Bad power board on iMac 2210, fix/replace with outboard PSU?

Discussion in 'iMac' started by Iron Mew, Feb 10, 2017.

  1. Iron Mew macrumors newbie

    Jun 6, 2014
    I was given by a relative the titular iMac, mine to keep if I could get it working. Symptoms: dead as a doornail.

    Took it apart, plugged it in and the diagnostic LED on the motherboard didn't come on. Tried to take voltage measurements at the power supply DC socket and everything reads zero. However, I confirmed that AC power does get to the board.

    So the power supply is dead - but what would kill one so thoroughly? Usually when I troubleshoot computers with power issues the PSU has bad capacitors and the voltage reads OK with no load but slumps below usability as soon as any load is applied, or reads wonky to begin with. But in this case output voltage is nonexistent - the PSU board seems completely dead. Even the standby rail reads zero. There is no inductor whine, nothing at all - if it weren't for AC power getting to the relevant pins I'd think the AC plug was broken. None of the caps are bulged, either.

    Assuming I can't fix the power board (though I'm hoping for a lucky shot), can this iMac be powered from an external PSU capable of delivering 12V at sufficient current? According to the pinout there is no 5V rail, and the 3.8V standby rail is only needed for soft-power, which I could do without since I'm perfectly OK handling powerups and shutdowns by just turning on and off the PSU.

    What worries me are the PSU temperature voltage on pin 4 and a tiny two-pin extra connector (with only one pin connected) that I can't figure out what it does. Also, I don't know if I can just group the 12V rails together on one thick wire - the reading on the back of the PSU mentions two 12V rails.

    Thanks for any help!
  2. Iron Mew thread starter macrumors newbie

    Jun 6, 2014
    I think I've found the issue - there is a SMD chip with an obvious crater in it. It appears to be a LM358. Middle left in this picture.

    Are LM358 failures a known fault? Replacements are available cheaply, but I wouldn't want to solder in a new one only to see that one release the magic smoke as soon as I power up.
  3. ljonesj macrumors 6502a


    Oct 20, 2009
    Kingsport TN
    Not sure but could have been a lighting came in on it or some how a voltage spike from the power company some how

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