[Collins] dipped mica caps do fail

[Al Parker]

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

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