Problem: Power on gives white screen (lit) but no characters. no button combinations alter anything.
Remove four screws at rear and split the unit like a book, hinged by the ribbon cables, see pic below.
The problem was the battery pack, which has been removed in the pic above. Applying power made the components to the right of the battery bay (especially the choke - a cylindrical object,) heat up, due to trying to charge a couple of shorted cells.
The proof of diagnosis ( I think this is true of all units with internal rechargeable memory batteries) is to remove the failed battery pack and replace it with a large electrolytic capacitor. - The black cylindrical component lying at an awkward angle ---> below.
The capacitor makes the circuit think it's got a battery and the unit boots up and the screen shows the start menu.
Theoretically I could just use the unit like this and leave it on all Summer, but the correct thing to do is locate a cell-pack like this:
The circuit board near the battery bay has been damaged by either water or fluid from the failed battery. The board was simply washed with isopropyl alcohol and will be re-lacquered to protect it in future.