So now, I need to figure out the distortion. I'll go upstairs and dig out another 50L6 and see what that does. I have checked voltages, but I'll do it again to be sure I haven't missed anything. I see I'm talking to myself today, but posting this will refresh my memory if I need it.

[electra225]

More progress!!

It was found that I did, indeed, have two 50L6 tubes with grid emissions. I finally twiddled enough that I was able to get a good, strong signal from the signal generator all the way to the speaker! Yea! It was also found that the upper end of the dial was dead/very weak due to the antenna trimmer being so far out of adjustments. People messing with stuff when they have no idea what they are doing.....

I found a VM-branded, RCA sourced, 22nd week of 1963 coded 50L6GT tube that fixed the mumbly audio. The two bad tubes will be confirmed in another radio when I get a minute to do that. I still have the metal 12SQ7 in the radio. I will put the original glass tube back in and verify that it is or is not a cause for weak audio. I currently have a fairly good radio thru the plate of the 12SQ7. The audio gets weaker rather than stronger in the output stage. I will troubleshoot that stage to narrow down the problem. The upper end still seems weaker than the lower end of the dial, but I need to get the audio right as a next step. And the audio tends to get a little squealy when the volume control is all the way advanced. That is typical of Zenith for me. The volume control on this set is not above suspicion, but that is "down the road" a bit. I'll get into the audio output section and see what's going on there.

[William]

Progress is always a good thing, keep up the good work.

Bill

[electra225]

Bill, remind me and I'll pull the VM 50L6 in this radio and get it to you for your VM collection....

I'm done with this project for the time being. It plays very decently for how it started out. Still a tad weaker on the upper end of the dial than I'd like. It also is right on the verge of oscillation when the volume is advanced on stronger stations. The trimmers in the IF cans should really be taken apart, cleaned and the mica insulation strips should be replaced. Same for the trimmers on the tuning capacitor. And....there is still something I am overlooking. This thing is right on the ragged edge of oscillation, and, even though it's a Zenith, I don't think it should be like that. Lead dress, a mica cap somewhere, something. It may never get better than it is now. I'm not going to play it much. Every wire and every component that has not been replaced probably should be to make this right again. It had several problems and had been tinkered with my somebody who didn't know what they were doing. And it's a Zenith, so it had three strikes against it when I started. I can finish repairing the cabinet then get it to Don to do his thing. I haven't decided on colors yet. Either red and white, or Garnet Red and Raven Black in honor of Art.

[TC Chris]

I found out how to "repair" the dead upper end of the dial. I ran an external antenna wire directly to the oscillator section of the tuning cap. So the trimmer in the antenna that was bypassed and removed is a wave trimmer of some kind.

Your friends at AES have a schematic online. It was not clear which trimmer you refer to. But I note that there is a loop for an external antenna/ground, and the schematic shows a jumper in place. Is the jumper a necessity for adequate performance of the loop antenna, which is part of the RF tuning circuit?

Chris Campbell

[electra225]

Hmmmm. What jumper? There is no jumper.....

That might explain why the antenna seems better at one end of the dial than the other. Another reason I may have a weaker upper end on the dial is that the antenna trimmer, "G" on the schematic, seems to be partially shorted. You can get it to almost where it needs to be then the radio just quits. Instead of pulling the trimmer apart and cleaning/fixing it (the proper method), I may experiment with adding a small value mica cap in series with the trimmer to get a bit more adjustment, if for only temporary. I overlooked the jumper, apparently. I'll look at the drawing again. Thanks, Chris....

[electra225]

Chris you are right about there being a jumper between the terminals of the external antenna. I sure missed it. Typically, I would expect the jumper to be drawn like an "S" so you could tell it was a jumper. I don't see a jumper on the external antenna terminals on the radio. That might explain why the high end of the dial seems weaker than the other end.

[electra225]

I used a jumper on the antenna terminals like is shown in the diagram with no appreciable change in performance. Perhaps a greater tendency to oscillation on strong stations with the volume control advanced. I really don't believe I have the alignment 100% yet. I may hook up the scope to it and see what I can see. The antenna trimmer on the tuning cap should really be taken apart, cleaned and then have the mica strips replaced so I can get more adjustment out of it. It goes so far, to "almost right" then it shorts out and kills the signal, thus the audio. There still may be something else I'm overlooking. I am 99% "there", to having a decently-performing radio. In any case, I can get the cabinet to Don so he can do this thing. I'm leaning toward Cascade Blue and Peach, like grandpa's '56 Century. Or Cherokee Red and Snowshoe White. I want the louvers on the front of the cabinet pinstriped. Then do the "Zenith" sign in gold with white letters, black with silver letters, something gawdy and showy. Paint the body of the cabinet in the darker color, with some shade of light color on the front. I have five bakelite radio cabinets that I'd like to hotrod. At least now they all work. I have a Zenith, a Bendix, a Trav-ler, and two Packard-Bells.

[TC Chris]

Maybe the trimmer cap is working to spec, but another component is out of spec and you're trying to pull the circuit too far. Like maybe the antenna coil? But wait, I just paused to consult the schematic, and the antenna loop is the antenna coil. Any damage on that loop? Or bent plates on C-1? Does it have those slits that allowed outer plate bending?

Chris Campbell

[electra225]

I tried a trick that my stepdad taught me. I ran the radio covered by a cardboard box. This allowed it to get really hot, to see if components would fail or change value when they got really hot. And, brother, did it ever get hot! When it ran probably six hours with zero ventilation, the top end of the dial got really weak. The lower end, under 900 khz was still pretty good, with good volume and sensitivity. The tuning does not seem to be overly broad. I don't know exactly what this experiment has revealed, but I think it is safe to say that I'm not done yet. Something, somewhere in this radio is not up to snuff. I'm guessing, based on performance, that it is in the front end. The antenna checks good and an external antenna doesn't help, and, in fact, may not be as good as the loop antenna. Have you ever taken a trimmer apart to clean and "fix" it? I never have. My gut tells me I need to make sure the antenna trimmer on the tuning cap is better than it is now. I have changed all the caps and have checked all the resistors. I have looked under a magnifying glass for broken/shorted/misrouted wires, none found. All the tubes except the 12K7, which is new, tests good and I don't have a spare for, have been subbed from a known good working set with no improvement. The tubes in this radio work well in the known good radio. This is an AA5, not a Concert Grand. I'm running out of things to check and ideas of what to check. I'll see if the upper end of the dial is stronger in the morning after it has cooled all night, or if I really broke something getting it so hot.