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Question on fuse-blowing power supply

Discussion in 'Troubleshooting and Repair' started by cyclones, Feb 13, 2010.

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  1. cyclones


    Feb 13, 2010
    I'd like to learn more about electronics repair and I am currently looking at a power supply that just failed the other day. It is out of a Power Mac G4 MDD a.k.a mirror-door. The computer was freezing up for a little while and then turned off. I was able to locate a used PS for it (new are no longer available) and the computer works.

    The PS immediately blows its fuse when plugged in (tried a new one). I checked for bulging caps but didn't find any, although they are the CapXon brand, which seem to always go bad. While looking over the board, I noticed what appear to be ballooned / bulging diodes. They are marked with DXXX as in D104, D107, etc. and they otherwise look like diodes. I've never seen anything like these. So far I counted 3, all in the same area.

    I'm wondering if these are a clue to what's wrong with this PS.

    Also, I'd like to know where I can learn more about testing and repairing electronics.

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010

    Learning electronics repair starting with a switch mode power supply is understandable (considering their ubiquity) but may be quite frustrating.

    Having said that, I believe we've had someone get one working recently, so maybe I shouldn't be so pessimistic.
  3. cyclones


    Feb 13, 2010
    Here's one I got the other day of the weird-looking diodes. I can try to get a better one on Monday.

    This thread here at is talking about the exact same power supply and has some pictures of it. I don't have the burned spots on mine however.

    Attached Files:

    • PS.jpg
      File size:
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  4. Resqueline


    Jul 31, 2009
    You mean the glossy white globular ceramic diode with a black band? That's a fairly common encapsulation, in particular for switching diodes.
    You'll neeed to whip out a DMM and start measuring, beginning with the mains rectifier. Any pins shorted means you found a fault. Then you may move on to the main power switching transistor.
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