Finding a switch column shorted to a power circuit

Steps

• Remove U20 from its socket (or circuit) and power the game on with all switch connectors plugged in.
• Measure DC volts at U20 pins 18 down to 11, which are switch columns 1-8.
• Anything above 12 V means that column is shorted on the playfield to the flashers, solenoid power or GI.
• If U20 fails after a few minutes of running, unplug all CPU switch connectors and run 15 minutes: if U20 survives, the fault is on the playfield (often an opto board LM339); if it fails, suspect U14 (U23 on WPC-S/WPC-95).

Systems

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

Related tips

• Finding which switch is shorted to a power circuit (U20 socket probe)
• Switch columns to U20 pins
• Testing CPU switch columns with the switch levels test
• Clearing a switch flagged in the power-on test report
• Tracing a column of dead switches from the manual chart
• Testing CPU switch rows and columns with a diode and clip lead
• A whole switch column dead or all closed
• Switch matrix dead after a screwdriver slip under the playfield

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