iMAc 27 inch late 2013 EMC 2639 no boot

[leovalpo]

No, still defective. Today I was unable to continue researching the defect, but I came to the conclusion that it is nothing related to the SSD, as I tried to boot with an external High Sierra SSD connected to the USB port.

You should try another power source.
Any experienced technician knows that these are not eternal and if the equipment has been operating with the original PSU for more than 5 years, it is very likely that it is the source of malfunctions, even more so if the iMac comes with a discrete configuration.
I bet iMac fails due to lack of power (typical failure).
Getting your hands on the motherboard is the last resort, if you don't know what you are doing, the motherboard can be completely unrecoverable.
If you are going to handle the inside of the iMac, wear thick rubber gloves, be careful of electrical shock, and remember to remove the AC cord from the outlet.

If you don't have another source to test, perhaps you can rebuild the original by replacing some components. If this is the case, write down the pcb number and send it to see if I have the schematic diagram for testing.

Greetings Colleague!

[Sergio Kub]

I don't have another card like this and I don't have another computer here like it.

[Sergio Kub]

Did you try CMD-S terminal mode? You need to power up without using the graphics card. I thought safe mode would do this, but it seems not.

I think that since you've re-installed MACOS, but it then crashed whilst starting up indicates that the graphics card is faulty.
I've seen some complete boards on eBay for around $40.

This iMac has a graphics chip integrated into the logic board and does not have a separate graphics card.

[pdblues2231]

it that the gtx775m video, that suck since I don't believe that it has any expansion slots.

[Sergio Kub]

An opportunity arose and I changed the power supply and the defect continued.

[Sergio Kub]

High Sierra External HD Boot

[Sergio Kub]

Using a USB stick with a High Sierra installer, the iMAc hangs on the Apple logo.

[Sergio Kub]

I can't boot with Linux, but with Windows 10 it Ok

[pdblues2231]

strange, are there any problems in the device manager?

[Sergio Kub]

On the Windows 10 screen I noticed that the image wasn't completely smooth... I think I'll do a reflow on the GPU

[pdblues2231]

at this point your option are limited, try the reflow perhaps, you will have success. if all else fails you may need a new board.