What each MPU LED flash proves
Flicker: reset circuit, +5 V, U9 CPU, U11 PIA and U6 ROM working. Flash 1: game ROMs U1-U6 pass their checksum. Flash 2: U7 6810 RAM. Flash 3: U8 5101 RAM. Flash 4: U10 PIA. Flash 5: U11 PIA. Flash 6: U12 555 display interrupt timer (skipped on Baby Pac-Man and Granny and the Gators). Flash 7: +43 V zero-crossing seen. A missing flash means the next item failed.
Systems
Related tips
- Reading the MPU flash code
- MPU LED locked on with no flicker: likely causes
- What a good MPU does straight after the last flash
- MPU stops before the seventh flash: check the 43 V at R113
- Good MPU stays LED-on with a PC power supply
- MPU boots only when U9 pins 39 and 40 are shorted
- Bally/Stern MPU LED self-test: what to expect
- Game reboots during play from slam switch noise
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)