The display passes the line test but goes cloudy when large lit areas or inverted graphics are shown.

Display looks cloudy on large or inverted graphics

Cause: Heat in the HV section: dried heat sink compound or loose MJE transistors, 5 W resistors out of spec, or cold solder joints.

Checks

• Check the three heat-sinked MJE transistors are tight.
• Measure the three 5 W resistors; more than 5% off means replace.

Fix

• Renew the white heat sink compound on the MJE transistors.
• Reflow the joints of the 5 W resistors, HV diodes and MJE transistors.
• If still cloudy, rebuild the HV section.

High voltage section: work with the game unplugged.

Systems

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

Related tips

• Random vertical or diagonal lines: 12 V missing at the display
• Repairing a missing display line with conductive epoxy
• One or two display columns stuck on
• Missing rows or columns on a pin-style display
• Blank display, garbage or diagonal lines: reseat ribbon cables
• Display misbehaves with good voltages: crystallised zener joints
• Dot matrix display goes blurry within minutes
• Vertical lines and garbage on the DMD mid-game: press the ASIC back into its socket

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