[Test-Equipment] Exploding capacitors ? (wavetek 114s)

[J. Forster]

Sure. Reforming is the process of applying a current limited voltage to an
electrolytic cap to reduce it's leakage and the other ravages of unpowered
storage.

It can be done in several ways and there are write-ups on the web. For example:

http://www.vmars.org.uk/capacitor_reforming.htm

I usually use a low current variable DC supply in series with a meter and often
a current limiting resistor. The R or current limit on the PS should be set to
limit dissipation in the cap being reformed. I typically use 0.5 ma for a 20 or
30 MFD 300 V unit and maybe 5 ma for a 500 MFD and up. You want to make sure the
cap does not get warm from the leakage current. IMO, heating from excessive
leakage is what destroys caps and sometimes makes them go BOOM.

Typically, I will raise the voltage on the PS from zero to about 1/20 or 1/10
the caps working voltage, unless that voltage makes it draw too much current. If
the current is too high, start at a lower voltage. Watch the meter and when the
current goes down to it's asymptotic value, take the voltage up a notch. I
typically allow at least several hours between steps.  Keep doing it until the
operating voltage is reached.

It is worthwhile to check the capacitance when done, BTW.  The procedure is not
always successful.

If the cap is a dead short, it can sometimes be recovered with a current pulse.

In general, if you are not striving for authenticity, it is not a bad idea to
replace caps that are bad. I am mainly concerned with radios that are 50 + years
old and are all original antiques. A bad cap in a Tek scope or the like gets a
new one.

-John

GeoffK wrote:

> For those of us who are not sure of the procedure you refer to for reforming
> caps, could you please set it forth here?
> Geoff K.