Jeff Walther wrote:
-------------
>
> > digikey also has excellent capacitors for
> >switching supplies (it's worth getting the best ones, the price
> >difference is small and they should last longer than you'll care),
>
> Which ones are the best? Is it a particular brand? Or can one tell
> from the listed specifications?
it seems i already threw my last digikey catalog out, they have the
somewhat irritating habit of sending them every 2 months (it does keep
the prices and selection current, but it's a management nightmare, i
know i have at least a dozen in storage). i think they were the
panasonic, and they have several varieties, but yes you can basically
tell by reading the descriptions. you of course want to look at rated
life, ESR (effective series resistance, which is what makes caps get
warm in the first place) and somewhat at size, if 2 with similar specs
will fit you're better off with the physically larger unit as it will
dissipate heat better, they seem to be getting smaller at a rapid pace,
though the specs at least say they are also getting more reliable.
electrolytic caps are by a huge margin the shortest lived class of
components, and the main part of most monitor and tv repair.
-------------
> Thank you for this detailed description. That's handy.
>
> BTW, do the caps age much if they're sitting unused? Way back when,
> Pre-Owned Electronics sold a repair kit for the Plus analog board.
with caps from long ago, they dried out in storage (they still had
liquid electrolyte in them, and needed it). electrolytics of the past
several decades are shipped dry, and well sealed except for a vent hole
(that seals itself, it's in a rubber like material) that keeps them from
being big fire crackers under normal conditions when they age (i have,
in my youth, seen a number of them explode rather excitedly when
connected to ac line current, best not to do that without great caution
and safety gear at the "safe" end of a long extension cord...). modern
caps do age a little in storage, but very little. i know the leakage
current is much higher at first after long storage (the catalogs say so
and i've seen it), but it drops down rapidly in the first hour or so of
having a voltage across it. i wouldn't go ahead and replace them unless
some look suspect, at which point it's often best to go ahead and
replace them all so as to get the ones that will fail soon. having said
that, i don't know what caps came in the kit, they might or might not be
"good" ones (you certainly don't need the very best, they will last
longer but the old macs are much easier on capacitors than some of the
newer, higher frequency switching supplies). the capacitors starting to
age is almost always what makes the diodes and other parts fail.
fortunately, the transformers in switching supplies very nearly never
fail (which is very good, short of detailed construction notes you'd
never find a replacement), with the exception of some flyback
transformers (even before computers clever engineers used the flyback to
not only generate the high voltage but all or many of the other voltages
a tv or monitor needed, even in the tube days), the main culprit with
them is likely high voltage and the inherent ozone, but heat is a
problem in some, largely due to the heavy insulation the hv needs no doubt.
-------------
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