If the 5v is dropping to zero on the driver board when the MPU is connected, then I'd think there is a short making the regulator shutdown.

If the MPU works on the bench then seems like it would be an issue with the connectors. Make sure the board can't short against the mounting rail or backbox ground shield in some way. Can check resistances from 5v to ground with the j4 connector on and off.

The isolated ground returns can throw off the meter readings depending on where you are probing at. There is an orange wire between the driver board top right connector and the rectifier board 20 pin plug that is very often a cause of low voltage. The schematic labels it "unreg return" or something like that.

Any chance these solenoid driver boards have had the 5V regulators changed for Chinese counterfeits? They can cause your symptoms being under-spec parts masked as something else.
Post some pictures of the solenoid driver boards and the MPU board.

You've got a burned resistor above Q5 on the MPU.

C23 & C26 appear to be original. Check the ESR or just change them - they are over 40 years old.

Wait for others to chime in

that last pic of a driver board has a 3 pin capacitor, can't tell if the wires are soldered to the correct legs or not, also purely my thing is i'd be swapping it out for a 2 pin capacitor.

with J4 not connected what voltage do you have at pins at 16 & 17 in the connector.

The regulators look genuine originals.

The driver board has been repin'd. I'm always suspicious of rework i didn't do. Check the wire colors to the wiring diagram page for the top right connector on the driver board. Same with lower left on the mpu.

Burnt resistor is for the 5101 ram power. I think it's 82 ohm 2w resistor.

What resistance do you measure between the ground and 5V test points on the MPU board? (measure this without any connectors hooked up).

You can see it's marked as "NC" meaning it's not connected. Likely just an extra pin to help anchor onto a PCB.

and there is the oversized fuse in the last picture, that the top cap would barely be making connection.

i believe the original fuse size isn't available but the top clip can be moved.

some of the 5x20mm fuse holders with single leads on each end will fit almost perfectly. pretty good option.

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That infers that there is a ground and 5V wire crossed over at a connector.
There are two ground wires and two 5V wires from the solenoid driver board to the MPU board. Inspect the cable colors:

SDB J3-14 grey-brown wire to MPU J4-16 (+5V)
SDB J3-15 white-green wire to MPU J4-17 (+5V)
SDB J3-18 grey-green wire to MPU J4-18 (ground)
SDB J3-19 black-red wire to MPU J4-19 (ground)

EDIT: Ooops, I didn't look at the picture close enough, I answered my own question.

I had +5V and ground listed on the wrong wires so corrected it in the above post - thanks for the heads up Rikoshay