Electrolytic Cap Reforming

Wed Aug 8 09:14:39 EDT 2001

I seem to hear it every time I express an opinion but here goes.

I do use a variac to bring up old unknown radios. I like to see if a radio
still works at all before doing anything to it. The variac may save the
transformer if the capacitors are a dead short. Some of the oldest and
nastiest radios work great - for awhile.

To replace or not to replace the caps? If an electrolytic cap is
mechanically and electrically sound then reforming it would be beneficial.
But there is no way to know this. I think usually these old caps have either
corroded internally or dried out and nothing is going to bring them back.
One school of thought is that old electrolytics are either bad or going bad.
I really enjoy rebuilding caps so would rather do this than clean up a bunch
of melted wax from a radio chassis! I have seen some of them blow up while I
was right there and it is pretty spectacular. With the electrolytic cans and
multisection cardboard tubes you can replace the guts and make it so it is
barely perceptible. I don't think it detracts from value if it is nicely
done. I would rather have a working radio with new caps (done right) than an
original radio waiting to burn my house down. (Overly dramatic)

Another way out. There is a good way to check the condition of electrolytics
and that is with an ESR (Equivalent Series Resistance) meter. This is an AC
Ohmmeter. I built the one described in Nuts and Volts magazine last year
which is based on an old circuit from Creative Electronics (no longer in
existence). The info on this meter including schematic and notes was
available on the Internet but I was not able to bring up the page today. A
company in Australia (Dick Smith Electronics) makes a digital ESR meter kit
for $50 or less. A lot of good links are available by doing a Google search
on "ESR meter" or "Creative Electronics ESR Meter".

The ESR meter works amazingly well and tests in-circuit so you don't have to
disconnect leads. I never even made a proper scale for the meter but after
some experience with it you don't really need it. (The closer to full scale
the better but it depends on cap value.)

This subject really gets into the "restoration philosophy" area and everyone
has their own views. Non-electrical things that may influence the decision
to replace caps are: How hard is it to work on? Are you keeping it or
selling it? Sentimental attachment. Is it made of metal or wood? (Remember
the fire thing.)

Russ WQ3X

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