Capacitors -- replacements?

William Donzelli wdonzelli at gmail.com
Sat Dec 11 11:50:17 CST 2010


> That's the punch line of "NOS" capacitors: if they are as old as the ones you're replacing, you're replacing crap with crap.  Many folks have noted that caps sitting on the shelf aren't any better off than those sitting in a power supply.

I would not damn NOS caps quite so fast. Capacitor technology really
improved in the 1960s, so the caps of the 1970s and especially 80s and
90s are really quite good.

With NOS caps, I think it all boils down to two things. One is the
age, being made sometime post-1970, after the cap makers really got
their act together. The other, and probably more important, is where
the caps came from. Storage conditions matter quite a lot. I suspect
that a huge share of electrolytic capacitor failures we see are not
due to whole improper/impossible reformation process, but rather by
the seals going bad and contaminants getting in. Thus, getting NOS
parts from a surplus dealer that keeps half of his stock in
non-climate control building is probably not a good place to get the
caps. Every year, the temperature cycles from freezing to baking, and
humidity might swing nearly 100 percent. Clearly, swings like these
are not good for any seal - electrolytic capacitors or your Great
Aunt's jars of tomato jelly.

My advice is to use caution when getting NOS capacitors - research the
source, ask about date codes, and when you get  them, inspect them,
margin test them, them burn them in.

--
Will



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