If you momentarily short pins 39 and 40 of the CPU together does it proceed to boot? If so, the power supply is probably not stabilizing quickly enough and the reset circuit is locking up the MPU.

There are a couple of things you can do:
1. Rebuild the SDB. Replace the caps, the header pins (especially the .100s) and repin the connectors. Also repin connector J4 on the MPU.
2. Replace the reset circuit components with a Dallas reset generator. There are several varieties available which hold the CPU in reset for a specified amount of time before allowing the MPU to boot. I think I use a 350ms chip whenever I need to rebuild a reset circuit and it's always worked for me. IIRC, the factory circuit holds the CPU in reset for 150ms, which is not much time. Using the Dallas chip you can expand that window, allowing for a marginal power supply to stabilize.

Are you getting the correct voltages on the MPU?

Have you installed newly programmed EPROMs on the MPU board?

Same result whether from a power up -or- pressing the reset button?

if you are only getting 11v dc on the 12v rail the filter cap is probably bad or the isolated ground return wire has resistance. It is usually more like 15v-16v with the cap in circuit and 11v with the cap out of circuit. A tenth of a volt drop on the 5v supply is a bit higher than what you'd expect on fresh connectors.

With a multimeter set on low range volts AC, measure across the 12v filter cap. More than about a quarter of a volt of AC is a sign there is excessive ripple. The blue film wrapped capacitors are probably all starting to go bad. The capacitance value might check close to right but they are likely leaking DC and has high ESR.