Can you post a video of the MPU booting? Specifically we need to see the flashes and if the battery is still installed and if there is any past battery damage.

Are you counting the initial flicker as a flash? The LED should flicker, indicating it has power and the board is going to boot, and then flash once, pause, and flash 6 more times.

1 flash means there is a problem with the program chips. 6 flashes means the 43v is not present. An old battery is a ticking time bomb that blows up the entire reset circuit.

This exact same thing happened to me a couple days ago. A wire from the left coin chute switch came loose and was shorting against the frame. Unplug the front door and if the problem goes away that's what your looking for.