Testing CPU switch columns with the switch levels test

Steps

• Backglass off, display lowered. Turn on and go to the Switch Levels test.
• Unplug J212, J206, J207, J208 and J209 from the bottom of the CPU board.
• Clip a lead to J209 pin 1 (the rightmost pin). Optionally hold a 1N4004 in its other clip with the banded end out; not needed with all playfield plugs off.
• Touch pins 1-9 of J207 in turn: switches 11, 21, 31 ... 81 should show closed. Columns 7 and 8 may be unused on some games.
• A column that never shows, or shows closed with nothing connected, means replace U20 (ULN2803).

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
• Switch columns to U20 pins
• A whole switch column stops working: replace the ULN2803 column driver at U20
• Why CPU U20 fails and how to replace it
• Row or column "ground short" errors
• WPC-89 CPU: one switch column still dead after a short: trace U14 to U20 to J206
• One switch does not work
• Switch matrix dead after a screwdriver slip under the playfield

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