Every switch in one row or column acts strangely, or one switch never registers.

Strange behaviour across a whole row or column from a switch diode

Cause: Each playfield switch has a 1N4004 diode. A diode shorted closed upsets the whole row or column; one that goes open makes its own switch dead.

Checks

• Game off, meter on diode test: black lead on the banded end should read 0.4 to 0.6; reversed, no reading.
• If it tests bad in place, cut one leg free and test again before condemning it.

Fix

• Replace with a 1N4004 (1N4001 or 1N4002 also work), band facing the same way as on a working switch.

Systems

• Williams System 9
• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• Testing CPU switch rows and columns with a diode and clip lead
• Test the switch column drivers with a diode jumper
• Test the switch row inputs with a diode jumper
• Row or column fault on the playfield
• Testing a switch diode out of circuit
• False switch readings only when several switches are closed
• Lamp lights twice or two lamps act as one
• System 80 switch matrix diodes

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Three, Clay Harrell