The System 11B CPU won't boot or blank, yet with a test ROM fitted all PIAs and RAM test good.

MPU won't boot even though a test ROM passes every PIA and RAM: bad game ROMs

Cause: The game ROM contents were corrupt.

Checks

• If the board passes with a test ROM and the problem follows into a second processor, suspect the game ROMs.
• If you have a programmer, compare the chips against the correct ROM images.

Fix

• Fit freshly programmed game ROMs for U26 and U27.

Systems

• Williams System 11B

Games

• Taxi

Related tips

• Won't boot even with a new CPU board and new chips: the new ROMs were bad
• Won't boot after restoration, new EPROMs fitted: try the original ROMs
• Game will not boot consistently: power-on reset (POR) circuit
• Occasional U10 PIA error at power-on, but the game plays
• Four beeps at power-up, GI only and no display: game has no batteries
• Displays lose digits and coils buzz once the game warms up
• Game will not boot after sitting off, but starts on the second try
• CPU keeps resetting: reset line at U15 pin 40

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