A new replacement CPU board won't boot, even with everything but power unplugged and the 5 V close to spec (about 4.9 V at the test points).

Brand-new replacement CPU board won't boot: press the socketed PIAs home

Cause: The socketed PIA chips weren't fully seated.

Checks

• Confirm the jumpers are set for the game and the ROM, CPU and RAM are fitted the right way round.

Fix

• Press firmly on the socketed PIA chips (power off) to reseat them, then power up.

Systems

• Data East/Sega CPU Version 3b

Games

• Baywatch

Related tips

• A row of lamps stays on solid, works if the plug is pushed sideways: repin the header
• CPU board PIA positions and jobs
• CPU still will not boot after the basics
• Program runs but the PIAs never respond
• Other CPU jumpers and what they set
• Bench-testing a CPU board on +5 V
• CPU boards are only downward compatible
• CPU EPROM jumpers J4 and J5

Source: Pinball Repair help group!! (Facebook group)