Logic-probe the lamp columns and rows

Steps

• Put the game in the lamp test with F3 fitted, then unplug 2J5 and 2J7 to take the playfield out of circuit.
• Probe each 2J5 (column) pin: all should pulse constantly. A dead pin points to its TIP42 or 2N6427 pre-driver.
• Probe each 2J7 (row) pin: each should switch on and off about once a second. A dead pin points to its 2N6122 or 2N6427 pre-driver.

Systems

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

Related tips

• Diode-test the lamp column and row transistors
• A whole lamp row or column dead or stuck on
• Tester LED check of the lamp columns
• Tester LED check of the lamp rows
• MOSFET lamp column upgrade
• Light a lamp row by grounding its transistor
• Lamp matrix column test at 2J5 with Leon's code
• Lamp matrix row test at 2J7 with Leon's code

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