The game plays normally for 10 to 15 games, then locks up in attract mode or during play until it is switched off and on. Later it stopped booting at all.

Random lock-ups in attract and in play: replace the soldered-in RAM chip

Cause: The original soldered-in AMI-brand RAM chip on the CPU board was failing. The 5 V supply measured steady even during a lock-up.

Checks

• Measure 5 V at the CPU board, including during a lock-up, to rule out the supply.
• Check the CPU and RAM configuration matches the board (a 6808 needs the extra RAM fitted).
• Look for old Scanbe sockets and tap the board with the coil fuse removed to find bad sockets.

Fix

• Remove the soldered-in RAM, fit a socket and install a new RAM chip.

Systems

• Williams System 6 / 6A

Games

• Tri Zone

Related tips

• Boots into audit mode every power-up with good batteries: bad 5101 RAM
• Which RAM chips go at IC13 and IC16
• Audit mode with good battery voltage

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