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Old 10-31-2005, 12:58 AM
Jonathan Jonathan is offline
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Zenith 7H822Z troubles

I received a Zenith 7H822Z (the "Z" indicates tone control) from ebay a while ago. It was recapped with molded black sprague caps instead of the paper zenith ones, but I replaced all of the caps. This set is an AC/DC set, and used a 22 ohm 3W ballast resistor to limit current flowing through it's stinky selenium rectifier. I replaced the recifier with a 1N4007 and the ballast resistor with a 100 ohm resistor which I had on hand. This may be part of the problem. Anyway, I replaced all of the black sprague and remaining zenith paper caps, as well as the electrolytics. I replaced the "micamold" 100pF line antenna coupling cap with a brand new silver mica. Since this was an AC/DC set, one of the tube filaments was burned out, which was it's 12AU6, so I put a new one in.

So after all this, I powered it up and all I get is buzzing and some low level static, but it's constant static. I'm almost sure it's a wiring issue, or could be the ballast resistor, or both, or some bad tubes. The power supply filter resistor is a 1k, right now 3W was replaced for no reason, but I'm going to get a proper 5W resistor the Sams says it requires. A picture of the chassis underside is here. More tinkering is required, but I was wondering if anything sticks out that I missed. I hear that this radio has excellent FM fidelity, and I'm quite eager to hear it.

Thanks again.

Jonathan

P.S. If anyone has the official Zenith servicing info on this set or the 7H822, please let me know as the Sams has some errata and that might have lead to a miswiring. I'll pay you to photocopy it and mail it to me. Thanks.

Last edited by Jonathan; 10-31-2005 at 01:18 AM.
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Old 10-31-2005, 01:53 AM
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wa2ise wa2ise is offline
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I had similar radios that had "silver mica migration" desease in the IF transformers. But that usually sounds like loud thunderstors. Do you get noise when switching the band switch? I'll have to dig around, but I might have a diagram in a Beitmans book.
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Old 10-31-2005, 10:29 AM
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Tom Bavis Tom Bavis is offline
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Sams #55 - I doubt I have a copy. Schematic is online here; http://techpreservation.dyndns.org/s...ics/Zenith.htm (djVu format - get the [plug-in from lizardtech.com)

Measure voltages - leakage in caps (and BETWEEN caps) inside IF cans can cause various problems.
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Old 10-31-2005, 10:50 PM
3Guncolor 3Guncolor is offline
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It is possible to over haul the IF transformers on that Zenith but and it a BIG BUT its not an easy job.
I have three of those and all off them had IF problems. I don't know if it's just me but i'm finding more and more IF cans in trouble.
Good Luck
Steve
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Old 10-31-2005, 11:11 PM
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Chad Hauris Chad Hauris is offline
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I have done the IF can rebuild and removed the mica sheets and trimmed the contacts inside the cans and added black tape to prevent shorts. I then added a 22 pf cap on the bottom of the chassis across the terminals to the FM IF's and 4, 22 pfs giving 88 pf on the AM if's (100 may be the ideal value for AM) and it tuned up just fine and gave good peformance.

To troubleshoot the suspect set I would put it on an isolation transformer (you can make one from 2, 110-12 volt transformers, connect 12 volt windings together, one 110 volt winding is input, other output).
You've got to use an iso trans to avoid hot chassis shock hazard.

Then take portable radio or cassette player headphone output, ground to isolated chassis, hot lead through .047/600 v cap. This is the test signal source, apply to volume control, output tube grid etc and see if you're getting signal flow.

Also check out all power supply voltages.
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Last edited by Chad Hauris; 10-31-2005 at 11:22 PM.
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Old 11-01-2005, 12:10 AM
Jonathan Jonathan is offline
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Thanks guys! I knew the dreaded silver mica disease might be a possibility. Problem is that I have no RF generators to do alignment, so I'll have to do it blind. Plus, I've only done it once with an RF generator, and a couple times blindly. Atleast I have a scope. When I switch the bandswitch I hear a pop. This'll just require some more parts and some more time alone with this one. What stumps me is the 60Hz buzzing after I replaced the electrolytic multi-can with fresh new ones. It MUST be wired wrong.

