[Test-Equipment] 2uF 5% Capacitors?

[Richard Knoppow]

----- Original Message ----- 
From: "Brian Goldsmith" <brian.goldsmith at echo1.com.au>
To: "'Discussion of Electronic Test Equipment'"
<test-equipment at mailman.qth.net>
Sent: Friday, June 11, 2010 1:54 PM
Subject: Re: [Test-Equipment] 2uF 5% Capacitors?


>
>
> -----Original Message-----
> From: J. Forster
>
>
> Film capacitors, except "Black Beauties" rarely fail.
>
> -John
>
>
>
>
> ****Black beauties do not use film as the dielectric but
> paper,this is why
> they fail.
>
> Brian G.
>

     I've dissected a few. They are sort of film caps. The 
dielectric is a plastic impregnated paper. The original BB's 
were also oil filled. The failure mode is at least partly 
due to the method of filling and sealing. The oil was 
introduced through an open tube at one end which also serves 
as the lead. The tube is sealed by placing the wire lead 
into it and soldering. If the solder joint fails the oil 
leaks out. The failed BB's I've opened were all completly 
dry inside.
     There may be another failure mode, namely distortion of 
the casing. A great many BB's have split or broken casings. 
Of course, the oil will come out of these also. I've foudnd 
that the capacitor elements were distorted. They should be 
pretty exactly tubular but the ones I've seen were all 
mashed in one way or another. That would affect the C and 
maybe other characteristics. I don't know if this is the 
result of shrinkage of the casing or if it happened in 
manuacture.
     BB caps were sold as deluxe, low loss caps capable of 
withstanding high ambient temperatures. Obviously, something 
happened in manufacture, whether poor design or some blunder 
in manufacturing technique is beyond me. However, they 
evidently began to fail shortly after manufacture as 
evidenced by the replacement of all BB's in Hammarlund 
SP-600 receivers about five year after the first ones came 
out. Hammarlund used other brands of metalized paper caps, I 
don't know if they fared any better but were also replaced.
     Sprague continued to make a similar cap to the BB, sold 
as the Telecap and referred to in advertising as "these 
Black Beauties", but I suspect the design was not the same. 
Unfortunately, BB's were pretty widely used, particularly in 
high-quality equipment such as Hewlett-Packard and General 
Radio. Not all go bad, I have one here that I replaced on 
general principles, that is intact mechanically and still 
checks good on the capacitor bridge although its not nearly 
as low leakage as a modern film cap.
     I am not sure of the failure mode of old paper caps of 
the wax impregnated type. I think they were probably not 
great when new. Even though a lot of them remain good its 
still probably good policy to replace them all in old 
equipment. BTW, I have found shorted ones.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com