Same problem here, also by water damage. PSU doesn't come up.
Everything looks ok except an apparently damaged (open) diode inside the PSU IC (VD1 in the attach). I bypassed it with an external diode but it still doesn't "convert". Voltage gets there. Anyone with further information?
I found the internal schematics of MA2810 and I assume the MA3810 is similar just different power handling (similar datasheets; datasheets are incomplete, no pinout nor internal diagram

), (reverse enginneered) schematics makes sense with such internals, see MA2810 internal diagram attached. Can't find any manual, schematics, whatever for this A/C model; can find for the newer models but not for these older ones

Unfortunately this PSU has also several outputs with other voltages and opto couplers and other stuff, I haven't looked into it in detail, which doesn't make the task of replacing it with something "off board" easy.
By the way, I have fixed this board before (another totally different problem, track corroded), and it can be powered up by 62VDC into the mains AC input, so it's an easy test. I didn't try lower voltages to see what's the minimum. Plug-in the LEDs board's 2 connectors, as one of them (the big red one) is for a mains fuse which lives in the LEDs board. The board consumes ~30mA with nothing else connected.