Hi, I have a similar problem with 3 different boards:
* 2 ea 820-00292 Imac 5K 2015 boards
* 1 Imac 21.5” 3.0Ghz GPU-555 2017 board
All 3 have no LED at all.
Checked voltage from PSU and it’s 12V OK
Tried 3 different PSU’s and all the same, and the PSU’s work with a normal board. SO ruled out PSU.
Checked voltage on first LED and 0.5V.
So some short in the trickle power circuit?
Any suggestions appreciated.
Am a newby on board level repairs but am interested to learn.
Thanks,
Ron