Ok, it looks like the main board isn't firing the inverters up at all. The plug that you were testing. It comes from the power supply board doesn't it. So it could be the main board or the power supply board dropping the Power on command. I know the main board doesn't mark the Pow_On line....Follow the cable back to the board it connects to. Look around the plug. On the board Sanyo usually marks the Pon or it may be labled as INV on (inverter on)
We need to determine if the Main board or the Power board is killing the power on line to the inverter.
We got our work cut out for us.
Right about now, I could use a picture of your entire chassis, a closer picture of your power board and one on your main board.
Let me see if I have an example of the reference or legends printed on some of the boards. Nope, the Sanyo markings are too basic to see an On/off line marked on them.
Ok, so this is what we'll have to do. Process of elimination.
The main board has one cable that connects to the power supply board. It's going to have the 5 volt standby, and a 12 Volt line or a 9 volt, or it may be 15. We will be looking for a 3.3 Volt or 5 Volt line that goes High and Low when the power button is pushed. You will find the line on a 10 wire cable connected between the Main board with all the input jacks, and the power supply board. You can eliminate any lines that provide the standby 5, the 9/12/15 lines, and of course the ground lines too. Anything else is CONTROL lines turning on the power supply/inverter or feeding back any inverter OK line. Look for one that fires up 3 times
Right now, I suspect the power supply board more than the Main board, simply because the power supply is more notorious for failures, but I like to check it as much as possible.