[sdiy] finding bad caps

[Paul Perry]

The thing with electrolytics is, they 'fail' in different ways.

At one extreme, you have "dried out" - very little or
no capacitance, so it just looks to the circuit as hough the
cap isn't there.
At the other end, you have a short circuit. Like an
exploding tantalum.

In between these extremes, you can have any lower value of capacitance
(as it dries out this gets lower) or even higher (if the foil is beign
corroded and momentarily getting thinner).
The capacitor may behave as though it has a resitance in series, or in 
parallel, or both.

Which is why it isn't simple.. and where in audio applications the results 
of a
failing or bad cap may be noise or lack of volume, in the case of the more
critical ones in switch mode power supplies, the results can be explosive.
The problem wiht the switch mode iones is that if the ESR (series 
resistance)
rises, it quickly overheats. Those kit cap testers were designed for this 
application.

And a faulty cap will likely behave differently at different voltages, which 
is
why you want to measure it at a 'working' voltage - but you can't really do
this while it is in circuit, for fear of damaging the gear you are working 
on.
Catch 22: if you take a cap out to test it, you might as well replace it!

My advice: if any bulge, or are otherwise found to be faulty, replace all
the caps of that brand in the unit. But only those.
And, (since I am very lazy) think a lot about what caps could be causing the
problem before replacing at random.

paul perry Melbourne Australia