[Milsurplus] 51J-3
Barry Hauser
barry at hausernet.com
Fri Jul 8 19:04:24 EDT 2005
Peter wrote:
> It's been a while..
>
> What is the best way to reform caps?
>
> Full voltage with high source impedance? half voltage? ramp up? over
> what time period?
>
One good way is to use a vintage capacitor tester that tests at operating
voltage -- usually up to 600 vdc. -- for the leakage test. To reform an
electrolytic, don't start at operating voltage, start way below. Depending
on the tester, it may have switch positions or a pot to adjust the voltage
and it's best to have one that has a built in meter instead of, or in
addition to, an eye tube. Start low, then switch over to the leakage test
and watch the current meter readings. As the cap is reformed, the leakage
will drop. When it gets down to a negligible reading, then step up the
voltage a bit and switch over to the leakage reading again. You can leave
the cap in the chassis and just lift the positive lead or better yet --
both. You keep doing that until you're up to operating voltage, e.g. 450,
and the leakage is virtually nil or acceptably low, depending upon your
persuasion. (The old cap checker manuals sometimes listed maximum leakage
allowable, and some don't believe in allowable leakage.) If the cap does
not get way down in leakage reading at some point, it's not reform-able. If
it does look good, then you can proceed to check capacitance reading and
power factor. If you have a digital meter that will measure capacitance,
you're better off taking the reading with that as many of the cap checker's
bridge/dial readings are frequently rough or inaccurate. If you want it to
be accurate, you'll probably have to re-cap the cap checker ;-)
You can do this with a separate power supply and meter(s), but the old cap
checkers are more convenient.
See below about the variac thing ...
>
>
>> telegrapher at att.net wrote:
>>
>> If you mean to reform the caps by applying power slowly from a Variac,
>> I'm not
>> convinced this is the best procedure for tube gear.
>>
>> Certainly reforming is essential, IMO, but without full filament on the
>> rectifier(s) the actual reforming voltage may be somewhat unpredictable.
>> It may
>> also be hard on the rectifier tube(s). usually leave the set unpowered
>> and
>> apply voltage from an external PS.
>>
>> FWIW,
>> -John
Bringing up an old rig with a variac can have some benefit, but re-forming
in place is a bit iffy. If you bring up the rig on a variac with rectifier
tube in place, it won't put out any DC until you hit 90 volts or more, so
I'm told, then the electrolytic is subject to that all at once. What some
do is temporarily (if not permanently) sub out the rectifier tube with a
couple of solid state rectifiers during the gradual power-up. However,
there may still be a risk of damaging the power transformer in the event of
a short. If you're going to start unsoldering things, you might as well
disconnect the cap(s) and check separately.
If you're really jittery, remove the caps altogether and place them in a
sturdy containment vessel, like an ammo box, during the reforming process.
That's a safety precaution in the event they have become allergic to
electrons during their long sleep and react to the awakening by exploding.
One general concept is that by applying power gradually to the whole unit,
if you hear any crackling, smell anything, or spot a puff of smoke (or
flame) you can back off the hammer and minimize the collateral damage -
before you let all the smoke out of the radio. I dunno 'bout that. I've
done it, but I suspect, in most cases, by the time you hear the snap,
crackle 'n pop, you've got yourself a pile of Rice Krispies.
Yer pays yer money, ya' takes yer chances and ......YMMV.
My 2 cents.
Barry
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