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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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Poorly connecting video cable? No chime no Boot, LED 1-3 OK

Hello,

I own a 2011 27" iMac EMC 2429 that's been doing me great service. I love the machine and would love to get it back up running.

Symptoms:
While operating normally a small move or other caused the display to go to a pixelated line like image. Force resetting results in black screen, no startup chime and generally no love.

Partial Diagnosis:
I have opened the machine, removed the display and followed the 4 LED diagnosis without screen.

After plugging in main power:
LED 501 lights up constantly, no issues

After turning on power button:
LED 502 and 503 light up constantly, no issues

LED504:
Is dark, but obviously my screen is disconnected at this point.

On Reinserting the screen and all cables:
Mac powers up and boots normally, no issues at all.

Anecdote:
I keep my iMac on a metal stand to get it to actual eye-level height, the moment I lifted it up there, the above symptom essentially repeated itself. Now it lays dead again with a black screen.

Theory:
While I never checked LED504 for proper operation, I suppose the screen and everything working as expected settles that.
My guess is that one of the cables is loose or partly faulty, and any random move of the angle of the monitor or the whole device itself ends up disconnecting something essential, resulting in a display error during operation (pixels and lines) and the display not being present preventing startup (no chime, nothing).

Ask:
While I know that the GPU failing on this machine is very likely, I am guessing this is more of a connector/cable issue?
While I'm familiar with the 4 cables of the screen, I wonder if there is a meaningful sequence of those and or other cables to check for and replace before going full GPU replacement or trying to diagnose problems at the inverter board.

Of course, all other suggestions highly welcome.
Thank you!

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Good afternoon,

I had an imac with this issue, and unfortunately had to dispose it because it couldn't be fixed. exact same model in fact.

The diagnosis i reached was GPU failure. Mine was the same but it displayed lines. no boot chime. Sorry to break it to you but there's only one thing i think could have failed, and that would be your DGPU.

If anyone can reach another diagnosis then sure, but i had the same LEDs and symptoms (well, except a pink screen) which to me screamed dead GPU (ATI radeon).

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Much appreciated answer. If this is the decision point to turn this iMac into a fancy screen, then my effort will go there instead. Making that decision easier is all I'm asking for.

I will not close this topic yet to allow other insights, but thank you for yours.

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Are you able to boot-up in Safe Mode? Using a USB wired keyboard press the Shift (⇧) or even diagnostics with the D key

Reference Mac startup key combinations

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@Dan

I bought myself a cheap windows keyboard to try this as well (Assuming that Windows = Command and Control = Option or the other way around).

And tried PRAM reset and Safe mode. But in my case the computer doesn't even go to the level of chime so to be honest this feels like I'm hacking around on a dead computer's keyboard.

So it either boots fully normally, or it's just plain dead.

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@czy93131 - Could you get Shift (⇧) or D functions to work?

PRAM or Safe Mode won’t help us here.

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@Dan

Not sure what the goal here is.

I can open the machine up here again and try to check on the LED04 with the screen attached (if one can see it from that angle).

But chances are I will either succeed (will boot normally) or just stay dead (whatever gets disconnected stays disconnected).

Are you hoping that I can read out something from Diagnostics mode when the screen is working and localise the problem through that?

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@czy93131 - These two options bypass the dedicated GPU, instead leverage the Intel CPUs graphics engine. Thus without opening the system we have ways to discover if a common failure is the issue behind your plight!

That is as long as everything is connected up and working properly besides the dedicated GPU. What we can then do is force the system to not use it. While the Intel graphics is not as good, you at least have a working system.

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@Danj

Thank you for the additional explanation. This clarified it immensely.

I am assuming that this doesn't widen the decision tree in the end but I will

1) open the iMac up again and replug/wiggle all the cables again to see if I can get it back into 'normal start up mode'.

2) Once that works, I will try the diagnostic or safe boot mode to see if any insights can be gained.

Just as a thought, the safe boot or diagnostic GPU bypass doesn't work if I physically unplug the whole video card, does it (just wondering if I can remove failure points).

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