MBA A1304 damaged by powered HD thru USB-bus

Discussion in 'MacBook Air' started by NicoNL, Aug 5, 2012.

  1. NicoNL, Aug 5, 2012
    Last edited: Aug 5, 2012

    NicoNL macrumors newbie

    Jul 23, 2012
    Don't ask me exactly how it happened, but I got this MBA to repair. The son of the owner put external power on to the USB bus, and now the thing doesn't boot anymore.

    It gave one long beep, but when you switched it on and off a few times the beep became shorter and then it just silently powered off. When waiting for a while it started to give one beep again when powering up.

    In the beginning I measured 5V after switching on. Just for a few seconds, but it was there. However, when I checked the resistance of the 5V power line this was less than 20 ohm, so I was quite sure about a short circuit. I desoldered the power-switch for the usb, the lower side mosfet of the 5V SMPS and two capacitors. After doing so the resistance even get lower to under 10 ohms (???). After soldering them on the board again I had no beeps anymore, and also the fan didn't turn on anymore. There was no 5 volt anymore, only a few 100mV when powering on...

    I think the one beep was possibly only a partly code, because the MBA is powering itself off, sometimes even before it starts beeping.

    When measuring the 3.3V I got a very big dip when pushing the power button... (But no dip in the 8V and 16V.)
    So I finally decided to desolder the TPS51220 (power control chip for 3.3 and 5 volt).
    Now I still have a low restistance/short circuit in the 5V-line. The resistance is about 3 ohms, so the short circuit is getting more severe and I'm failing to locate it... (I also desoldered the lower side mosfet again, because it possibly could be switched on, since the gate is now floating.)

    Is there somebody who knows which parts to look at next? I should think there's not so much left on the 5V on a MBA logic board, but well, I don't have the schematics... Maybe someone has the schematics of a 2009 A1304 too?

    Many thanks in advance!

    PS: At some points I have tried to feel if there were some parts that heated up, but it switches off so quickly, that I had also got no result that way around...

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