On power-up the CPU's diagnostic LED keeps flashing in long bursts and the board only occasionally boots, though a test ROM runs and the PIAs, RAM, sockets and game ROMs are new.

System 11B CPU flashes its LED and rarely boots: faulty 74LS139 at U12

Cause: The 74LS139 decoder at U12 had failed.

Checks

• Scope or probe the read/write and chip-select lines when the board fails to boot and compare with a good boot.

Fix

• Replace U12 (74LS139).

Systems

• Williams System 11B

Related tips

• System 11 CPU board won't boot after cleaning: worn ROM socket contacts
• System 11A-11C diagnostic LED flash codes
• Boot stops after the ninth LED flash: IRQ fault
• How to tell a System 11C CPU from an 11B
• System 11A-11C CPU LEDs: normal state
• Early System 11 CPU in a later game gives meaningless LED codes
• Leon test EPROM does not run: check the 6808 and its pins
• Which System 11 CPU boards fit which games

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