Demo unit only powers on when plugged in.

My demo unit only powers on when the power cable is plugged in. It automatically powers on, but it shuts off as soon as I remove the cable. When I switch it off, it comes on again after two seconds. Battery charges normally until it's full. I checked the battery voltage, everything is normal. I tried with another battery - same problem, so it's not the battery what's causing this.

I guess Nintendo made some kind of modification to the mainboard. Maybe a jumper, maybe a chip that needs to be replaced, I have no clue.

I know that there are people out there with the same problem and I would really appreciate if someone knows how to fix it.