Power Basics (Multi Mega)
Start by applying 9V DC into the jack of the power board and measuring for the following voltages with the power switch on and off (the pin 2/3/4 change).
The DC jack sends power through the fuse near it, and over to pin 9 of the output.
The DC jack power also goes to a 7805 LDO regulator, and into Q202 transistor (seen in the top right of the image above).
The 5V output goes to pins 5/6 and is a permanent feed.
The output of the 7805 goes into the Q202 transistor also, and the Q202 output sends the 5V to pins 3/4 when pin 2 is pulled to ground.
The power switch on the console has its off position pulled up to 5V, and when turned on sends that 5V to the base of transistor Q1, which in turn turns Q1 on, pulling pin 2 of the power boards pin to ground, which turns on Q202 on the power board, passing 5V into pin 3/4.
When the power switch is on, all 3 pins (On, Common and Off) sink to about 1V as the base of the transistor Q1 pulls the resisted 5V rail low and turns on.
In short, the power switch sends a pulled up 5V to Q1 which enables it and sinks pin 2 of the power board to ground, which is the base of Q202 on the power board, which sends the 5V from the LDO to the pins 3/4.
So, to check if the power board works, have it disconnected from the console, send a wire from pin 2 to pin 7 (Ground) and check if all your voltages are output (mainly the 5V on pins 3/4).
The pin 3/4 of the power board is the main 5V power of the Multi Mega and you can bypass the entire power board temporarily and the top PCB to just test if the bottom PCB on its own is generally functioning.
Simply inject 5V directly to pin 3 or 4 of the bottom board, and have your AV cable installed.
The bottom board should draw around 600mA with just the 5V rail powered up.
With this, you should get the console to boot to the BIOS screen without text (as the top board is not connected).