Dead, except for 3 blinks

When I plug in the charger, the red LED flashes 3 times, but absolutely nothing else happens. The 3rd flash is a bit shorter than the first 2. Nothing on the screen whatsoever.

The phone draws 0,3 A during the flashes, then drops to zero.

All kinds of reset procedures have been tried, so this is most likely a hardware failure.

Being an electronics technician, I still want to attempt to repair. Any hint on where to start looking? The microscope reveals no visual damage, and I have no schematics.