Telling a playfield switch short from a CPU board fault

Steps

• Unplug the four playfield switch connectors J206-J209 from the lower right of the CPU board; leave J212 in so switch 24 stays closed.
• If the error clears, the short is probably on the playfield (the CPU can still be at fault).
• If the error stays, the problem is on the CPU board.
• Confirm with the diode-and-jumper row and column test in the T.1 switch test.

Systems

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

Related tips

• Narrowing a switch matrix fault to the CPU board, wiring or opto boards
• Switch matrix chips on the CPU board
• Testing CPU switch columns with the switch levels test
• Testing CPU switch rows with the switch levels test
• Find the short before refitting a repaired CPU
• Testing CPU switch rows and columns with a diode and clip lead
• Whole switch rows read closed and U20 keeps failing: replace U23 on the CPU
• "Ground Short Row" message

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