[R-390] A question about silver-mica capacitors failures.

[David Olean]

Hello Jacques

It is hard for me to say, but I have looked at a few failures and the 
capacitors are fine at very low voltages, but die as the voltage is 
increased.  I have one SM capacitor that is fine at 8 volts, but starts 
leaking when the potential approaches 12 volts p to p.  I would think 
that elevated voltages would aggravate the situation. but I am seeing a 
small but equal number of failures from the R-392 with 28 volts DC on 
the plates vs. the R-390 or R-390A with about 200 volts.  The usage of 
the two receivers can be quite different as well.  R-392s were strictly 
tactical radios and were used infrequently, whereas R-390A s had both 
tactical and fixed station type environments where DC voltage was 
applied for months at a time.  In my mind, the jury is still out!  It 
would be nice to test a bunch of NOS postage stamp caps from the 50's.

Dave K1WHS

On 3/29/2020 7:28 PM, Jacques Fortin wrote:
> It was flagged by many contributors to this site that the number of silver
> mica capacitors failures is rising.
>
> As the failure mechanism seems to be understood (silver migration within the
> capacitor package), it is not clear for me if this occurs only when a
> potential difference is applied to these capacitors, or if it can be
> triggered only by the time, even if the radio containing those capacitors
> was left unpowered for years.
>
> I will appreciate to know your thoughts about it.
>
>   
>
> 73, Jacques, VE2JFE
>
>   
>
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