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AndrewT 13th February 2005 01:10 PM

reforming electrolytic capacitors
Hi all,
I am about to resurrect a number of poweramps from storage. I have read numerous times that storing electrolytics reduces their performance/voltage capacity.
Is there a simple and foolproof method to reform the electrolytic without taking the caps out of circuit?
I have a variac to hand if this helps.
regards Andrew T.

Sch3mat1c 13th February 2005 05:08 PM

If they've been in storage more than five years, they may need to be reformed.

Just bring up the voltage slowly, over an hour or so. A variac will work. As long as the circuit isn't wired to complain about low voltages, it'll work.


AndrewT 13th February 2005 06:41 PM

Thanks Tim, sounds like lo volts & lo current is all that is needed.
regards Andrew T.

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stevekendal 13th March 2010 06:25 PM

I mess around repairing and restoring vintage valve radios and high power amps here in the UK and never use a variac, preferring to follow the practices used by those who have been doing it longer than me. That is, you hit them with full voltage straight off and see what happens. About 80% of the sets I power up seem either to be ok with the original power supply electrolytics left in, or else they fail within an hour of use. This is of course with high-voltage types (250 volts and more and I've never actually measured the capacitance. (I dont have a meter for that either). Just my experience with dozens of the dusty old relics. Steve. :drink:

amc184 19th March 2010 09:43 AM

You're pretty much sorted by having the variac, like everyone else said, just run the amps at a lower voltage for a while. A couple of caveats though:

- Many switch mode power supplies don't like being run at lower than normal voltages. Again, some may self destruct, but in SMPSs this isn't that rare. I would only use this method on amps with linear power supplies.
- Some amps that use linear supplies still react very badly (and possibly self destruct) when run at a lower than normal voltage. I'm pretty sure a couple of Carvers were like this, but it's pretty rare.

timpert 19th March 2010 11:00 AM

I can see something in stevekendal's recommendation to just power up and see what happens. Capacitors that have stood long enough to need re-formation are probably also dried out and/or unreliable. So if they can't handle being powered up normally, they probably need replacement anyway. Only problem is, when they pop, they make a bit of a mess. Ramping up with a variac can indeed prevent them from going off, bit IMHO it is not more than a crude trick to temporarily revive a capacitor that is already well underway towards failure.

So, I'd recommend taking the larger electrolytics out of the equipment, and ramp up the voltage towards their rated working voltage using a resistor such that the RC time constant is about 10 seconds. If, after a few minutes, a significant leakage current still runs (more than 1V drop across the resistor), treat the cap with suspicion and replace it. If the cap survives this test, it is probably safe, but it may still have dried out. This is very likely with capacitors that are 20+ years old and exposed to high ripple currents and/or elevated temperatures. When in doubt, replace.

bobc47 11th August 2010 03:06 PM

Reforming electrolytics
back when I worked in the power supply industry we occasionally had to reform NOS electrolytics. We usually had to do batches of several dozen at a time. the method below protects against catastrophic failure and limits current to safe levels. If your dealing with high voltages, BE CAREFUL, getting yourself across a high voltage cap will ruin the rest of your life!

The method i always used was to supply the full dc voltage through a resistor to limit the current to about a milliamp - 150 volt would need a 150k resistor. i would leave this on for a day and then measure the voltage across the resistor to see how things were going. Assuming nothing looked bad I would solder a resistor to limit the current to 10 ma (for 150 v use 15 k). I would leave this for another day. If the cap shorts out the resistor may well smoke but it will prevent exploding caps and highlight the bad caps pdq.

If your setup allows you to easily disconnect the plus side of the caps you can look at the voltage to see how fast they decay as the caps dischrges through it's own leakage to get an idea for the caps "health". Any cap that had excessive leakage was discarded, the rest were discharged and put to use.

indianajo 11th August 2010 07:06 PM

Electrolytic caps have rubber seals to keep the water in. Historic rubber is dirt on a stick. You can reform historic caps, and they might work a while if they haven't leaked already, but the rubber is still ancient and the ozone has been working all that time to ruin it. That said, when I run tube stuff that hasn't been used a couple of years but the caps are under 20 years old, I run it at half line voltage for a while first. I built a box with fuses in and out for my variac, but it was a trash can reject, and no matter what the pointer says, it puts out half the line voltage- which is good enough for my purposes to gently warm in the electrolytics. No cap explosions yet! See the leslie cap explosion thread of for what can happen with the "plug it in and see if it is okay" method. That cap was fifty years old, and was removed molecule by molecule with a towel and scrub brush.

noSmoking 4th April 2011 08:29 AM

Another way to test a cap is to use the variac and a volt meter across the terminals,set on dc and run it up slowly,if it stops taking the juice stop and see if it's getting hot or use a temp meter on it,if ok keep turning it up till at working vdc and check it often if it starts to get warm it's going bad.if not turn it off after a while and watch the meter,if it's not loaded it will sit for a while then start to discharge, all of caps have internal leakage,just a better one goes down real slow,a bad cap real fast,
cap meters are on ebay for under 50.00
Hope that helps,

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