Check the battery blocking diode D17

Steps

• With batteries fitted, measure the unbanded side of D17 to ground: about 4.4 to 4.8 VDC.
• Measure the banded side: about 0.5 V lower, 3.9 to 4.3 VDC.
• Measure IC19 pin 22 (pin 8 is ground), or TP7 on System 6/7: about 3.9 to 4.3 VDC.
• Voltage on the unbanded side only: replace D17 with a 1N4148 or 1N914. No voltage on either side: fault is the batteries or holder.

Systems

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

Related tips

• Firepower 2732 change when IC15 is a 74139 (diode and pull-up variant)
• Test a playfield switch diode
• Battery power not reaching the RAM: diode D2
• Spare diodes for coils and switches
• Settings lost although the batteries are good (blocking diode D2)
• Settings are lost at power-off even with fresh batteries: bad battery diode D2
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks
• Bench-testing a -133 MPU: swap the R113 diode for a resistor

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