I wrote:
Playing with the machine some more last night, the power off pin is
definitely being set low, so it looks like it is the servo chip that's
broken. I now have one on order (I'm amazed they are still available).
All that's left is to hope I can remember how to use a soldering iron!

For the benefit of anyone googling for this subject in the futu

Yes, it was the servo amp ic (IC601, CX20084, on the corner of the
board next the battery compartment) that was broken. Replacing it
has fixed the problem.

I had some private correspondence with people who serviced this model
in the past. Their recollection was that it was usually this ic that
broke if power was connected with the wrong polarity.

In addition, I discovered that the ground of the external power-in is
connected to the system ground only by a black-covered wire. A diode
can be placed in series with the wire (cathode to the pcb, anode to
the wire) and will protect against wrong polarity. It is not even
necessary to cut any pcb traces. The diode is not in circuit when the
machine is being powered from battery and so it will have absolutely
no effect on battery life.


-jonathan

--
Jonathan H N Chin, 1 dan | deputy computer | Newton Institute, Cambridge, UK
| systems mangler | tel/fax: +44 1223 767091/330508

"respondeo etsi mutabor" --Rosenstock-Huessy