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Model A1418 / Late 2012 / 2.7 & 2.9 GHz Core i5 or 3.1 GHz Core i7 Processor

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Opties

iMac not booting after thermal paste replacement

I just replaced the thermal paste on my iMac, and now only one diagnostic LED is lighting up, witch means that it’s getting AC voltage… I know it’s not the power button because I heard the little click from the PSU when I pressed it.

Here’s a picture if this could help:

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Thanks,

Ryan

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Which one do I do?

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Which one do I do?

Update (03/04/2023)

Okay I just extracted the CPU from the iMac to make sure that none of the pins in the socket were bent, and I noticed that the CPU itself got bent. I have done a lot of thermal paste/ CPU replacements and never seen this. I think it happed because I took the hsk off and the cpu came with it, and it got crushed. Could I use an i7-3770 or do I need a 3770s

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I just noticed that when the iMac power button is pressed, the fan spins to life for around two seconds and shuts off. Then, when you unplug it the fan spins again and stops. Don’t know if this will help me figure it out though.

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@ryan39167 - That’s a good sign of a POST issue. As you started off with a working system, I really would check the CPU is seated properly as well as check the socket pins. As if the CPU doesn’t respond SMC will crowbar the power supply.

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I do see one of the BT/WiFI antenna lines is not connected in your image. Can you post a few pics of the display cables both and both sides so we can rule them out.

I’m suspecting you didn’t seat the CPU properly and/or the socket pins got bent in the process.

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I know it is not the display because only one diagnostic LED will light up.

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@Ryan - Given you repasted the CPU you need to go bac and reseat it as it’s likely not in the socket properly.

I had pointed to the cable only as that was the easier first check. As if the cable is damaged you can cause this reaction as well, but if your sure then it’s the CPU not connected.

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Yeah, thanks! I realised that the whole CPU was bent lol

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@ryan39167 - Sorry to hear that 😩

If the socket is OK, you should be able to get a replacement CPU. You’ll need to stick within this systems series CPU’s as the systems firmware will only support them.

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So does that mean that I can’t use an intel core i7-3770, and have to use an i5, or something?

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Ryan first you want to go back and double (or triple0 check your work. Always make sure that everything is properly connected and back in place the way it should be. If you removed the CPU to apply new thermal past, check to make sure it is properly inserted etc. If you did not remove it, it'll be okay. Next, you want to check the power supply output. I know you already know this, but LED 1 coming on and LED2 not coming on is still non-specific. It can be the power board or the logic board.

If the second LED is not visible when the power button is pressed:

Verify that the power button is connected to the power supply.

Verify power button functionality.

Verify the cable connection between the power supply and the logic board.

Verify the power supply.

Verify the logic board.

Of course you already tried to reset the SMC and PRAM but, just in case:

Resetting the System Management Controller (SMC)

The System Management Controller (SMC) is a chip on the logic board that controls all power functions. If the computer is experiencing any power issue, such as not starting up, not displaying video, sleep issues, or fan noise issues, resetting SMC may resolve it.

Intel-based Macs: Resetting the System Management Controller (SMC).

Note for iMacs: If you press the power button while inserting the power cord, the iMac will enter a mode in which the fans run at full speed.

Resetting Parameter RAM (PRAM)

PRAM stores certain system and device settings in a location that OS X can access quickly. Exactly which settings are stored in the computer’s PRAM varies depending on the type of computer as well as the types of devices and drives connected. To reset PRAM:

1. Shut down the computer.

2. Locate the following keys on the keyboard: Command, Option (Alt), P, and R. You will need to hold these keys down simultaneously in Step 4.

3. Press power button.

4. Immediately press and hold Command-Option-P-R keys.

Important: You must press this key combination before the gray screen appears.

5. Hold down keys until computer restarts, and you hear startup chime a second time.

6. Release keys.

Right now it sounds to me like a bad power supply. See if you have 12V coming of it. That would correspond with LED #2

Measure this on the logic board connector where the power supply connects to:

Pin 1==> GND

Pin 2==> GND

Pin 3==> GND

Pin 4==> PP12V ACDC

Pin 5==> PP12V ACDC

Pin 6==> PP12V ACDC


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I have a votimeeter, but where do I connect negative?

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@ryan39167 To common ground when you measure pin 4,5, and 6. When you measure pin 1,2 and 3 place the red on a known +12 V pin.

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Does the iMac exterior work as ground?

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@ryan39167 yes that should work. Better yet, use one of the screwheads for either the power supply or the logic board, that have a metallic appearance

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[image|2881205][image|2881206]

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Ryan zal eeuwig dankbaar zijn.
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