After a restoration the CPU board shows a steady +5 V LED but no blanking or diagnostic LED activity, even with only the power connector attached. Voltages, fuses and the reset line all check out.

Won't boot after restoration, new EPROMs fitted: try the original ROMs

Cause: The newly burned game EPROMs were not accepted by the board, even though they verified correctly on the programmer.

Checks

• Confirm +5 V and +12 V at the CPU and that the processor reset pin goes high after power-up.
• Put the original ROMs back in their correct sockets (use photos taken before the work to get the order right).

Fix

• Run on the original ROMs, then re-burn or replace the new EPROMs before trying them again.

Systems

• Williams System 11B

Games

• Black Knight 2000

Related tips

• Won't boot even with a new CPU board and new chips: the new ROMs were bad
• MPU won't boot even though a test ROM passes every PIA and RAM: bad game ROMs
• Game will not boot consistently: power-on reset (POR) circuit
• Resets continue after diode repair: power and connector checks
• CPU keeps resetting: reset line at U15 pin 40
• Occasional U10 PIA error at power-on, but the game plays
• "Adjust Failure" at power-on
• "IRQ Failure" at power-on

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