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Model A1311 / Mid 2011 / 2.5 & 2.7 GHz Core i5 or 2.8 GHz Core i7 Processor

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Not powering on whatsoever!

I tried SMC reset, opened up the 2011 21.5” iMac, reconnected the power cables.

I only see LED #1 turned on that indicates power. I reseated the CPU as well and even replaced the CMOS battery.

Doesn’t turn on - No fans spinning, no chimes / sounds and no display. What could the issue be? How do I find out what’s gone wrong?

A few questions:

  1. Could a bad GPU prevent it from turning on?
  2. Could a bad CPU be the issue?
  3. Will it turn on without HDD?
  4. How do I test the power supply unit and will the diagnostic led turn on in case of bad power supply?
  5. Could it be the Logic Board?




Update (02/11/2021)

@danj When I disassembled the mac's gpu, I found that the thermal paste leaked onto the chip. The heat sink is black on top. Is this normal?


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Answer this question I have this problem too

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Can you give us some back history. What happened, was it working and then stopped on its own? What kind of apps are you running.


@danj I am not sure about that, i forgot. Was it necessary?


@btwtg - Like a doctor we often need to understand what happened. Often it can give us a lead on what needs to be looked at.

I love this movie! Awakenings If you haven't seen it watch it.

Now getting back to your issue ... What does this movie tell you about the condition Encephalitis Lethargica that many doctors couldn't break through as they just had too little to work with. What did Oliver Sacks see that the others missed?

So... We need to have a better understanding of the cause to be able to know which way to go.



it was most probably turned on, running, no malware, default OS and it just turned off and didn't start after that . The fans don't spin / hdd doesn't turn on when I press Power button. I almost forgot about it until I recently found it lying in the loft. This happened more than an year ago. I know that its been long and I should have brought the issue out at that time but i wasn't interested in mac's.

So, how do I know where the problem is? I removed the HDD and replaced PRAM Battery.

No matter what, only LED #1 is lit.


@danj also, how do I test the Power Supply using multimeter?


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2 Answers

There are two directions here and I can’t tell you which way to go.

1 - The power supply or logic has failed

2 - The GPU board has shorted out

At this point you need to test the logic board voltages. You’ll need to get the schematics and boardview drawings for your system so you can check the voltages.

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so, how do i test the GPU?


@btwtg - checking the voltages


@danj and How Probable is each case?

Using a multimeter or do I take it to a 3rd party repair centre?

Are the logic boards and power supplys from ebay reliable (pre-owned ones)? What place do u recommend to buy from?

ALSO, If the PSU is bad , will the diagnostic led#1 turn on?


@btwtg - Lets try not to machine gun the running comments please. Use the edit function to append to your comment if you forgot to ask something.


Its a matter of your skills are you able to read schematics and understand analog and digital electronics? If not there is no shame in bring your system to someone with deeper skills.

I don't buy from Amazon or eBay as I don't want to support theft. I only buy from recyclers who make sure the system is clean and is traceable back. I would go to these folks The Book Yard UK

The PSU trickle voltage is only telling you you've got AC to the power supply. nothing more.


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Well you’ve got some work here!

You’ll need to carefully clean off all of the old TIM being careful not to pull off the tiny SMD devices also on the chip carrier. I would use a Rechargable spray can loaded with 85% or better isopropyl alcohol] Using the snorkel you can dig out the TIM that gets under the SMD devices (a strong portable shorting issue)!

You’ll need to get some fresh TIM onto the the raw GPU silicon chip without going over the edge. It doesn’t take much as you can see the provious application was way too much! The four VRAM chips will need new thermal pads as well. You really don’t want the gapping TIM (white thicker paste) as its just not very good. Carefully dry fit the heatsink and using a feeler gage or some thin strips of card stock measure the distance between the VRAM and the heatsink so you get the proper fit. Here is what I would use:

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what about the black on the contact surface of heatsink? is it normal?


After cleaning with isopropyl alcohol use a fresh pink pencil eraser (still soft) wipe the pins with the grain..


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BTW TECH will be eternally grateful.
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