What the blanking signal does
Blanking is a watchdog line from the CPU to the driver board (interboard pin 37). The driver outputs are gated with it, so solenoids, CPU lamps and displays only work while it is high. A timer (556 on System 3-6, 555 on System 7) is reset by pulses from IC18 port PA2 every 0.8 ms while the program runs; if the program stops, the timer runs out and blanking drops. Low blanking is usually a symptom of sockets, PIAs or the interconnector rather than a fault in the blanking circuit.
Systems
Related tips
- Measure the blanking signal
- Add a blanking LED to a driver board
- Solenoid fuse blows at power-on and solenoids fire: driver board logic chips
- Slings and pop bumpers feel sluggish: backbox boards loose with missing mounting screws
- Slings and pops all burned at once: CPU lock-up from the 40-pin interconnect
- System 6 and System 7 driver boards can be swapped for testing
- Coil locks on and new transistors don't help: a failed 7408 on the driver board
- Kicker coil locks on again after a new TIP122: dead 7408 gate on the driver board
Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Two, Clay Harrell (pinrepair.com)