1. I, too, have shared Bob K6UJ's experience with an electrolytic cap
exploding, and the mess was awful enough to make me conservative about the
issue of replacement. The goo between the foil seems to be somewhat corrosive,
was slow to clean off, and damaged some of the metal it got on.
2. I'm cheap, and have used many surplus electrolytic caps. Along with Glen
K9STH's suggestions, I suggest that anyone also using surplus caps connect them
in series and place some DC across for a few hours, then see how much they
leak. I toss anything that gives off any more than a few watts -- I've seen
leakages of 20 - 30 mA at voltages of around 300V (6 - 9 watts) on surplus caps
-- and know others who are even more conservative than I about throwing them
out. They can be so much cheaper than new that I think it is cheap insurance
being conservative. Along with Glen, I have found that frequent usage tends to
make electrolytic caps last longer, not go bad faster.
3. Finally, particularly if you use surplus caps, I suggest that AFTER THE
CAPS ARE THOROUGHLY DISCHARGED, every so often it is worthwhile to simply feel
them after they have been under power for a few hours, and replace the whole
batch if any one of them is particularly warm.
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