[R-390] R-390A IF deck caps in T501 or T502

[David Wise]

Yes, drift in a mica cap is usually a precursor to more drift or other failure modes like loss and leakage.

These molded mica caps might or might not be silvered, so it's not quite correct to call them "silver mica".  Newer caps in dipped packaging are always silvered.

Molded mica caps are usually a book-sandwich of mica sheets and foil sheets, compressed by the molding operation.  I don't remember what metal the foil is, but it doesn't solder; instead, metal contacts are crimped onto the ends of the foil sheets.  As the years pass and water etc gradually makes its way inside past the lead wires, the electrical connections between the foil, the end piece, and the lead wire corrode.  Eventually, one or more foils become imperfectly connected, which lowers the capacitance and raises the loss (dissipation factor).  Often it is sensitive to temperature change or mechanical stress.

I don't know how dipped silver-mica electrodes are connected to the outside world, but if they are crimped, they will eventually fail the same way, and the only reason they are regarded as more reliable today is that they are less far along on their wear-out curve.

Not an expert either, but I've read more,
Dave Wise
________________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Larry H <larry41gm at gmail.com>
Sent: Tuesday, March 19, 2019 1:51 AM
To: R-390 Forum
Subject: Re: [R-390] R-390A IF deck caps in T501 or T502

Well, after numerous interruptions, I finally finished changing the 200 pf
caps in IF xformers T501 to T503.  Yes, all 3 have the same cap
configuration, a 200 pf silver mica and a 39 pf across each coil.  It was
easy to change all the caps with out removing the IF xformers.  The
resonant point on each coil is still quite stable, even during warm-up.  As
far as I can tell, only one was bad and 2 had drifted slightly.

I'm not an expert on SM caps, so does it mean that the 2 caps that drifted
were on their way to being bad?

Regards, Larry