Silver Mica ‘Disease’ in a Zenith H723Z

[Gerry O’Hara]

So youve been told that silver mica capacitors are the most reliable in vintage radios and that it is not even worth checking them? Well, they may indeed be the most reliable, but they are not infallible! Occasionally they suffer from the so-called silver mica ‘disease’. This can cause difficult to trace symptoms such as loud crashing sound, crackling, distortion and weak signals. This effect is caused by the creeping (by electrostatic means) of the silvering from where it was originally applied on the mica dialectric to where it should not be, eg around the edges of the mica (in a discrete silver mica capacitor) or between separate areas of silvering on the same side of a mica sheet, as in the silver mica sheet/pad capacitors ‘integrated’ into the IF can construction in the Zenith illustrated here. Below is a brief email string between myself and Phil, members of the CVRS, concerning this phenomena, which we thought would be of interest to others. I have included some photos to illustrate the points made. For more detail, check out the following website: (the first post is at the top of the string).
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Gerry,

Another thing we could cover is this business of “silver mica disease”. It affects the tuning caps, I think usually later model AA5’s. I don’t think many folks have even heard of it, even those with a lot of experience under their belt, so might be interesting to some others.

Check out https://www.google.ca/search?source=ig&h … gle+Search

Phil
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Hi Phil,

I have first-hand experience of the silver-mica disease in a 1950’s Zenith (Model H723Z) – see attached – I took these photos with the intent of writing an article myself (thinking this was an undiscovered issue!), then I came across the articles on the web… The cracking in the Zenith was amazing before I replaced the mica sheet caps with discrete ones per the photo. The set now works very well. Still may be worth a write-up or at least a tech tip as you say. Maybe a post on the forum?

Gerry
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That is an excellent example of the problem.

There must be a lot of radios with this problem, but to a lesser degree. I was just thinking, I bet you took the can apart because it was bad enough to become problematic enough to make you look for it, but how would we know if it just wasn’t “up to par” ? the radio might still play ok, but with less than great selectivity and maybe some crackling? Thunder storming? Is that about it for the symptom? – and how would we know which can might have that wafer, hate to pull the can apart if it doesn’t need it.

A list of radios that have this style of caps would be handy to have.

I guess it is no worse than the old ones which could have a paper cap in them, they can be bad too I assume.

Anyway thanks, interesting pictures. I think I had that problem with a Crosley D25, dashboard radio, that would be about 52 or so as well. I swapped the IF can from a junker and was happy that it fixed the problem. I don’t remember if I opened the can. Yes we should be talking about this in the forum, we need the posts.

Phil
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Hi Phil,

Yes, I had never heard of this problem before. I eventually isolated the noise to the first IF transformer with much checking. As you say, there is a reluctance to remove all the wiring from the transformer connections to remove the can from the chassis in order to gain access. When I opened it up, I wondered where the caps were at first! (as I had never really looked at this vintage of domestic radio before – most of my experiences were with 1930’s domestic radios or communications sets which had all used discrete mica caps). I soon figured it out and it then dawned on me what the problem could be – once I saw the mica plate it was obvious. At first I tried just scraping the fine silver lines away and re-assembled/tested the result, but, although an improvement the fault persisted. I ended up removing the transformer for a second time and fitting a plastic washer in place of the mica plate and two discrete capacitors across the primary and secondary coils. The schematic did not have values marked for these caps, so I measured the coil inductance and calculated what the cap should be to resonate the coil inductance at the IF frequency. I think this came out at 120pF, which seemed about right so I fitted these values, checking the resonant frequency with a GDO (one of the photos shows this) – they tuned up like a charm.

GerryDownload DSC00111 [1024×768].JPG. (Caution: This file may not be virus scanned.)
Download DSC00114 [1024×768].JPG. (Caution: This file may not be virus scanned.)
Download DSC00113 [1024×768].JPG. (Caution: This file may not be virus scanned.)

• This topic was modified 1 week, 1 day ago by R. Williams.
• This topic was modified 1 week, 1 day ago by R. Williams.

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