A wavy hum bar, roll or bounce remains after replacing the main display filter caps, and may change as the game warms up.

Hum bar or roll persists: small HV filter caps

Cause: The small high voltage filter capacitors have failed: C6, C9 and C10 (.1 mfd 500 V) on the WPC-S and earlier controller, C29 to C31 (.01 mfd 200 V) on the WPC-95 audio/visual board.

Checks

• Inspect and test those capacitors.

Fix

• Replace them with the same values.

High voltage section: work with the game unplugged.

Systems

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

Related tips

• Dot matrix display with dark gaps or dim areas: outgassed glass
• Moving vertical bars across the DMD with known-good display parts: CPU board fault

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