In the all-lamps test a row or column lights twice per sweep; or in the single lamp test two lamps flash instead of one.

Lamp lights twice or two lamps act as one

Cause: A shorted or backwards lamp diode lets the lamp light from another row or column.

Checks

• Step through the single lamp test with the Start button and note any step lighting two lamps.
• Test the diode at the suspect lamp.

Fix

• Replace the lamp diode with a 1N4004, band the right way round.

Systems

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

Related tips

• Lamps in a row or column light twice per sweep
• Two lamps flash together in the single lamp test
• Test each lamp column driver with a test lamp
• Testing lamp-row drivers with a test lamp
• Testing lamp-column drivers with a test lamp
• One controlled lamp never lights, bulb is good
• Strange behaviour across a whole row or column from a switch diode
• Testing CPU switch rows and columns with a diode and clip lead

Source: Repairing Williams System 11 Pinball 1986 to 1990, Part Two, Clay Harrell (pinrepair.com)