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Strange problem with primary caps

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    #1

    Strange problem with primary caps

    I ran into a problem I haven't seen before while rebuilding a small Enhance ENP-0812A power supply.

    These use all Teapo caps. I recapped everything but the two Primary caps, a pair of Teapo SK series 220uf 200v caps.

    Using a variac on the AC input, the psu would run normally until the input voltage got to about 90-100v, then it would make this weird cracking noise and vaporise the 4A primary fuse.

    I noticed one of the primary caps was ever so slightly bulging, so I replaced both with new Chemicon SMG series caps. The psu works perfectly now.

    Is it possible the Teapos were arcing over internally and causing the fuse to blow? It was strange, no problem until a certain threshold voltage, then a sudden dead short.
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    #2
    Re: Strange problem with primary caps

    probsbly yes. Caps can easily short out, and if there is enough from the oxidlayer is left, may be it can survive a lower voltage.

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      #3
      Re: Strange problem with primary caps

      Primary cap oxide breakdown of that kind can usually only happen if the protective varistors that shunt them are missing or open. Maybe the varistors failed after a previous large surge?

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        #4
        Re: Strange problem with primary caps

        i remember this type of failure in a old twistlock in a rca tv.
        the snapping sound led me to believe it had a crt arc but never saw it flashover the crt socket spark gaps.
        i knew the cap was bad as there was image distortion from ripple.it got bad enough the relays in the protection circut started clattering.last time i fired it up to check with the scope it did it several times and i could tell it came from the cap.
        new cap fixed all problems.
        iirc it was a ctc 74 "tailgate"chassis.

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          #5
          Re: Strange problem with primary caps

          If the cap's dielectric was breaking down well below its rated voltage, the MOVs wouldn't fire. However, MOVs do deteriorate slightly with every surge they clamp, so another suspect, had replacing the I/P caps not fixed the problem would have been to replace the MOVs.
          PeteS in CA

          Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
          ****************************
          To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
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