Won't boot when internal display is connected - Can it be the cable?

Hi folks!

A friend brought me her iMac A1418 (EMC 2544) for repair. Now, that’s a long story, and I think it is significant, so here are the key points:

  • She brought it to some store for a hard drive upgrade. After the HD upgrade, she brought it back home, and used it for a few days. But one night, while the computer was still turned on, the display fell to the front.
  • After that, the computer would still be bootable, but would display nothing on the internal screen. No visible “ghost” images (though I have not tried with a flashlight at the time). Diagnostic leds 1 to 3 would light up, but never led 4. The computer was still usable with an external display.
  • After inspection, I identified that the L9100 ferite bead had totally blown up. I suspect that when the display fell, it must have pulled the display cable out partially, causing either a short or an intense discharge.
  • I (well, another friend…) changed the L9100. But after that, when trying to boot the computer, it would completely shut down, similar to a circuit breaker that trip because of a power surge. To make this clear: if I plug the computer in the wall, diagnostic LED 1 turns on. Then when I press the power button of the computer, instead of having LED 2 and 3 light up, LED 1 actually turns off, and wont turn on again until I unplug the computer and plug it again. This happens only if the internal display is plugged. If I completely unplug the internal display, the computer can be booted (and can notably be used with an external display).
  • Suspecting that the L9100 replacement might have been unsuccessful and could be causing a short, I ordered and replaced the logic board, but the result is still exactly the same.

So that’s where I am now. What should I consider next? Changing the complete display assembly is out of my friend’s budget, so if that’s the only option, we’ll have to abandon this quest.

On close inspection, I can see that the display cable’s connector (the end connecting it to the logic board) has been chipped on one side (see picture). But could this reasonably explain the “circuit breaker trip“ symptom I described previously? Unless I’m wrong, it seems to be pin 1 of the cable, and from the schematic I mentioned previously for the L9100, we see that pins 1 to 6 on that connector are connected together… So even if that cable’s pin was messed up (which I’m not sure of), wouldn’t the cable still work correctly?

I also checked the control board on the display panel, but can’t find anything visibly wrong there. And as far as I know, I have no way to test anything there since the logic board will not provide electric power to the panel until later in the boot process.

Could it be the backlight led strips? They pull more power than the display panel itself, don’t they? Would it be safe to try to boot the computer with only the display data cable connected (without the backlight cable), or do I risk agraving things?

Thanks for your help.

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