Taken from pinrepair:
Booting a CPU board "On the Bench".
The CPU board can be booted without the rest of the game using just an external 5 volt power supply. Just connect the CPU board's A1P1 connector to a computer switching power supply and power up (A1P1 pin 1=+5 volts, A1P1 pin 4=ground). If the CPU board LED flashes, the CPU board has booted. Though pretty useless just like this, it will give you some idea if a Gottlieb System 3 CPU board is dead or not.
If you would like to get fancy, the next board needed is the dot matrix controller board. With this board connected to the CPU board via a ribbon cable at A1P3 to A8P3, the dot matrix controller board can be booted too with these input power sources:
A8J1 pin 9 = +5 volts DC
A8J1 pin 5 = 20 volts DC
A8J1 pin 10 = ground
A8J1 pin 1 = 58 volts AC
A8J1 pin 2 = 58 volts AC return
A8J1 pin 3 = 95 volts AC
A8J1 pin 4 = 95 volts AC return
This will allow you to connect a dot matrix display to A8P4 with a ribbon cable. Also a power cable from A8P2 (dot matrix controller) supplying +5,+12,+62,-100,-112 to the dot matrix score display is needed:
A8J2 pin 1 to DMD pin 6 (+5 volts)
A8J2 pin 2,3 to DMD pin 4,5 (ground)
A8J2 pin 4 to DMD pin 7 (+12 volts)
A8J2 pin 5 to DMD pin 8 (+62 volts)
A8J2 pin 7 to DMD pin 1 (-112 volts)
A8J2 pin 8 to DMD pin 2 (-100 volts)
With this all connected, the game will boot and go into attract mode (showing all the dot matrix display animations). This can be very useful for diagnosing problems on the CPU board.