Two or more lamps come on at the same time when only one should.

Several lamps light together

Cause: A shorted lamp diode or shorted socket, often from changing bulbs with the power on and grounding the lamp drive.

Checks

• Test the diodes on the affected sockets.

Fix

• Replace the shorted diode or socket. Change bulbs with the game off.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Lamp numbers that are the same on most games
• A group of lamps is out
• No lamp matrix voltage
• Light a lamp row by grounding its transistor
• A whole lamp row or column dead or stuck on
• All controlled lamps out: bad spade connections on the bridge rectifier
• All matrix lamps out, everything else works
• Match and player lamps light at random in play: faulty replacement driver board

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)