What the opening flicker tells you
The flicker shows the CPU came out of reset with good +5 V and a two-phase clock, fetched its start address and began running the self-test in U6. If the LED locks on instead, suspect U6, U9, U11, the reset parts (Q1, Q2, Q5, VR1), C23 or Q20 on the solenoid driver board, or corroded traces between them.
Systems
Related tips
- MPU LED locked on with no flicker: likely causes
- Single reset parts that hold the LED on
- Good MPU stays LED-on with a PC power supply
- MPU on the bench always stops one flash short
- Bally/Stern MPU LED self-test: what to expect
- Reading the MPU flash code
- Testing the MPU reset circuit at U9 pin 40
- MPU U9 voltages that show a working reset section
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)