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Thread: Variac procedure for reforming power supply capacitors

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    Default Variac procedure for reforming power supply capacitors

    I'd like to try to reform the electrolytic caps in a Sol-20's power supply as replacements are surely made of pure unobtainium. What's an effective (and safe) procedure (voltage increase over time) for this? Also, should the power supply's outputs be loaded or not?

    Thanks very much,
    Dave

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    The load doesn't really matter, at least not that I can determine.

    However, if those old caps are dried out, as is the case in the majority of post-1960s electrolytics, you're not going to save them.

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    I usually reform caps out-of-circuit using a current limited power supply. You can also just use a resistor and a regular power supply (DC wall wart, computer power supply, etc) as long as you can get it up near the working voltage you intend to be at. Like Chuck(G) said, if they're dried out there's no saving them, but a lot of the larger can-type caps do survive after a reform.

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    Quote Originally Posted by DaveL View Post
    I'd like to try to reform the electrolytic caps in a Sol-20's power supply as replacements are surely made of pure unobtainium. What's an effective (and safe) procedure (voltage increase over time) for this? Also, should the power supply's outputs be loaded or not?

    Thanks very much,
    Dave
    For a Sol-20 once you know they big metal cans are not shorted you will need a power resistor load because of the crowbar circuit. You don’t want to do this with the main board attached. Then use a variac and slowly bring it up over the course of a day to operating voltage. Checking the voltage periodically to make sure they don’t short out or the transformers have a problem. I had a sol-20 with a bad transformer that started smoking around 40 or 50 volts, I was able to find an exact replacent, but it wasn’t easy. I have also had bad transistors go poof in the bring up process that needed replacement on a sol-20. However I can tell you after reforming the ESR on the caps was well within spec.

    I should add I have a few Sol-20 systems so not all of these things happened on the same one.

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    The better approach is probably to disconnect the caps, put them on a bench supply and slowly crank up the voltage, while watching the current and temperature.

    But as always, there were some really bad examples. Right now, I'm trying to remember if it was CDE or Mallory that had a line of capacitors from the 1970s labeled "long life" or something like that. (I've got one on a Lambda rack supply, so I can go check). Those didn't age well--every one I've ever come across has been an electrically empty can. Electrolytic capacitors do not suffer idleness gracefully. If you want to preserve the look, consider rebuilding the old caps.

    One of the enemies of old electrolytics should be familiar--rubber degrading. Most large can-type 'computerlytic" caps have a rubber-based seal. When that fails, the innards start losing moisture. As time progresses, that causes an increase in series resistance (ESR) and the capacitor develops more heat, accelerating the drying process. AFAIK, there's no way to "re-hydrate" a cap with failed seals.
    Last edited by Chuck(G); July 6th, 2018 at 11:59 AM.

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