At power-on both CPU LEDs flicker, then come back on and stay on.

LEDs flicker then stay on: flipper or game ROM problem

Cause: A bad flipper ROM or game ROM, or its socket. LEDs that come on solid with no flicker point to a locked CPU instead.

Checks

• Reseat or replace the ROMs at IC17, IC20 and IC14 and check their sockets.

Fix

• Fit freshly burned EPROMs in new sockets.

Systems

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

Related tips

• CPU LEDs flash once, then come back on and stay on
• "Flipper ROMs" have nothing to do with the flippers
• Which System 3 to 7 games use which flipper ROM system
• CPU LEDs (or System 7 digit) never light at power-up
• CPU LEDs come on and stay on with no flash
• System 3 CPU board ROM layout
• How a good System 3 to 7 CPU behaves at power-on
• Why the CPU LEDs stay on when the program does not run

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