Quoted from barakandl:

Lets get the core idea out there of what the blanking does and how it works.
The blanking signal is used as a safety protection to make sure the computer is running. One of the display PIA outputs also goes to a timer IC. The way the timer is set up, as long as the display PIA signal going to the to the timer input toggles high/lowfast enough (the time interval is set by a cap/resistor values) the timer output (the blanking) will stay HIGH.
The blanking being HIGH assumes that the computer is running OK and the display is turned on. If the display is not operating, the display PIA that stimulates the blanking timer is not pulsing so the blanking stays LOW.
Now that we know how the blanking goes high and low. How does the computer handle that as a safety feature to turn off lamps, solenoids, etc? Through the use of 7408 AND gates. 7408 is a two input AND gate. That means each gate has two inputs and one output.
[quoted image]
Here is our AND symbol. Pretend this gate is in the solenoid section of the mpu/driver. Lets call 'A' the solenoid PIA signal and 'B' the the blanking signal.
You can see the truth table there. The blanking must be high and the solenoid pia must go high for the 7408 output 'X' to go high and turn on the 2n4401 predriver to fire the coil.
The same principle is applied on the lamp columns, AND gated with blanking/PIA using a 7408 to predriver a 2n6427

Quoted from wdennie:

Wayout, so what does the crystal do?
I have a space shuttle sys 9 that wouldn't boot, the crystal had no output, replaced it and game booted.