Checking the switch matrix with a logic probe

Steps

• Game on in the Switch Levels test with J212 and J206-J209 unplugged; power the probe from the board.
• Probe J207 pins 1-9 (columns): every pin should pulse. No pulsing means U20 is bad.
• Probe J209 pins 1-9 (rows): every pin should be high. A pin that is not high points to its LM339 (U18 for rows 1-4, U19 for rows 5-8).

Systems

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

Related tips

• How the switch matrix reads a switch
• How the Williams switch matrix is scanned
• WPC switch matrix basics
• Switch matrix chips on the CPU board
• Narrowing a switch matrix fault to the CPU board, wiring or opto boards
• Finding which switch is shorted to a power circuit (U20 socket probe)
• Using the switch edge test
• Switch 24, the always-closed switch

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