[Collins] dipped mica caps do fail

[Jbrannig at verizon.net]

I was rebuilding a Ranger and had a number of problems with the VFO....all 
traced to bad Mica capacitors

Jim

-----Original Message----- 
From: Al Parker
Sent: Saturday, June 21, 2014 4:23 PM
To: collins at mailman.qth.net ; BoatAnchors at theporch.com
Subject: [Collins] dipped mica caps do fail

Hi folks,
We don't expect dipped micas to fail, but some of us have seen it
happen occasionally.  Here's another example:
In the 75S-1 which I've posted about recently, the S-meter has pegged
downscale on warmup, coming back to zero pretty consistently after a few
minutes.  After hunting a bit, checking both IF tubes pin voltages at
turn-on and after a few minutes, it appeared that the 2nd IF had a
slightly positive voltage on the grid, and the tube was drawing too much
current.  A good pointer at the grid coupling cap from the previous
stage.  That cap was 100 pf brown dipped mica, like we regularly use to
replace the red/pink rectangular micas, etc.
I put the bad cap on my bench megger, it started showing leakage as
soon as I brought up the voltage, and showed about 20 megs at 400vdc.
But it was erratic, and settled down (up) to higher values fairly
quickly with steady voltage of 4-500.  I think they're rated at 400vdc.
it also measures about 55pf vs 100.
73,

Al, W8UT
www.boatanchors.org
www.hammarlund.info

"There is nothing -- absolutely nothing -- half so much
worth doing as simply messing about in boats"
Ratty, to Mole
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