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.