Dead 5k iMac - anyone help?

[Ant84]

Hi guys,

Late 2015 Retina iMac 27 won’t power on at all. Was told there was a power cut and since then it wouldn’t boot up. I’ve removed the display and I’m looking at the bare machine. The first LED indicator on the Logic board lights up when power cable is connected and stays lit. Second LED indicator flashes for a second when the power button is pressed but the system won’t boot and the light immediately goes out.

I’m really unsure whether this is likely to be a board issue or maybe something PSU related or maybe some other component causing an issue.

Any suggestions welcome. Thanks.

 

[Journey]

Second LED indicator flashes for a second when the power button is pressed but the system won’t boot and the light immediately goes out.

I've only fixed a few of the 27" models from that era, but the symptoms you describe are that of the PSU, with an outside chance of the logic board being killed/damaged during a spike. Check the voltages and continuity on the PSU to the board when trying to power it on using a DVM/DMM, if they are reading nominal then re-seat the RAM etc. and try again.

You can also disconnect the SSD/HDD and other power drawing items that may be causing the fault if you have no luck, as one of these devices could cause the machine to not power on correctly. Make sure you've unplugged all peripherals as well, keyboard and mouse etc.

 

[Ant84]

I've only fixed a few of the 27" models from that era, but the symptoms you describe are that of the PSU, with an outside chance of the logic board being killed/damaged during a spike. Check the voltages and continuity on the PSU to the board when trying to power it on using a DVM/DMM, if they are reading nominal then re-seat the RAM etc. and try again.

You can also disconnect the SSD/HDD and other power drawing items that may be causing the fault if you have no luck, as one of these devices could cause the machine to not power on correctly. Make sure you've unplugged all peripherals as well, keyboard and mouse etc.

I replaced the PSU with a brand new one and the issue remained exactly the same. I’ve also re-seated the RAM and disconnected the mechanical hard drive (assuming it’s a Fusion drive so would also have a blade drive - which I haven’t touched yet).

So I’m assuming it’s a dead logic board?

 

[Journey]

So I’m assuming it’s a dead logic board?

Ugh, sounds very much like it could be. It's hard to diagnose a machine when it's not in front of you, and I could miss something really obvious when replying, but a spike may have blew a fuse on the logic board, or worse and fried some components.