The CPU board does not boot.

CPU will not boot after battery corrosion

Cause: Corrosion under the battery holder can short traces together (in one case address line A10 to chip select CS1).

Checks

• Inspect and test the traces under the battery holder for shorts and breaks.

Fix

• Remove all corrosion and repair the traces.

Systems

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

Related tips

• Move the CPU batteries off the board
• Boots straight into the adjustments menu: corroded battery holder
• Check the battery blocking diode D17
• Audit mode with under 3.6 V at the RAM
• System 3 with no batteries boots into audit mode
• Battery holder looks fine but makes no contact
• Lights up but dead after storage: leaked batteries on the board
• Game powers up in audit mode

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