Thanks again.
I wasn't able to find a schematic.
I've had a look at the motherboard and as far as I can see there's one obvious problem, which is a mosfet, between the DC in and battery, that's clearly burnt. See here:
https://drive.google.com/open?id=1-oR2P4TZAn1AcNW9iRrahs-i9upuAlzB
Is...