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Model A1419 / EMC 3070 / Mid 2017 / 3.4, 3.5 or 3.8 GHz Core i5 or 4.2 GHz Core i7 Kaby Lake Processor (ID iMac18,3) / Retina 5K display. Refer to the older iMac Intel 27" Retina 5K Display (Late 2014 & 2015) guides as the system is very similar.

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Why is it not turning on?

I upgraded the CPU in my iMac to the i7-7700K and everything went pretty smoothly. I put it back together and tested to see if everything was working. I plugged it in and pressed the power button and the fan came on but the screen did not. I double checked that all the cables were plugged in and looked things over. When I put it back together the second time and plugged it in only the 1st diagnostic led turned on and nothing else. Now after putting the original CPU back in the first two leds are on along with the fan. The fan spins at full speed and the display is still black. What can I do to fix this?

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Put the orginal CPU back in, does it work then?

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I had already tried that but it didnt do anything but I just swapped it back the original again and now the second led is on too but nothing comes on the screen and the 3rd led is still off.

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Are you sure you correctly mounted it in the carrier?

What thermal paste did you use?

Did you use a ESD mat and wrist strap?

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I have double checked that the CPU is mounted correctly which is why the second led is now coming on. I used the artic silver thermal paste that ifixit sells and I carefully followed the guide. I did not see a mention of an ESD mat in the guide so I did not know to use one.

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At this point I think you buggered up the logic board somehow. You'll need to bring it into an Apple Store or authorized service center to let them fix it at this point. The logic board is not available on the open market, only as a spare.

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Mitchell Rudoni will be eternally grateful.
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