Settings lost even with good batteries and holder.

Battery power not reaching the RAM: diode D2

Cause: Blocking diode D2 (1N4148 or 1N914) has failed. It sits beside D1 (1N5817): right of U9 on WPC-S and earlier, just below the battery holder on WPC-95.

Checks

• Game off, new batteries, black lead on the backbox ground strap.
• The non-banded side of D2 should read about 4.3 V and the banded side about 0.5 V less.
• Voltage on one side only, or the same on both, means D2 is bad.
• Then check U8 pins 26, 27 or 28: about 4.3 V with the game off (pin 28 is opposite pin 1, which is marked by a dot).

Fix

• Replace D2 with a 1N4148 or 1N914 (a 1N4001 works in a pinch).

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• New CPU batteries go flat quickly
• Settings are lost at power-off even with fresh batteries: bad battery diode D2
• Settings lost although the batteries are good (blocking diode D2)
• Check the battery blocking diode D17
• Fit a remote AA battery pack with a blocking diode
• Battery or memory cap won't hold settings
• Test a playfield switch diode
• Fitting a remote AA battery pack

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Three, Clay Harrell (pinrepair.com)