I checked all primary ICs voltages: always 0.
PowerOn signals goes into a custom card, which seems to act as a controller.
nearby there's a current sense and other stuff.
Then, output signals from this custom card seem to go into the optocoupler,
to the primary side. No voltages around the optocoupler...
I begin to suspect the custom card.
But I can't desolder it. It's in a bad place, I need to desolder other stuff and
only with a small desolder pump ...