[sdiy] old capacitor ID

[Roy J. Tellason]

On Saturday 19 January 2008 17:38, Scott wrote:
> Hi, I'm working on fixing an old 50's amp, and want to replace the
> electrolytic capacitors.  A few are obvious, but then there are a few
> more that have a line marking one side, but I just read something about
> how they might not really be polarized.
> Could you look at these pics and let me know if you know what kind of
> caps these are, and if they are likely to need replacing?
>
> http://www.sdiy.org/wicked1/oldcap1.jpg
> http://www.sdiy.org/wicked1/oldcap2.jpg

Those are _paper_,  or perhaps real early mylar or similar.  The line on one 
end is the same as the marking in the second pic -- it's the outside foil,  
so if for example you had one end of the cap grounded that's the side you'd 
ground,  but for most cases its not really going to make a whole lot of 
difference.

Molded caps aren't all that much of a problem,  mostly.  Not something that 
I'd worry about replacing unless you had some definite indication that the 
part had a problem.  I've seen a very few of them get "leaky" where you have 
say plate voltage on one side of the cap and what should be a negative grid 
voltage on the other side,  that ends up positive,  creating distortion and 
shortening the life of the tube.  A simple DC voltage measurement will 
usually tell you if that's going on.  Many times they're used as bypass caps, 
which isn't going to be as critical an application.

Now if you find the old wax-covered units,  _those_ I'd replace without a 
second thought.

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