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

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Machine turns on but no chime, no PRAM response, black screen ...

I tried the usual PRAM reset, no response, I connected to external screen, no signal, I tried T mode , no icons on neither internal or external screen. I removed the native LCD, to check the LED lights, I have 1 and 2 but no 3 nor 4. That would mean logic board is toast, right?

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If you disconnected the display LED’s 3 & 4 will also won’t light. So you need to make sure the display is wired up to be sure.

You likely have a burn’t GPU board.

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OK, I will hook up the display and try and peer in behind to see if I can make out how many LED lights are coming on...

Dan, with the Display hooked up all four connectors, I still only have 2 led lights. Does that then confirm your most likely scenario of a fired GPU? I had jumped the gun last night and had ordered a good used logic board...but if need be I can replace or send in the GPU for repair as well...

LED 3 - Indicates that the computer and the video card are communicating. This LED will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement.

LED 4 - Indicates that the computer and the LCD display panel are communicating. This LED will be ON when the computer is turned on and video signal is being generated. If the LED is ON and there is no image on the LCD display panel, the LCD display panel or inverter might be installed incorrectly or need replacement.

@Dan, Given the new info I provided : with the display connected, I still only 2 LEDs your original guess remains, that I need to replace or repair the GPU, and that it is unlikely to be the logic board? There is no way to tell which one is the culprit then at all, other than trial and error?

Your GPU board has failed if LED #3 is off - "Indicates that the computer and the video card are communicating" which is false here so its bad.

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