75 What about electrolytic capacitors? Can they be re-formed?
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This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
75 What about electrolytic capacitors? Can they be re-formed?
Roy Morgan sent me a drill for re-forming old caps. Keep in mind
that some caps won't come back to life. The "wets" from the early
thirties generally have internal problems and corrosion, and a lot of
the axials have dried out internally. Note that a "dry" electrolytic
has a moist gauze with electrolyte inside---what makes them "dry" is
that the electrolyte doesn't slosh around. "Dry" can types, like the
Mallory FP series, often will come back to life with a re-forming. I
used the procedure that follows on a 20/20/15 mike 450V Mallory FP with
a date code of June 1945 that probably last saw power in the 1960's,
and the cap came back to usable condition. Here's Roy's procedure:
To Re-form electrolytic capacitors:
With the "patient" set off, set the external supply at the rated voltage
of the cap(s), and feed the old set at the input to it's B+ filter through
a 100K, 2W resistor. The old caps will slowly come up to voltage as
their elecrolytic layer re-forms after long storage. You may want to
unhook bleeders or screen voltage dividers if present in order to get no
dc load other than the caps. Once re-formed up to nearly the cap rating,
increase the external supply voltage to the point where increased voltage
only increases the current drawn (the electrolytics begin to "leak".) You
can vary the series resistor depending on the voltage of the cap you're
trying to reform.
If the final cap(s) voltage is high enough, it doesn't need to be
replaced. If it's too low, put new one(s) in (leave any original cans in
place for appearance, and substitute new axial lead ones under the
chassis.)
Some caps take only a few minutes to re-form. Some take a day or so! Be
patient. Your Adjusta-Volt or Variac can be well-used for this if your
external supply is solid state, or has a separate hv supply transformer.
I have one good for 900 volts no-load having 5R4's and separate filament
transformers. This lets me re-form 500 volt electrolytics if I need to.
With a 500 volt supply, and a number of 100k or 200k resistors, you can
re-form a number of caps all at once. Measure the voltage on the caps as
time goes on with a high-input-resistance meter (VTVM or solid state
DVM). Allowing an electrolytic to idle with a small leakage current of 1
to 5 ma won't hurt it, so if the thing re-forms to it's limit during the
night after you've left it on the re-former, no harm is done.
Most electrolytics in good health will leak at a voltage from 125 to 200
percent of the continuous rating. If the leakage voltage is only a little
above the needed circuit voltage, or is below about 110 percent of the
cap's rating, then you can excpect it to not live too long. New axial
lead caps are fairly cheap, and are good peace of mind in my opinion.
(I didn't have a separate power supply. What I did was disconnect B+
from the caps and feed the rectifier output through 100K resistors to
each section. With a 670VCT plate winding, and only a few ma. current
draw, an 80 will come very close to delivering 500 volts peak (1.41*370
is a little over 500. Once the caps settled down, I put 20K's in the
circuit to pull them up even further---they had about 480 volts on them
at the end).
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