[Milsurplus] BC-348-R Paper Capacitors

[hwhall at compuserve.com]

According to data in my Sprague TO-5's manual, new paper caps of 0.1uF or less ought to have an insulation resistance of at least 5000 megohms (or so it was back when paper & wax caps were part of the state of the art).  

The rule of thumb used was that paper caps should have a minimum insulation resistance 
R  = 1000 megohms divided by C in microFarads.  With the exception that no paper cap be required to have more than 5000 megohms insulation resistance.


 Wayne
WB4OGM

 

-----Original Message-----
From: k2cby <k2cby at optonline.net>
To: milsurplus <milsurplus at mailman.qth.net>
Sent: Fri, Feb 26, 2016 12:56 pm
Subject: [Milsurplus] BC-348-R Paper Capacitors



The BC-348-R uses two varieties of .01 uF postage stamp capacitors. 
Part numbers beginning 12- are micas and seem to be in good shape.
Part numbers beginning 11- are rectangular black paper capacitors are .01 uF 10% capacitors rated at 500 volts and used in non-critical plate and bypass applications. I discovered that one of these had shorted and taken out the dropping resistor associated with it. This prompted me to pull out one of the other paper caps.
My Heathkit capacitor checker showed “bad” with any voltage above 50 or so applied. In the bridge mode I could not get a null, but my digital capacitor meter showed .01 uF – right on the money.
I then decided to see “just how bad” and connected the cap to a variable dc supply. With 250 volts applied (the BC-348 uses just a bit over 220 volts B+) the cap drew 220 uA, corresponding to a resistance of about 1.16 Meg.
I checked a couple of other “11-series” capacitors with the same result.
In today’s world a capacitor with these specs wouldn’t pass muster. Before I yank out and replace all of them, I wonder whether these were passable values for a bypass capacitor in the 1940s with “a war on.” 
Any thoughts?
 
Miles B. Anderson, K2CBY
16 Round Pond Ln.
Sag Harbor, NY 11963
k2cby at optonline.net
 
 

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