Occasional resets, weak flippers and dim lamps that bridge, capacitor and flipper work did not cure; AC inputs to the bridges read at the bottom of their range.

Resets, weak flippers and dim lamps from a failing DMD board

Cause: A marginal high-voltage section on the dot matrix controller board, or a worn out-gassed display, draws enough current to pull down every supply from the transformer.

Checks

• Inspect the big resistors and diodes beside the heat-sunk transistors on the DMD controller for overheating and darkened board.
• Try a known-good DMD controller board.

Fix

• Rebuild the controller board high-voltage section.
• Replace an out-gassed display.

The display power section produces roughly 100 V and more, and its capacitors stay charged after power-off.

Systems

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

Related tips

• Resets caused by a failing dot matrix display
• Flickering dots after repairing an older REV 6 DMD controller
• Occasional display glitches: run the display RAM test
• -125 V and -113 V read the same and the display is dark
• Vertical lines and garbage on the DMD mid-game: press the ASIC back into its socket
• What the WPC dot matrix controller does
• Dot matrix display blank or faulty
• Dot matrix display supply voltages

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