Odd display behaviour that comes and goes.

Occasional display glitches: run the display RAM test

Cause: A failing RAM chip on the dot matrix controller board.

Checks

• Enter diagnostics and run the Display test (often T.11); it should report no errors.

Fix

• Any page or data errors: replace the RAM on the controller board, or replace the controller board.

Systems

• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Dot matrix display supply voltages
• Dot matrix display blank or faulty
• Dot matrix display power connector pinout and voltages
• One or two display columns stuck on
• Resets caused by a failing dot matrix display
• Dot matrix display faults
• -125 V and -113 V read the same and the display is dark
• Resets, weak flippers and dim lamps from a failing DMD board

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)