[R-390] Postage stamp mica caps and AGG problems

[David Wise]

Thanks for posting your findings, Tisha.

Can you measure a few abnormal micas on a 1kHz impedance bridge?  Is DF normal?
(Most of these caps are so small that their ESR may not perturb DF much until you go RF.)

These days the only paper caps I expect to pass are brand-new XY line bypass, and hermetic-sealed.
Everything else is shoot on sight, unless (a) I have a curatorial wish to preserve the instrument's
history and (b) the cap is used where leakage is permitted and failure won't cause collateral damage.
That's paper only; plastic film is trusted except for a few corner cases.

I'm losing patience with old electrolytics, except for the premium ones.  Parts I left in because they
measured okay last year are failing now.  Life's too short to mess with them twice.

Dave Wise
________________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Tisha Hayes <tisha.hayes at gmail.com>
Sent: Tuesday, February 18, 2020 4:28 AM
To: R390A
Subject: Re: [R-390] Postage stamp mica caps and AGG problems

I have been seeing about a 25% capacitor-fail rate on NOS paper/wax when
tested with a Secore LC75. Most of them will easily pass testing with a
DVM-type tester that only puts a few (tens) of volts across the cap and
doesn't do anything to measure ESR. While mica caps fail at a lower rate
that has been observed as well.

Micas are weird; near rated voltage tests don't always show the problem,
capacitance values look somewhat good, ESR.. didn't show anything; but in
an RF test at a few hundred KHz they are no longer resonant when compared
to control samples.

I try to test at least at 75% of rated voltage (up to 600 volts on 1KV
caps) and failures sometimes happen with spectacular, fireworks type
results. A few times the cap would wander around in value, up and down,
seemingly in a random way as internal areas of the cap are changing
characteristics.

I don't even mess with reforming any more; other than for curiosity's sake;
I might be able to get a cap to not be as leaky but the recovery is
temporary at best and the next time the cap goes on an excursion it becomes
a hard failure that takes out resistors, or with electrolytics in the power
supply, a choke or transformer.

*Ms. Tisha Hayes, AA4HA*

*Sr. Engineer, 4RF USA.*
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