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모델 A1419 / Late 2012 / 2.9 & 3.2 GHz Core i5 또는 3.4 GHz Core i7 프로세서, ID iMac13,2

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How to troubleshoot an iMac 27 late 2012 that wont power up?

So I decided to upgrade my iMac with a blade ssd. I followed ifixit guide carefully. Everything went fine, there were no trouble at all, so I decided to put a new thermal paste in the cpu while i'm at it. The reassembly of the cpu didn't go as smooth, but I did managed to assembled it. But as I reassembled the machine, the iMac wont boot up. There is no sound coming from the iMac. The LED #1 on the logic board is lit, but nothing on the others when I push the power button.

My initial thought is that I have toasted the cpu, as it was the only step I had trouble with. Worst case scenario is that I have toasted the logic board. Is there anyway to check which unit is the cause of the power up failure? Can I assume that the power supply is fine when there is light on the LED #1?

Symptoms before the ssd upgrade: The iMac had trouble waking up from sleep. It went on "hibernating" mode and it took a while to start up again. I tried pram and smc reset andd it didn't help. Which is one of my reason to upgrade to ssd.

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Hello,

What kind of difficulty did you encounter with the CPU reassembly? It may shed some light into whether that's the cause or not.

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The more info the better!

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After the first reassembly and the iMac won't start up, I disassemble the whole machine again to see if I missed a wired or anything alike. I disassembled the heatsink from the cpu again. But I find it very difficult to know if cpu sit aligned in the socket as the only thing that's holding it's the two very thin hatch from the cpu and the socket plastic. So I have no idea when putting the heatsink on, if its sit perfectly in place. And the spring from the heatsink (step60 on ifixit) is very hard to screw back, so I might have moved the heatsink and thereby misplacing the cpu on the socket. This uncertainty make me think that this could be in this step that I screw up.

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Update:

I had bend some of the pins while doing the cpu thermal paste. I have since make the pins straight again and it now power up and I now have 2 LED light on the logic board, but at the same time, I get a loud error sound instead. That kind of error sound you get when you forgot to put the ram inside. And yes I had working ram inside. And I have also checked cpu, ram and psu for error on a working iMac. From that I isolated the problem to be a faulty logic board, but than again iam not a professional and can only do this much. I have spend a lot of hours assembling and disassembling the iMac that I could probably get a job in one of those assembling factory.

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Did the CPU have pins? If so you may have accidently bent one.

Generally, if it was heat related the computer would have run a short while and then automatically shut down.

(An aside - You may also have put on too much paste)

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The pins are on the logic board. They are all bend in one direction, I don't know if it is normal though. Should they all stand straight?

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Yes the pins should all stand straight!

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No, do not assume the power supply is good. All you know now is that the power supply is receiving juice. If #2 lights up then you know the PSU is good.

So that’s the first thing to check out and make sure all your connections are secure.

On this model the PSU is the most common failure in my experience.

iMac Intel 27" EMC 2546 Power Supply Replacement

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Did you ever get a resolution? Almost identical scenario just happened to me, I find it hard to believe its the PSU, simply because before the repair (literally 3 hours ago, the computer booted perfectly fine. Only 1 led turns on, I get nothing, I'm borderline devastated. I loved this thing! Help!

Thank you in advance!

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In My case I had bended the pins while changing repasting the cpu. I managed to power it up now, but it has other problems. I now get 2 light LED on the logic board, but get beep sound as soon as the machine power up. I had a spare imac I could test the cpu, ram and psu on, so I could exclude those problem. From what I can find on google it seems that the graphic card is not communicating with the logic board. In my case the logic board is faulty and unfortunately its not cost effective to do logic board repair in Denmark. It’s cheaper and safer to buy a used iMac than risking repairing a problem child.

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I had issue with crucial Ram failing before, if you have those then try to swap the ram block one at the time to isolate the problem. And the psu Can power up before failing, especially if you have heard buzzing Sound before, you know that the psu is beginning to fail. This is from My experience with Thunderbolt screen with bad psu.

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