Hooked up all the cables, yet you had the jumper in place?
If you hook up all the cables, you actually REMOVE the jumper, then you test the P-on line first to see if it's jumping up and down voltage wise....then you test the BL-on to see if it jumps up and down.
The point is this. You jumpered your Power board. It stayed turned on. Your voltages were rock solid....unchanging. Your 18 was 18, your 10 was 10, your 5 was 5.
That's a good power board.
You should even see your LCD panel light up....not show a picture, but the lamps should light.
Then when you remove the jumpers and plug it back into the MAIN board....you monitor the Pon line first when you press the front power button.
If it's going Hi Lo Hi Lo Hi Lo....that's a bad main board.
The same thing MIGHT occur on the BL on line, but the P-on MUST stay HIGH or ON before the BL-on can even activate.
So P-On is the KING of TURN ON.....BL-On follows behind him...