Narrowing a switch matrix fault to the CPU board, wiring or opto boards

Steps

• Have the game manual: its inside cover shows the switch rows and columns, wire colors and connectors.
• Enter the T.1 switch test and decide whether one switch, a row or column, or opto switches are affected.
• A single dead switch is usually the switch itself. Several in one row or column usually mean a broken daisy-chain wire under the playfield, killing everything downstream.
• For a whole row or column, or a ground row/column short error, unplug the playfield switch connectors from the CPU board and test the rows and columns with a diode and jumper.
• For opto switches, unplug the under-playfield opto board (and on Indiana Jones through Demolition Man the trough opto board) and see whether the switch test changes; reconnect them one at a time to find the bad board.

Systems

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

Related tips

• Switch matrix chips on the CPU board
• Telling a playfield switch short from a CPU board fault
• How the switch matrix reads a switch
• How the Williams switch matrix is scanned
• WPC switch matrix basics
• Testing CPU switch columns with the switch levels test
• Testing CPU switch rows with the switch levels test
• Checking the switch matrix with a logic probe

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