[Boatanchors] Navy RAL Filament Voltage: Solved

[Rodger Singley]

I am still pretty trusting of most mica caps since the failure rate is pretty low.  When I restored my Pierson KP-81 I replaced all of the mica caps in the front end because that module is so painful to access (makes working on a SX-28 front end look like a Kindergarten project).  About 6 months after restoration the receiver failed and it was due to a shorted mica cap in the IF section but that was one of the caps I replaced with current production dipped mica caps.  It was a 500 volt part that never saw over 10 volts but it failed dead short.  I am glad it wasn’t one of the caps I put in the front end.



A lot of the “mica look alike” caps that are of a high value are actually the old wax paper types in a flat pack that look just like the mica construction from the outside.  Most failed micas do so because of silver migration which can occur when there is a high DC differential across the capacitor but they normally don’t develop the high resistance leakage common to older foil/paper types sealed in wax.  Older Tektronix tube type scopes with the open silver potted ceramic terminal strips suffer the same issue when there is heavy dust or pollution buildup on the strip causing fingers of silver to migrate across the barriers when there is a large differential across them.



The Soviets produced a neat little run time meter that uses mercury migration between two electrodes to display the amount of run time for a device.  Perhaps some future vintage restoration person will come up with a calculation of the number of hours a radio has run based upon how much silver migration has occurred 😊



Rodger WQ9E



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________________________________
From: boatanchors-bounces at mailman.qth.net <boatanchors-bounces at mailman.qth.net> on behalf of Rob Atkinson <ranchorobbo at gmail.com>
Sent: Monday, March 19, 2018 4:54:50 AM
To: Joe Connor
Cc: BoatAnchors QTH
Subject: Re: [Boatanchors] Navy RAL Filament Voltage: Solved

I shotgun out those micas among other parts when I'm working on a rig.
Not worth the repeated trips to the bench every time one goes, and
they will sooner or later.  At least half leak off the bat.

73

Rob
K5UJ

On Sun, Mar 18, 2018 at 8:32 PM, Joe Connor via Boatanchors
<boatanchors at mailman.qth.net> wrote:
> Thank you to all who responded to my request for help yesterday. I just found the problem that caused my heater voltage to hover around 4.8VAC, not 6.3VAC.
> There are four RF bypass caps on the heater circuit for the detector tube. They're .01 caps listed as molded micas. On mine, one was shorted. Once I disconnected it, I had the proper heater voltage. Needless to say, I'll replace all four of these caps.
> Again, thanks for all the advice.
> Joe Connor
>
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