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모델 A1419 / EMC 3070 / Mid 2017 / 3.4, 3.5 또는 3.8 GHz Core i5 또는 4.2 GHz Core i7 Kaby Lake Processor (ID iMac18,3) / Retina 5K Display. 시스템이 매우 유사하므로 이전 iMac Intel 27" Retina 5K Display (Late 2014 & 2015) 안내서를 참고하십시오.

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CPU Upgrade Issue - No power

I decided to upgrade my 2017 iMac 27 in two parts this weekend: NVMe and SSD first, then CPU.

First part: 970 EVO NVMe with heatsink installed. Recognized @ Link x4 and great speed. SSD installed and MacOS Catalina reinstalled. Very happy so far.

Second part, a day after: Removed CPU, installed i7-7700k. Struggled a bit with heatsink like everyone it seems. But once done I felt confident that everything was done right, so I re-assembled everything. But the power switch didn't trigger anything!

  • LED 1 is on as soon as I connect the iMac to the external power cord.
  • Power switch cable seems intact and connected to PSU. PSU and logic board cables all disconnected/reconnected with great care and seem intact.
  • Tried to short both power switch pin to start up the Mac. Nothing happened.

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Does anyone have any suggestions or comments on what the problem may be?

  • I've ordered a new PSU for safe measure. I can't find a spare power switch button (the only part available that has it is an entire empty housing!).
  • Can't find a spare power supply control cable (between PSU and Logic board) for a 2017 iMac. But there are some for the 2011-2013 iMac which looks the same as for the 2017 version.

Tempted to order that. Does anyone know if the 2013 and 2017 use the same power control cable?

After discussing with friends, one plausible explanation could be that my CPU upgrade went bad. Such as static electricity frying the Logic board. Or the CPU is bad. Even though I've been careful all the way. However, if I understand the explanations regarding the logic board LEDs function, it seems that not even having the LED 2 or 3 on would mean my issue isn't yet entirely related to the Logic board or the CPU. Anyone can comment on this?

I would be very grateful for any suggestions (at least it'll boost my morale, as right now if the new PSU doesn't work, I would have probably to sell the iMac for parts!!)



Update (10/15/2019)

Just received the new PSU (see picture). Installed carefully (making sure I discharge any static electricity from my hands prior). Reconnected all wires, after inspecting them (see picture). And unfortunately I have the same result: the power switch, which seems to be 100% intact (see pictures), doesn’t trigger any response. Fan doesn’t spin. No second LED.

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Could it be the main power cable socket (buried under the bottom bezel of the imac housing)??

Any suggestions welcomed!

Update (10/15/2019)

Disassembled the Mac again. Look very carefully for damaged cables, or sockets… None found! Took a picture of the wires coming from the main power socket (see below). Everything looks fully functional.

Could this be a back Logic Board??

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Update (10/16/2019)

I can finally breath: Removed the new CPU, installed the old CPU and I’ve got the board to boot up. Did not perform a full reboot with the LCD on. I will make another attempt at upgrading tomorrow with another CPU, so I had only the PSU and Logic board mounted.

Is this a Bad CPU or it wasn’t mounted right, I can’t say for sure. But the Fan started to spin at max speed without stopping, so that got me thinking and decided to go for the i7-7700 instead of the 7700k!

Now, I got to improve my tension spring loading technique. I concur with others that the tension produced by this spring is excessive (Judging from the residue of thermal paste which had been completely squeezed out to the sides when I removed the CPU). So I’ve kept a more important gap between the mounting spring and the logic board, than what I had originally measured (<1mm). See pictures.

Hopefully my next post will be about completing this job… Wish me luck.

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Update (10/17/2019)

Final update: Finally got the system to work.

How: The short answer is you have to unbend the metal spring underneath the CPU!

There is way too much tension in this spring, this put so much strain on the CPU that it bends it. Some pins are then no longer connected. When the LED 1 or 1-2 lit up and nothing more it just means that.

I’ve tried a i7-7700k twice, reinstalled the i5-7600, then tried and kept a i7-7700 finally. So I’ve got plenty of trials to figure all this out.

I will now answer questions :)

답변되었습니다! 답변 보기 저도 같은 문제를 겪고 있습니다

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So does your logic board work with all 3 cpus? My logic board doesn't work now that I upgraded to an i7 4771 on my 2013 imac.

The issue is mounting the CPU is a bear! @sebmcadet issue was the tension of the spring and mounting the screws without distorting the CPU chip. This is where the method of tightening the screws is tricky! Just like replacing a tire you can't focus your effort on one screw screwing it in all the way before you do the others otherwise your wheel won't be mounted properly. You need to use a cris-cross pattern slowly tightening each screw down so the pressure is even as you go! Reducing the spring tension is a bit risky! The other risk is damaging the CPU chip as well if stressed

The other issue is having the CPU shift on you so you damage the socket pins!

i7-7700K works well on my 27" 2017 (I've replaced the i5-7500 3.4Ghz)

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선택된 해법

Sadly, you need to roll back your CPU to check your logic board. I suspect you either have a bad chip or in the process you damaged one of the socket pins or lastly, mis-aligned the chip.

해당 답변은 도움이 되었습니까?

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I was actually thinking of doing that. Just removed the Logic board and been staring at it for while.

Tightening the heatsink on this board was not trivial. Can the logic board pins be visually checked for damage, and what to look for? Mis-aligned seems impossible as you have two notches that would prevent it, correct?

--- Would love to have a bad chip. But 1. I may have touched the CPU's underneath and 2. When trying to screw the heatsink the spring plate underneath the board was sliding in all directions. Will post again when I re-install and test the board. Thanks for your prompt reply.

@danj Just posted an update above!

Reduce the amount of thermal paste! You only need a little bit so a film across the whole chip surface is covered.

Thanks Dan. Yes, I figured that out. It gets all squished to the sides everytime. So i've used less and less.

Having the exact same issue after installing 7700. Mac looks dead. Roll back to old cpu and system boots. How much did you end up bending the rear support spring and did you modify the screw down distance?


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I’m also having a similar issue, but I’m upgrading from a 7700K to 9900K. macOS DOES recognize it as a processor since it’s the same processor in their 2019 iMac, but I can’t get it to boot. I’m going to run some experiments and see if it’s the pins on the socket itself.

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Any update? Thanks!

The computer is still gutted, but I did some digging and while the SOCKET is identical from the 7700K & the 9900K, but the CHIPSET is different. Translation: While the i9 will physically fit, it won't connect. The pins mark to different readouts. I wish I knew this before, but the documentation on these iMac upgrades are extremely fragmented.

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Hi, I’ve already f**** up 2 CPU’s because of that stupid spring. An i7-7700k and the original CPU. How bent were the CPU’s and can you still use them, even if they are slightly bent? Not looking forward at playing another 200-300 bucks.

Thanks in advance.


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You also mention to bend the spring on the underside of the logic board. By how much?

I bent mine back just by using my fingers. Tried it fingers crossed, didn't work at first, was just about to start cursing when - with power on (don't try this at home) I just loosened the screws just another bit. And it turned on, would you believe it! So yes, it can be bent back. Use your fingers and look down the edges to see if they're straight, and then be very careful on installing it back if that's possible at all. Personally, I'd try next time to turn the spring around and see how that works out.

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