Bally/Stern MPU LED self-test: what to expect
The green LED next to the battery reports the MPU self-test. A good board gives a brief faint flicker, then flashes once per passed test: seven flashes in all (six on Baby Pac-Man and Granny and the Gators), after which the LED stays dimly lit while the game strobes the switches. The count stops at the first test that fails.
Systems
Related tips
- What each MPU LED flash proves
- MPU LED locked on with no flicker: likely causes
- Testing the MPU reset circuit at U9 pin 40
- 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
- MPU locks if switched off and quickly back on
- Game reboots during play from slam switch noise
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Two, Clay Harrell (pinrepair.com)