Credits and high scores are lost at power-off and the displays show garbage even after clearing settings, though the RAM chips get 5 V on pin 22.

Stern MPU-200: memory won't save and displays show garbage: two 5101 RAMs

Cause: U8 and U13 must both be 5101 RAM. One had been replaced with a 2101, which is pin compatible but not meant for battery backup, and one chip had been fitted backwards.

Checks

• Check that U8 and U13 are both 5101 chips and both face the right way.
• Old AMI-brand 5101 chips are often unreliable.

Fix

• Fit two new 5101 RAM chips (a matched pair) and a fresh battery, or an NVRAM adapter.

Systems

• Stern MPU-200

Related tips

• Power and ground pins of the main MPU chips
• Stern M-200 MPU jumper settings by ROM set
• MPU LED gives only two flashes at power-up: replace the 5101 RAM chips
• Stern M-200 occasionally will not boot and the MPU reset button does nothing
• Stern M-200 game fires coils and counts displays at power-on
• M-200 flashes seven times but the game misbehaves or repeats one number in audits
• Stern games from 1980 on need an M-200 MPU
• 5101 RAM speed grades and which MPUs need what

Source: Pinball Repair help group!! (Facebook group)