In these pictures it appears that the large power plug with the blue wires, on the LPSU side, is plugged in shifted one pin to the left.
By my logic this would have created a reverse polarity condition and fed a positive high voltage (24v) to the optoisolator in question and ground. Not good.
Can you meter the the board?
Please and check continuity (if short exists) with the board powered off. Have the SD Card removed from the board entirely and no cables connected to it.
In this diagram I show:
- Ground (Gnd) with Black Square
- VMOT Rail with Red Square
- 5v with the Green Square. Touch it with the probe from the right so you do not touch any other pin, and be careful not to touch the SD card casing.
- 3.3v with the Orange Squares - either on the SERIAL Header pin #2 from the left, or the head of the AMS1117 component - whichever you can access easier.
Set the meter to continuity mode to check for shorts at these 3 locations:
VMOT - GND
5v - GND
3.3v - GND
If there is a short between any of these 3 rails please let me know and do not proceed.
If there is no short, please plug the power brick into the board, observe if any LEDs turn on, and do the following voltage measurements:
With the board powered on, and your multimeter in voltage measurement mode (mine has maximums of 2, 20, 200v and I choose the 200v mode for this) please place and keep the negative probe on Ground.
Place the positive probe on VMOT. You should see a steady 24v.
Place the positive probe on 5v. You should see a steady 5v.
Place the positive probe on 3.3v. You should see a steady 3.3v
For each of those, please tell me what happens.