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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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All of the 4 Logic Board lights are green but screen does not turn on.

Hi, I wonder if you can help me.

I have a iMac 2011 21.5" and I have recently replaced the thermal paste on the CPU & GPU as it needed to be replaced.

All of the cables are attached properly as the 4 LED lights on the logic board are green when the iMac powers on.

The only problem is the screen is black and even when the computer chimes the screen never turns on, BUT the 4th LED on the logic board is green indicating the screen is connected properly.

What can I do? What does it mean?

Thanks

Update (03/23/2021)

Here is my GPU I can see a tiny amount of thermal paste in-between some of the SMD devises. Does it need to be isopropyl alcohol to clean effectively the GPU as don’t have that in stock?.

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The little paste that dribbled can easily get cleaned up. You'll need to carefully clean the GPU chip of its paste and being careful not to touch the gapping paste or best yet I would clean it off and replace it with thermal pads. The gapping paste is not as good as the the pads.

I'm more concerned on when you apply the fresh paste you don't over do it.

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Hi Dan, I have put new thermal paste on the GPU making sure I didn't over do it and still a black screen.

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Sadly, its looking like your GPU board has failed.

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Oh no, I have had the iMac powered on for 2 hours and the heat pipe is warm that connects to the GPU I don't know if it shows it's still working? It's a bit strange that the GPU has died when I opened the iMac, what might have caused that?

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ESD is one possibility. In the winter early spring the air is quite dry in the northern hemisphere with the cold.

You could have damages something in the process. It's also possible just kismet.

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What happens when you shine a flashlight into the screen?

Place the flashlight on the glass at a sharp angle, you should see the faint image of your desktop and its icons. If you do they your backlight logic need repairs which is why your onboard diagnostic LED's are all lit as the backlight is not monitored by the onboard diagnostics.

Update (03/22/2021)

You’ll need to open the system up and pull the GPU board off to clean the thermal paste off.

You’ve either over done the paste so now the SMD devices on the chip carrier has paste on them or you have a bad board.

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I’ve marked a few of the small SMD devices that often get covered with TIM which can cause issues.

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Hi Dan,

Thanks for your response, I have tried using a flashlight at a sharp angle and still nothing. I have heard online it might be the LVDS cable port or the screen but I was so careful on opening the screen as I have repaired another iMac before. What do you think?

Thanks

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@mrt100 - Lets check a few more things before we make that jump.

What does an external display show using a mDP or TB connection?

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@danj - I have tried connecting it using thunderbolt but it said "no signal".

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Then you know there issue is not the LVDS cable or the connector!

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Oh yes your right! Thanks Dan, I'll try that out, does SMD devises mean the different components on the Logic board? Would I need to replace the thermal pads on the GPU as well as what I did last time was to try and reshape the original thermal pads on the chips?

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