The DMD picture rolls sideways, bounces or shows a wavy bar.

Wavy hum bar, bounce or roll on the display

Cause: Failing display power filter capacitors: C4 and C7 on the WPC-S and earlier dot matrix controller, C28 and C42 on the WPC-95 audio/visual board. Originals are 150 mfd 160 V.

Checks

• Inspect and test the two filter capacitors.

Fix

• Replace with 150 mfd 160 V, or 220 mfd 160 V (going up is fine, never down). Avoid physically large heavy caps whose weight can crack the pads.

These capacitors can hold a high voltage charge; discharge before handling.

Systems

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

Related tips

• Dot matrix display blank or faulty
• DMD fully lit with no picture while the game plays: bad DMD panel
• Moving vertical bars across the DMD with known-good display parts: CPU board fault
• WPC DMD shows garbage and a RAM address error: reseat the ribbon cables first
• Display shows garbage or nothing though the glass is fine
• Blank display, garbage or diagonal lines: reseat ribbon cables
• WPC display and DMD controller board versions
• Display intermittently garbled after new driver board and cables: the DMD itself

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