CPU board will not run; data bus activity stops at the buffers.

Data buffer check: 8T28s at IC9/IC10 (System 3 to 6)

Cause: A failed 8T28 bus transceiver at IC9 (D4-D7) or IC10 (D0-D3).

Checks

• With a logic probe compare the CPU side and bus side of each buffer: pins 2/4 with 3, 5/7 with 6, 9/11 with 10, 12/14 with 13.
• IC9 carries D4-D7 (IC1 pins 29, 28, 27, 26); IC10 carries D0-D3 (IC1 pins 33, 32, 31, 30).
• System 7 uses one 74LS245 at IC9 instead; compare pins 2/18, 3/17, 4/16, 5/15, 6/14, 7/13, 8/12 and 9/11.

Fix

• 8T28s are hard to find. On pre-System 6A boards with EPROMs (no 3624 ROMs), bypass them (see the next entry). On System 7 replace the 74LS245.

Systems

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

Related tips

• A System 7 CPU board can run any System 3 to 7 game
• Which CPU boards can run which system firmware
• Dead CPU: the order to replace chips
• Check the CPU address and data bus for shorts
• Run the System 3-6 built-in CPU test (SW1)
• Reflowing surface-mount chips on the switch matrix board
• Identifying a System 3 to 7 CPU board
• What changed on the System 6A CPU board

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