Score displays stopped working after the CPU board was taken out and refitted.

Displays dead after the CPU board was removed

Cause: A display ribbon cable plugged in one pin off.

Checks

• Look at each ribbon connector on the CPU board; the red stripe must sit at pin 1.

Fix

• Re-seat the cable correctly; normally nothing is damaged.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• Same character position dead on every display: digit strobe wire at MPU J2
• System 9 versus System 11 display data
• UDN display driver chips
• Characters missing from a display
• Display board chips shown as 4050 are really 4049
• Telling a shorted UDN chip from a shorted display glass
• Missing or stuck segments from a bad SRC pack on the CPU board
• Same segment missing across one 16-digit display

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Three, Clay Harrell