Chosen Solution
I have a late 2015 iMac I purchased second hand, I upgraded to 512GB NVMe drive and I was installing Montery when the power went off. Now I’m getting no power on and no diagnostic lights.I have checked with two other power supplies which are known to be working but no cigar. The power supply is receiving power. Nothing was shorted and I can’t see any damage. What would have cause this?I’m dreading the quote from the tech shop that can do micro soldering
I would pull the drive to see if it is messing you up.
Could 100% be the drive. Take it out or put in the old one and see if you get a light. I will also note that since it is necessary on this model to pull the whole board out to put a blade style SSD in it could definitely be a board issue as well. In order for that LED to light, you need the PP3V3_S5 power rail to be functional, so there is some doing on the board that is necessary before that light will pop on, even if the Power supply is good. Most of the components involved in generating that power rail are along the bottom edge of the board. So it is plausible something in that area was knocked loose when you took the board out or put it back in. I would take a look in this bit of the board specifically. In case you can’t tell, this is the lower corner below the cutout near where the RAM slots would be on the flip side of the board. I would check both sides. That specific chip generates the rail, but there are some components on the back which are part of the line that tells this chip to generate this rail. So either could be at fault.
