The dreaded "Silver Mica Disease"
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Hello everyone,
Well, I think I have it. I'm working on a Zenith 7H822 AM/FM radio. I've done the tubes, caps and resistors. When I first turn it on I get a crackling that sounds like lightning on AM only. After a few minutes it goes away. Does it sound like I have it?
If so, how do I determine which IF can has the bad one? I saw a thread on this phorum a while ago that shows how to replace them, but I guess I could just replace the can (I have some spares). I just need a way to locate the bad one.
Thanks.
http://www.nostalgiaair.org/Resources/309/M0025309.htm
Eric
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Sounds like you have it Eric. To test I believe you can put a voltmeter on the secondary of the transformer. If you get a 0 or low negative reading you are OK. If you get a positive voltage reading you've found your culprit.
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I have a Zenith AM/FM radio that has the same problem if that is what it is maybe I will fix mine to.
(This post was last modified: 01-15-2014, 10:37 AM by vinzer.)
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Thanks, Jontz. I'll check it out. I'll let you know how it turns out.
Eric
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Vinzer, I love your updated location! As a Northerner who heads South of Indy semi-frequently I won't miss driving through town on the old "bypass".
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Eric
This disease is only cured by shots. In the backside. Once a day, early in the morning. Until you stop hearing scratching sound.
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Thanks Morzh, I think I can learn to live with it. Kind of like a bad back. 
Eric
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If you have one bad can, good idea to replace the caps in both, since if it isn't already, the other one will go soon.
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Thanks Brenda, I thought about that.
Eric
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It could also be a dirty tube socket, especially in a set using loctals or miniature seven pin tubes.
Regards
Arran
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I found the problem!  First of all I have to tell you that I'm working on more than one radio right now and the problem was with an Airline AM/FM Model# 15BR-1535B not a Zenith. It was pretty simple really, once I found it. There was one of those black capacitors, a .02mf, with 5 color bands on it that connects across the a/c line. I thought that it was the kind that doesn't have to be replaced...WRONG!
The more I do, the more I learn.
Thanks everyone for your help. Maybe what I learned can help someone.
Eric
The Villages, FL
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Glad you found it Eric! It presented almost exactly like silver mica, but just goes to show how hard it is to tell what's wrong when you are working on a radio, let alone just guessing from the internet. Congrats!
(This post was last modified: 01-16-2014, 06:35 AM by jontz.)
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Eric
Let me get it...you DID NOT replace all capacitors? Big mistake if this is the case.
Another thing is, of all things this very capacitor HAS to be taken out and replaced with X-grade capacitor (line filter).
And, if it is before the power switch on the Mains' side (as is often the case) - place a 200K resistor across.
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Wow Morzh, ease up!  As I stated, I thought this was the type that "didn't go bad". Like the mica caps. Anyway, like I said, "The more I do, the more I learn".
I believe in learning by my mistakes.
Eric
The Villages, FL
Member: Philco Phorum, ARF, ARCI & Radiomuseum.org
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