The MPU LED comes on at power-up and stays on with no flicker or flashes.

MPU LED locked on with no flicker: likely causes

Cause: Damaged reset circuit (Q1, Q2, Q5, CR5, VR1, in the battery corrosion area); bad U6 program ROM; ROM jumpers wrong for the ROMs fitted; bad U9 6800 CPU; bad U11 PIA; bad sockets on U6, U9 or U11; open ground traces. The reset circuit and the sockets/grounds are the most common.

Checks

• Check ground continuity at U11 pin 1 and +5 V at U11 pin 20.
• Check U6 pin 12 for ground and pin 24 for +5 V.
• U9 grounds (pins 1 and 21) are less often affected, being away from the corrosion.

Fix

• Work through the reset circuit tests, then the chips and sockets, as in the following entries.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• MPU boots only some of the time
• Testing the MPU reset circuit at U9 pin 40
• Single reset parts that hold the LED on
• Game reboots at random: R11 heating Q5
• Quick test of reset transistor Q1
• Converting a -133 MPU to a -35
• MPU VR1 is an 8.2 V zener; the manual's "1N9598" is a misprint
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)