Good +5 V, clean board, but the CPU does not boot.

CPU still will not boot after the basics

Cause: Most often the CPU chip at 3D has died; the RAM at 5D is the next most common.

Checks

• Try a known-good CPU and RAM.

Fix

• Replace the CPU at 3D with a 6802 (the 6808 is hard to find and either works).
• Replace the RAM at 5D (6264 or 2064C 28 pin, or 6116 24 pin).

Systems

• Data East CPU Version 1/2
• Data East CPU Version 3
• Data East/Sega CPU Version 3b

Related tips

• Battery voltage at the RAM but memory still lost
• PIA LED lights, goes out, then lights again
• U15 CPU and U25 RAM substitutes
• CPU chip types: 6800 on System 3/4, 6802 or 6808 on System 6/7
• No sound, only amplifier hiss with the volume up
• Replace a failed 6116 RAM with a 6264
• Electronic spares worth keeping for System 11 boards
• Test program does not run at all

Source: Repairing DataEast 1987 (Laser War) to Sega 1995 (Batman Forever), Part Two, Clay Harrell (pinrepair.com)