The game resets when the knocker fires (and sometimes then shows random lines on the DMD until power cycled), with light static and a growing hum from the backbox and a glitching display. The lighting connectors are burnt.

Resets when the knocker fires, hum and DMD glitches: cracked power board joints

Cause: Cracked and cold solder joints on the power driver board, including one leg of bridge rectifier BR2, plus burnt GI header pins.

Checks

• Follow the usual WPC 5 V reset checks: test the bridges, filter caps and regulator.
• Inspect the joints on J101 and J102, the bridge rectifiers, F113 and the regulator under magnification.
• Look for burnt GI header pins and connectors.

Fix

• Reflow J101, J102, all bridge rectifiers, F113 and the regulator; replace burnt GI headers and connectors and stitch any burnt pads.

Systems

• WPC Dot Matrix

Games

• The Party Zone

Related tips

• Resets or dead banks of coils or lamps from cracked bridge or cap solder
• Resets remain after rectifiers, caps and connectors are done
• Resets and GI that will not dim after BR2 was replaced
• What a WPC game reset is
• Watching the supplies while the game resets
• Raising the WPC/WPC-S +5 V to about 5.15 V (last-resort reset fix)
• Resets continue although the bridge and diodes test good
• Resets: +5 V at the driver board test point is low

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