On the bench only the 5 V LED lights; the board doesn't run, even with new ROMs, RAM, CPU and a known-good clock and reset.

System 11 CPU board won't boot after cleaning: worn ROM socket contacts

Cause: Several contacts in the game ROM sockets (U26 and U27) were bad, so address lines to the ROMs were open.

Checks

• Use a diagnostic test ROM and watch address line A8 (CPU pin 17) for the expected flash pattern.
• Remove the ROMs and let the processor run free; check each address line is toggling with a logic probe.
• Inspect the ROM sockets closely for damaged or corroded contacts, even on a board that looks clean.

Fix

• Replace the U26 and U27 ROM sockets.

Systems

• Williams System 11B

Related tips

• System 11A-11C CPU LEDs: normal state
• Early System 11 CPU in a later game gives meaningless LED codes
• Boot stops after the ninth LED flash: IRQ fault
• Which System 11 CPU boards fit which games
• Running the CPU board on the bench
• Checking the CPU clock signal
• CPU RAM at U25
• Which System 11 CPU boards swap between generations

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