[ARC5] Use of a Variac

[Tom Lee]

I'm a big believer in re-forming electrolytics if you don't have 
replacement caps handy. I have saved many pieces of kit that way. Not 
all caps are salvageable, but many are. The key is to do exactly what 
you describe: Free their terminals, then apply voltage with a 
current-limited suppy. The open-loop variac method can work (and has, 
for many), but it has its risks of permanently damaging the cap. The 
current limit is key. Set it so that the max power dissipated by the cap 
is below a half-watt or so.

How do you know when you're done? The standard pass/fail criterion is 
that the capacitor should have a self-leakage that corresponds to no 
more than 1% loss of the stored charge per second at the working 
voltage: Ileak < (0.01per second)*C*Vmax, where Ileak is in amps, C is 
in farads, V is in volts.

If the leakage can be brought down below that limit, the cap is probably 
going to give good service. I most recently did this on a Tek 485 scope. 
I have new replacement caps waiting in a box, just in case, but the 
thing has been going strong for well over a year since re-forming.

--Tom

-- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 4/23/2020 13:39, Richard Knoppow wrote:
>    I think the idea of the Variac to bring voltages up slowly is based 
> on not having filter caps short. I don't think it really works. What I 
> used at -hp- and have now is a metered Variac. If something isn't 
> right the current meter will come up too fast and I can hit the off 
> switch quickly. Still won't protect something that has a shorted power 
> transformer. I am also not a believer in re-forming electrolytic caps. 
> While there is a long military treatise on this I think there are some 
> assumptions made about how caps fail that are wrong. If you want to 
> reform the caps do it with a variable, current controlled, DC supply 
> with the caps out of the device. A metered Variac is very useful 
> because you can tell right away if something is drawing excessive 
> current, or maybe too little current. A dim light tester is better 
> because it will show immediately  if something is shorting.
>
> On 4/23/2020 1:18 PM, Robert Eleazer wrote:
>> We frequently hear people suggest that you bring up the input voltage 
>> on an older piece of equipment with a variac to allow capacitors to 
>> fromate and so forth.  Of course this remains as a somewhat 
>> controversial approach.
>> I have a URM-127 that I need to recalibrate the frequency dial and in 
>> the process of researching that procedure I found the following:
>> /a. /If the signal generator has been stored in excess of two years, 
>> perform the following pre-test procedure to assure proper operation 
>> of the power supply filter capacitor.
>>
>> (1) Operate the signal generator for 30 minutes at 57.5 VAC, 60-400 
>> Hz, 1 phase input.
>>
>> (2) Adjust the input to the required value using a variable 
>> transformer between the power supply and the power cable. Check input 
>> voltage prior to connecting to the signal generator.
>>
>> (3) Remove the variable transformer from the power cable and connect 
>> the power cable to the electrical power source.
>>
>> Wayne
>>
>> WB5WSV
>>
>>
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