Late 2015 iMac 27" A1419 Not Booting After CPU and SSD Upgrade

[le9508]

I recently tried upgrading my Late 2015 17” iMac to an Intel i7-6700K CPU and an NVMe SSD. However, when I try to turn on the computer, three weird things happen. First, I do kind of hear the startup chime, but it is very distorted. Second, the main display remains black and does not turn on. Third, the fan ramps up and starts spinning at full speed.

The following video shows all of these problems

https://drive.google.com/file/d/1M3suMFfTo_j6DvDRldxemxEBWXunfFqE/view?usp=sharing

I took off the screen and on the main logic board, the troubleshooting LEDs #1 and #2 turn on normally. LED #3 also turns on, but is a little fainter than the other two. LED #4 does not turn on, but this is normal since I removed the screen.

I reconnected and disconnected the speakers, but this had no effect at all. Strangely enough, removing some of the ram did reduce the distortion of the sound. The next thing I am going to try is to take the whole thing apart and reseat all the connectors. I think that the CPU should be fine since LED #2 turns on and I hear the startup chime.

I tried searching for this issue, but it seems that nobody has experienced these exact symptoms yet. Any help or insights would be greatly appreciated!

 

[Nguyen Duc Hieu]

I recently tried upgrading my Late 2015 17” iMac to an Intel i7-6700K CPU and an NVMe SSD. However, when I try to turn on the computer, three weird things happen. First, I do kind of hear the startup chime, but it is very distorted. Second, the main display remains black and does not turn on. Third, the fan ramps up and starts spinning at full speed.

The following video shows all of these problems

https://drive.google.com/file/d/1M3suMFfTo_j6DvDRldxemxEBWXunfFqE/view?usp=sharing

I took off the screen and on the main logic board, the troubleshooting LEDs #1 and #2 turn on normally. LED #3 also turns on, but is a little fainter than the other two. LED #4 does not turn on, but this is normal since I removed the screen.

I reconnected and disconnected the speakers, but this had no effect at all. Strangely enough, removing some of the ram did reduce the distortion of the sound. The next thing I am going to try is to take the whole thing apart and reseat all the connectors. I think that the CPU should be fine since LED #2 turns on and I hear the startup chime.

I tried searching for this issue, but it seems that nobody has experienced these exact symptoms yet. Any help or insights would be greatly appreciated!

If you put back the old CPU and the system booted, then it definitely the i7-6700k caused the problem.

 

[le9508]

If you put back the old CPU and the system booted, then it definitely the i7-6700k caused the problem.

You’re right, it has to do with the CPU.

I put the original i5 back in and got the computer to boot. I also managed to get the i7 to boot after reseating it, so I think it was just a matter of the CPU not making contact with the socket somewhere.

 

[le9508]

Update: I managed to get the computer to boot with the new processor, but only when the SATA cable is unplugged for some reason. The startup chime is still distorted. I really have no idea what the cause could be, this is so strange...

 

[le9508]

FINAL UPDATE: It turns out that the CPU I was using was defective. I returned that one and bought another i7-6700K. Everything is working perfectly now, including the SSD upgrade.