MPU is locked up which can be from any large number of causes.
Visual inspection always good starting place. No IC chips with a folded over leg. Blown out parts. Deep gouge breaking a trace. Solder splash....
Check at the CPU chip reset and clock. The clock circuit fails fairly often in the original boards and will lock up the MPU.
Rotate in known good ICs. Verify the ROMs.
After that stuff it gets more technical, but probably how I would approach it.
So many locked up boards have magically started to work after I replace all IC sockets, rejumper to 2732, and install new roms. Not really something I would recommend tho unless you are well practiced at working on PCBs. 40 years later the IC sockets which may not have been good to begin with are known to be unreliable and a pain to diag. The IC socket may start to conduct from the pressure on the chip leg/dmm lead.
You have some voltage drop on the 12v compared to the driver board (14.5) and MPU (13.4) Should not be bad enough to stop the MPU from booting up but may hint at connector issues.