[Boatanchors] [DrakeRadio] Questions about silver-mica capacitors

[Richard Knoppow]

     Someone on one of the lists I am on tested a bunch of very old 
silver mica caps and found them all good.  Might mean nothing since it 
was uncontrolled.
     Beside "Whiskers" there is a problem of oxidation of the silver at 
the edges of the plates.  I think this is due to incomplete sealing of 
the Bakelite cases but might be from something else.  Dipped micas do 
not seem to have this problem but there my be more different than just 
the case.
    I have found several silver mica caps that either caused warbling or 
just jumped at some temperature.  The rapid change in capacitance is 
called "scintillation" but I can't cite any specific articles or papers 
calling it that. There is a tendency to assume silver mica caps are 
always extremely stable and overlook them when trouble shooting.  BTW, 
not all mica caps are of the silver plated kind.  For instance the 
working standard caps made by General Radio are of the stacked type, 
with alternating layers of mica dielectric and foil electrodes.  I think 
many high voltage mica caps are of the same general construction.

On 9/9/2015 4:14 AM, Jim Wiley jwiley at gci.net [DrakeRadio] wrote:
> A question for the various group(s) :
>
>
> We have all heard reports of silver-mica capacitors failing due to
> "whiskers" forming on the metallic surfaces and subsequently causing
> shorts.  Does anyone know if this problem is dependent on whether or not
> the capacitor is in use or not?
>
> In other words, does the problem occur even it the part is simply stored
> in a box somewhere, or does it require the application of operating
> voltages (either DC or RF) for the problem to manifest itself?   Or it
> it a combination effect, with the rate of failure being faster (or
> slower) when in use, but the problem will still happen at a faster (or
> slower) rate than when the capacitor is simply sitting in dry storage?
>
> Next, does this problem occur in all types of packages or just some,
> such as the common "epoxy dipped" vs. "postage stamp" (or other type)
> configurations?
>
> Finally, is there a way to reverse the effect?
>
> Inquiring minds want to know.
>
> - Jim, KL7CC
>
>
>
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL