[Milsurplus] Fwd: Re: BC-348-R Paper Capacitors

[Bruce Gentry]

Slow cooking to drive out moisture doesn't always work. The foil 
windings in the condenser corrode from the moisture, and the products of 
the corrosion impregnate the paper and make it conductive. Also, the 
paper may contain acids  that do the same. Once this happens, drying the 
paper out will not eliminate the leakage completely.

       Bruce Gentry. KA2IVY

On 2/26/16 5:04 PM, AKLDGUY . wrote:
> Should be a lot better than 1 Meg, even by 1940's standards. Paper is
> absorbent. The fact that moisture got in means that it can be driven
> out. What has happened is that old sets don't get turned on, in some
> cases for decades. Retain those old caps but run the set more often.
> The heat in the set will drive out the moisture, slowly but surely.
>
> 73 de Neil ZL1ANM
>
> On 2/27/16, Bruce Gentry <ka2ivy at verizon.net> wrote:
>>
>>
>> -------- Forwarded Message --------
>> Subject: 	Re: [Milsurplus] BC-348-R Paper Capacitors
>> Date: 	Fri, 26 Feb 2016 15:51:33 -0500
>> From: 	Bruce Gentry <ka2ivy at verizon.net>
>> To: 	k2cby <k2cby at optonline.net>
>>
>>
>>
>> The leakage you are seeing is very excessive even by the standards of 70
>> years ago. Paper condensers are not junk, but they don't age well. For a
>> radio that could easily be destroyed within a year or so of manufacture,
>> using less expensive condensors made of more plentiful materials made
>> sense.  AudioPhools actually search for good ones, and they are also
>> made today for specialized applications. There was a brand of condenser
>> called Micamold, it was a paper condenser in a molded plastic case to
>> look like a mica and make the buyer think they were getting a mica at a
>> bargain price. Other manufacturers also made them. I always get rid of
>> them because they may be good now, but can fail without warning.
>>
>>
>>         Bruce Gentry, KA2IVY
>>
>> On 2/26/16 2:56 PM, k2cby wrote:
>>> 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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>>
>>