New batteries go flat within a few days, or heat up and leak.

Batteries drain in days or get hot

Cause: A shorted D17 lets the batteries power the whole CPU with the game off, and lets the board charge them with it on.

Checks

• With the game off, measure CPU IC1 pin 8 (+5 V pin); any voltage there means D17 is shorted.
• Out of circuit, on diode test: black on the band, red on the other end reads 0.4 to 0.6 V; reversed should read nothing.

Fix

• Replace D17 with a 1N4148 or 1N914.

Systems

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

Related tips

• Lights up but dead after storage: leaked batteries on the board
• Move the CPU batteries off the board
• Replace batteries of unknown age right away
• System 3 with no batteries boots into audit mode
• Battery holder looks fine but makes no contact
• Boots straight into the adjustments menu: corroded battery holder
• CPU will not boot after battery corrosion
• Clean battery corrosion off a CPU board

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