Finding which switch is shorted to a power circuit (U20 socket probe)

Steps

• With U20 already socketed, pull the ULN2803 out and turn the game on; ignore the test reports.
• Meter on DC volts, black lead on the side rail.
• Probe each pin of the empty U20 socket. Anything above about 13 V (18 V, even 70 V) means that column is permanently shorted to power; check its eight switches.
• If nothing reads high, hold the red lead on a socket pin and press each playfield switch in turn; a jump above about 13 V marks a switch physically touching another supply (flasher, solenoid).
• Repeat for every pin of the socket.

Systems

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

Related tips

• Finding a switch column shorted to a power circuit
• After a new U20, work every flagged switch once
• Switch columns to U20 pins
• Checking the switch matrix with a logic probe
• Clearing a switch flagged in the power-on test report
• Switch matrix dead after a screwdriver slip under the playfield
• Row or column "ground short" errors
• Using the switch edge test

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