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iMac Intel 27" EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz) ID iMac10,1, EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz) ID iMac11,1

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iMac 2009: screen goes black, logicboard of graphic card at fault?

I tried to upgrade the CPU on my imac 2009 27" (EMC 2309). It was a blunder because after I put back everything, the screen goes black within a few seconds after power on. Sometime the screen stays until the login window appears, sometime during the boot progress bar, sometime just right a few seconds. I can still connect to the imac from a laptop. When I plug an external monitor to the imac, the login window appears on the external display as usually.

I thought it's because of the new CPU, so I put the original CPU back. But the problem remains after that. It seems I broke my imac? Is there a way to find out what is broken: the logicboard, the graphic card, or the board on the display?

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If you still have the original Grey CD/DVD's find the one that has the diagnostics and try to run it.

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@tonyh2 "When I plug an external monitor to the imac, the login window appears on the external display as usually" have you checked to see if it is your backlight that turns off? In a darkened room shine a flashlight against the screen and see if you can make out the desktop. Have you reattached the LED thermal sensor?

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A system that winks out quickly could be many different issues and the hardest to nail down without using spares to swap in.

What happens when you try booting up in Target Disk Mode? Restart and press the T key to enter. You should see a FireWire Icon on the screen does it stay visible?

Reference: Mac startup key combinations

If it stays running then you've proved the discreet GPU needs to be looked at. If not then you need to look at the thermals as the system is likely sitting down from a heat build up. That could be the thermal paste and pads on the GPU as well as the CPU and the systems logic board or displays thermal sensors, or a bad fan . There are other possibilities besides thermals but given the system was working? Before you went in that's what I would focus on.

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I tried various startup keys from the link above but it's always the same: the screen turns white on power on, then after a few seconds it goes black regardless which key is being pressed.

The external monitor works just fine. Does it mean I can rule it out? Can a faulty graphic card drive an external screen?

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@tonyh2 - White screen tells me you messed up on the CPU. What did you try to put in?

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I tried Intel Core2 Duo E8600 @ 3.33GHz (bought on ebay). The orig. CPU was Intel Core2 Duo E7600 @ 3.06GHz.

However despite the screen being black, the imac seems to boot up and can be accessed via external monitor, or remote desktop. Doesn't it mean the CPU is OK?

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@tonyh2 - Well, it's a workable CPU for this system, it's possible the CPU is bad.

You see the Target Disk Mode leverages the CPU's graphics not the ATI Radeon HD 4670. So it should have displayed an image.

That only leaves the display cable connection. Refer to Step 7 in this guide iMac Intel 27" EMC 2309 and 2374 Display Replacement if you damaged the socket or cable in any way that can be your problem.

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Tony will be eternally grateful.
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