I have the exact same problem, at the same period, with a TZ and Dr who.
One row and one column on the SM are lit (squares) in test mode. I replaced the Dr who CPU with a new one, no change.
When the pin is on, if I remove the 4 SM connectorson the CPU (205, 206, 208, 212), the switch matrix in test mode does no longer show any square, I deduct the CPU is good.
When the machine is off, I remove the same 4 connectors, at boot it tells me check fuses f114, f115.
I received and read the litterature about the voltage sent by the driver to the CPU, my 12v is ok on Tp3 and 5vdc ok on TP2.
Some very focused support (no general articles or comment) would be helpful .
Thanks a lot in advance