Checking the CPU data lines and buffer U16

Steps

• Data lines are U15 pins 26 to 33; each should be active at about 2 to 3 V.
• A dead line with a good CPU means a short on the line: bend the pin out and see whether it now shows a signal.
• All data passes buffer U16, which takes inputs on pins 11-18 and drives outputs on pins 2-9. Signals going in but none coming out means U16 has failed; one or two missing outputs points to a short on those lines.
• Cut the shorted line temporarily to trace the chip causing it, then repair the cut.

Systems

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

Related tips

• Checking ground and +5 V on the CPU board
• Which System 11 CPU boards fit which games
• Running the CPU board on the bench
• Checking the CPU clock signal
• CPU RAM at U25
• CPU board test switches SW1 and SW2
• What each 6821 PIA on the CPU board controls
• Testing buffers U11 and U13 (74LS244)

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