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Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor, ID iMac12,2

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Backlight off after "pop" noise

Hello everyone,

I need help with this one as I am dreading the cause to be the graphics card again….

A week ago while using my iMac it made a “pop” noise - much like a capacitor breaking. The screen went black. Using a torch I could see that the LCD was indeed working, just the backlight of the LCD turned off.

I went through the Technicians diagnostic steps.

I’ve replaced the Backlight Board, no change.

I’ve replaced the complete LCD panel, no change. This also changes all cables in the process.

I have a feeling that the main power board might be at fault here, but the diagnostic steps end here. Can anyone tell me what voltage I would need to see and when at the Backlight Board power supply side? There are 6 pins, I understand that pin 3 should be live to enable the backlight?!

  1. Do I need the LCD connected to be able to take a valid measurement there?
  2. What kind of voltage do I expect?
  3. Are there schematics available to give me an indication?

I’ve replaced the graphics card once already. I hope it is not due to it again (backlight enabling voltage etc…).

Any input is mutely appreciated.

Many thanks,


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Found the issue after replacing components bit by bit.

The power supply replacement resolved everything.

This is not part of the iMac diagnostic paper! It appears that the power supply to the backlight board was not sufficient.

Haven’t found the faulty item on the power supply board just yet. “pop” noise would suggest capacitor has faulted.


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Resolved by replacing power supply board.

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Rusty Travelboy Rob zal eeuwig dankbaar zijn.

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