Thanks again guys.

Jonathan
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Old 11-01-2005, 06:28 AM
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Chad Hauris Chad Hauris is offline
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You can do alignment by ear. Just take the weakest station the set will pick up and follow the order of the adjustments in the Sams. Once that station comes in strong, then tune in another weak station and repeat adjustments. It's important to work with a weak station so you can hear the difference you make as you adjust. You can use this method for AM and FM. On FM as a final step adjust the discriminator or ratio detector stage for best sound quality. You shouldn't have to adjust it too much.

I have had to use an RF generator though when a set was so messed up that I couldn't pick up the local station at say 550 khz and had to generate a very strong test signal so I could start adjusting.
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Last edited by Chad Hauris; 11-01-2005 at 06:30 AM.
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Old 11-27-2005, 02:12 AM
Jonathan Jonathan is offline
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Thanks again guys. I finally got the damn thing playing. I had the wrong power supply filter restistor (in place of an inductor), 3W instead of 5W, and it just got too damn hot. I also had a higher value ballast resistor, 100 ohm instead of 22 ohm, thinking it'll work fine. Infact, both resistors did the job, but I replaced both with the correct parts. The 1N4007 diode works great inplace of the stinky selenium. The B+ was within tolerance much more than before, but still I gor the same 60Hz buzzing. I disconnected the electrolytics just to test if they were actually doing anything. They were! As the tubes heated up, the buzzing got so loud I thought it was going to ruin something and quickly cut power. I then maticulously checked over every capacitor, resistor, etc for any wiring error and found none. So, testing some grid voltages I noticed I finally got something on AM. It was a faint station. My meter lead was acting as an antenna! So I connected the original antenna and I got stations on AM! woohoo!

The audio quality was horrible, but I feel it really needs an alignment and a recapping of the IF cans. On FM, I still got a 60Hz buzzing. I tuned for a bit and once an fm station came in, the buzzing was gone. I get very little stations on FM, but the FM antenna is connected to the power line via a 100pF mica, and the FM signals don't pass well through my isolation transformer. Even so, when I get no station, i get the 60Hz buzzing. This to me indicates I need an alignment, do you think it's true?

The one thing that I don't like is the quality of the speaker. It sounds like crap. I don't know if it's the IF cans or both, but it sounds terrible. Can any of you guys suggest a direct drop in replacement or will I have to make my own mounting bracket?

Thanks again.

Jonathan
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Old 11-27-2005, 02:00 PM
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Chad Hauris Chad Hauris is offline
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You usually don't need an alignment on a set this "new". I have only had to align very old sets like from the 30's and 40's, and you will need to align if IF can caps are replaced.

In your case though there is likely a circuit defect...usually when there is the IF can problem you will get NO stations at all or you will get only AM but with a lot of crackling noise. Since you are picking up stations it leads me toward thinking of a bad tube problem...Often times the 12AT7 or 12DT8 FM RF tube will go bad, also make sure your IF tubes are OK.

If you still have buzz double check your tubes, you may have heater to cathode leakage in a bad weak tube...
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Old 11-29-2005, 02:19 AM
Jonathan Jonathan is offline
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Thanks Chad,

I think you might just be right. It doesn't really crackle at all, and AC is getting in from somewhere, so it has to be a weak tube. The 35B5 I replaced with a known good one just for kicks, and the 19T8 seems fine, and I just replaced the 12AU6 since it had a burnt out filament when I got it. Either the IF tubes or the RF tubes. I'm going to test for shorts, AC has to be leaking in from somewhere.

Thanks.

Jonathan
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Old 12-01-2005, 12:00 AM
Jonathan Jonathan is offline
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Well, to my suprise, no heater to cathode shorts. I tested all pins and no shorts. My only guess could be weak tubes. I have no tube tester, so can it be some by measuring voltages on the pins? I don't have complete service data, but the Sams tells me some voltages. Other than the bad IFs or bad tubes, I have no idea what else could be causing this. Anyone have any suggestions?

Thanks.

Jonathan
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