A screech starts at power-on and after each drain, stopping only when a game sound is triggered.
Stern SB-100 sound board screeches until a game sound plays
Cause: A failed 7402 at U1 on the SB-100 board.
Checks
- Swap or replace U1.
Fix
- Replace the 7402 at U1.
Systems
Games
Related tips
- Damage after the black and white plugs were swapped
- Right sling locks on at power-up and a pop is dead: bad 7402 at U45
- Coil fires when tested but not in game play: TTL driver chip
- Flippers and special coils all dead
- A coil or flasher comes on and stays on at power-up
- Diverter coils dead and flippers live at game over: 7402 at U50
- How a coil or flasher is switched: PIA to TTL to pre-driver to driver
- Special solenoid signal paths SS1-SS6 on the CPU board
Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)