Rodney Pont wrote:
There is only about half a dozen electrolytic caps and it's only a
single sided board so I'll take them out and test them.
Ok, do you have an ESR-meter? I want to highlight, that the capacitance
measurement doesn't tell reasonably the condition of the cap, as the
capacitance very rarely goes out of tolerance when the cap goes bad.
It's the ESR (equivalent series resistance) instead, like I've already
mentioned. It increases due to aging.
If you have an ESR meter accessible, good. But if not, your chances to
deal with this case a
-to buy one
-to build one:
http://ludens.cl/Electron/esr/esr.html
-to just replace all the caps with new ones
-to test the caps by using a function generator and an oscilloscope
instead
The latter method is done by tuning the gen to a high freq, something
around 50kHz, with 100-200mV amplitude, connecting the cap in series
with the gen output and scope input, and scoping if the cap causes
significant attenuation by comparing the amplitude with and without the
cap in series.
You said not
the mains filter cap(??), there is a 400V 1000uF (or 100uF can't
remember which) in there and I assume that's the mains one
Yes.
and it may
have a slight bulge on the top. Any particular reason not to go near that one?
Not for not going near it. I just meant that if you decide to do the
method of replacing all the caps (if you haven't equipment for testing
them), then I would leave the mains one alone, as they fail very
rarely, and are also the least critical. So the small ones in chopper
circuits are both far faster to decay with age, and also much more
critical, and a small ESR on those can cause an appliance to completely
cease working, while the mains one in many cases outlast the device in
good condition, and might work well even with slightly increased ESR.
I've never experienced a mains cap having to be replaced except in over
30 years old antique equipment.
But however, I'm still not claiming that it couldn't fail, and if you
are suspecting it, replace it too if you're still gonna do the
replace-them-all-method. But if you are gonna make ESR measurements, of
course, the mains one should be measured as well, and treated according
to it. But some caps (especially those bigger ones) look like "little
bulged" just normally.
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