The usual causes of lamp problems

In rough order: a burnt bulb (check it for continuity), a wire broken off a socket or upstream in the chain, cracked header pins on lamp boards, a diode broken off a socket or cracked on a board, a corroded socket (reseat the bulb, bend 555 socket tabs, or replace the socket), a blown fuse when several lamps are out, a burnt driver-board connector (mostly GI), a failed TIP107 locking a whole column bright, and a shorted diode or bad TIP107 making two lamps act as one.

Systems

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

Related tips

• Testing a lamp diode
• Triacs switch AC lamps
• How the WPC lamp matrix works
• Testing lamp-row drivers with a test lamp
• The three WPC lamp circuits
• How the WPC lamp matrix is switched
• An entire lamp row or column is out
• How the lamp matrix drives a bulb

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