One or more lamp rows stay lit.

Lamp rows stuck on: the 7406 at IC19

Cause: Most often the 7406 at IC19 on the driver board, which handles six of the eight rows (IC12, also a 7406, handles the other two), rather than the row transistors. The row 7406s fail far more than the column 7408s.

Checks

• Diode-test or logic-probe IC19 and IC12.

Fix

• Replace the 7406; socket it.

Systems

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

Related tips

• Lamp matrix row test at 2J7 with Leon's code
• Damage after the black and white plugs were swapped
• Switch matrix column test at 2J2 with Leon's code
• Switch matrix connector pins and their chips
• Lamp row still dead after a new TIP122
• A whole lamp row or column dead or stuck on
• Tester LED check of the lamp rows
• Diode-test the lamp column and row transistors

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