For skilled repair men and women: iMac G5 LCD drawing without the inverter hooked?

Discussion in 'PowerPC Macs' started by gregounours, Apr 27, 2010.

  1. gregounours macrumors newbie

    Apr 13, 2010
    I have a tough one regarding an iMac G5 ALS 17" 2Mhz I am fixing. First I want to point out that I am familiar with the innards of these ancient beasts as I have fixed a couple 1st gen iMac G5 as well as iSights and Duos.

    When I plug in the cord in the PSU, the 1st diagnostic LED comes on but when trying to start the computer the second diagnostic LED comes briefly on and then goes off. Nothing else happens, no chimes, nothing. According to Apple KB this is the sign of a faulty PSU. Yet, I have tried replacing the PSU and I get the same result.

    Now the weird part is that if I unhook the LCD cable the computer starts normally (it chimes) with the display off. What is most puzzling is that it will start with the inverter hooked up but the LCD display cable off but will not start with the inverter entirely taken off but with the LCD cable on. It seems that it is the LCD itself that prevent the computer from starting. I thought the inverter would draw current but not the LCD itself so it's probably something else.

    Anybody had anything similar happen to them? Could it be that the inverter is faly but it only draws when the LCD is hooked? Could it be some kind of short in the LCD or something that make it draw even with the inverter unhooked?

    All thoughts, comments, ideas welcome.

    Thanks in advance

  2. KeriJane macrumors 6502a


    Sep 26, 2009

    I also have one of these great digital photo frames (iMac G5)

    It also had PSU troubles.

    Darn, I just threw out a spare G5 inverter board. :eek:

    Anyway, here's how things appeared to function:

    The PSU provides power and power good signals to the mainboard mostly similar to a standard computer. It also provides 24v to the inverter IIRC. The Inverter ONLY provides power to the fluorescent backlights, usually 500v.

    Unplugging the inverter might either set off some kind of fault detection OR be causing a marginal PSU to malfunction.

    The LCD has two kinds of connections: The LCD cable that carries the data and power for the actual LCD, and the high-voltage wiring for the backlights.

    If you were unplugging the low voltage data cable, there could be either: 1- a fault in the display OR 2- Either the PSU or the mainboard can no longer properly supply power to it. The most likely cause is described below.

    If you were unplugging the high-voltage wiring to the backlights, a failed backlight bulb is indicated.

    The G5 iMacs ran hot. Especially, the tiny PSU ran hot. A very common problem is failed or "blown" capacitors on the mainboard or in the PSU.
    The capacitors on the mainboard are easy to check- the tops should be flat with no sign of leakage. There are 20 or so that tend to fail on the board and when they do the top tends to bulge outward or burst.

    The PSU also has failure-prone capacitors inside. (that's what was wrong with my G5) You really have to disassemble it to look at the caps. Any bulged ones are really bad and causing problems. Clean ones are probably OK but not necessarily.

    This picture shows several bad caps. The ones on the far right and left are OK, the bulged ones in the middle are bad.

    Good Luck,

    PS. Many G5 iMacs were plagued with faulty capacitors from Nichicon, a reputable company that made some mistakes.
    Also, Electrolytic Capacitors are sensitive to heat and the G5 iMacs ran HOT. That tiny PSU doesn't even have a fan, relying upon convection and system airflow from the other fans.

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