I
finally got back to this and after a bit of looking found a service manual. It's strange.
As we left off you apply AC, the power supply comes up, the high voltage comes up and then the high voltage kicks out and you got a panel that runs totally blind with no error blinks. You got audio, you got a solid green light and you have working controls.
As it turns out that's not entirely true because what is happening isn't listed in the docs as a fault. If there's a power issue the front indicator will start cycling green and red rapidly and you have to power cycle it and hold the Input button on the front panel to reset the fault monitor. I never does that. The power supply is a Pioneer PS-250 and has its own smarts for fault monitoring on a daughterboard. When the high voltage drops out it starts blinking an LED code. (one long and one short before repeating)
Only if you unplug connector PD (note how one pin is ALARM and its also the larger DC voltages) will the panel fault out with a power issue. So the power supply is catching something that the rest of the panel doesn't see.
I have tried several salvaged power supplies now and they all do the same. The service manual does not have a schematic for this power supply and a separate schematic so far has not been found. Additionally, troubleshooting information for the high voltage side is missing from the service manual.