[Collins] dipped mica caps do fail

[Carl]

Ive traced many dipped silver mica cap problems to circuits that have a 
voltage applied that causes the silver to migrate and eventually cause a 
short or intermittent internal arcing.

It doesnt have to be much voltage, the ones in a DX-100 VFO are readily 
prone to problems and well out of tolerance.

Ive had dipped SM failures in Collins, Hammy and Halli, National, and 
several other brands of RX and TX.

IF tube grid emission is fairly common in 7 pin miniature tubes running at a 
high B+ and screen voltages and resultant current.

Carl
KM1H

----- Original Message ----- 
From: <Jbrannig at verizon.net>
To: <anchor at ec.rr.com>; <collins at mailman.qth.net>; 
<BoatAnchors at theporch.com>
Sent: Saturday, June 21, 2014 5:41 PM
Subject: Re: [Collins] dipped mica caps do fail


>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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