Testing CPU switch rows and columns with a diode and clip lead

Steps

• Unplug the playfield switch connectors from the CPU board and enter the switch levels test.
• Columns: clip a lead to J209 pin 1 with a diode in its other clip, non-banded end in the clip; touch the banded end to each pin of J207. Switches 11, 21, ... 81 should register.
• Rows: clip a lead to J207 pin 1 holding the diode by its banded end; touch the non-banded end to each pin of J209. Switches 11-18 should register.
• With all playfield plugs removed the diode is optional.
• A dead row or column here means CPU U20 (or U14/U23); also look for battery corrosion (green parts) on the CPU board.
• If every row and column works, the fault is on the playfield, usually an opto board.

Systems

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

Related tips

• Testing a switch diode out of circuit
• Fitting a remote battery holder to a WPC CPU board
• Optos test good but still don't work: the opto board
• Switch diode failures and what they look like
• Testing a leaf switch diode in place
• "Check Fuse F114/F115" (F106/F101 on WPC-95): step-by-step
• Fitting a new switch diode the right way round
• Testing a microswitch diode without unsoldering it

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