Tracing a column of dead switches from the manual chart

Steps

• From switch test and the manual, list the dead switch numbers; usually they share a row or column.
• Read the column's wire color and CPU pin from the top of the switch matrix chart (the guide's example: column 4, green/yellow, J206/J207 pin 4).
• Check every pin of J206/J207 (rows: J208/J209) makes good contact. These are .100" connectors, originally IDC; replace a damaged one with a crimp-style .100" Molex connector.
• Under the playfield find the end of the chain: the switch with only one wire of that color, usually nearest the player. Check its wire, diode and diode wiring.
• Follow the wire back switch to switch; a break is the most common cause.
• If nothing is visibly broken, buzz the wire from the CPU connector pin to each switch; the wire can be cut inside its insulation where the playfield pinches it.
• If the wiring is good, check continuity from the first switch to the U20 pin for that column (column 4 is U20 pin 15); if that is good too, replace U20.

Systems

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

Related tips

• A whole switch column dead or all closed
• Testing CPU switch columns with the switch levels test
• A whole switch row or column is dead
• Finding a switch column shorted to a power circuit
• Testing CPU switch rows and columns with a diode and clip lead
• A whole switch row dead or all closed
• Switch matrix dead after a screwdriver slip under the playfield
• Row or column "ground short" errors

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