Fixing Bad Power Supply Voltages.
Of course check all power supply fuses first using a DMM and removing the fuses from their holder.
If the high voltage (+/- 100 volts) is missing or low or high, the score displays will not work. The HV section of the power supply will need to be rebuilt. The input 90 volts AC (3J1 pin 4 and pin 9 pre-Taxi) comes directly from the transformer, so there should be no issues with that (unless power supply HV fuse(s) are blown).
If the +5 volts is high, suspect a bad 2n6057/2n6059 voltage regulator on the power supply.
If the +5 volts (J6 pin 7,8,9 pre-taxi) is low, check J6 pin 6 (pre-taxi) for +12 to +15 volts DC. If this goes lower than 11 volts, there isn't enough "headroom" for the power supply to make +5 volts, and the game will not boot due to low +5 volts. You can also check the input AC voltage at 3J1 pin 10 and pin 11 (pre-taxi, should be about 12 volts AC). If the input 12 volts DC is low at J6 pin 6 (pre-taxi), put a DMM on low AC volts, and place the test leads on power supply capacitor C10. This should show less than .3 volts AC. If it is greater than this, replace cap C10 (15000 to 18000 mfd 20 volts). If J6 pin 6 (pre-taxi) is still below 12 volts, replace the BR1 bridge rectifier on the power supply. This should fix any low 12 volt DC input problems.
If J6 pin 6 (pre-taxi) is +12 to 15 vdc, this means the BR1 bridge rectifier on the power suppy is good as is cap C10. Next this power goes to IC1 chip (723PC) and Q5 (2n6059/2n6059) to make regulated +5 volts. This should be 4.9 to 5.2 vdc (J6 pins 7,8,9 pre-taxi). If this voltage is low, first replace Q5 (2n6059/2n6059).
Lamp matrix voltage on pre-Taxi power supplies include 3J4 pins 5,6,7,8 (+18 vdc) and 3J4 pins 9,10,11,12 (ground). On Taxi and later power supplies, the lamp matrix power does not go to the power supply board at all (it is wired directly to the CPU board at 1J4 pins 1,2,8,9 and grounded at 1J5 pins 1,2,6,7). If the lamp matrix voltage is missing, the lamp matrix won't work. Power for this is supplied by a bridge rectifier mounted to the metal backpanel of the interior backbox, and the huge blue capacitor and fuse next to it.
On pre-BigGuns sys11 games, the solenoid voltage on the power supply include 3J3 pins 6,7,8,9 (+34 vdc) and 3J4 pins 1,2,3,4 (ground). This is the lower voltage solenoid power. Power for this is supplied by a bridge rectifier mounted to the metal backpanel of the interior backbox. The high voltage (50 to 70 volts DC) solenoid power is supplied by the smaller flipper power board beneath the power supply in the backbox. On sys11b games Big Guns and later, the Auxiliary Power Board has both the 35 and 70 volt solenoid voltages (there is no flipper power board and there is no backbox mounted low-power solenoid bridge rectifier). Also the power supply will *not* have a 3J4 connector for solenoid power.
The Auxiliary power board on sys11b games Big Guns and later handles the solenoid voltages completely: J10 pins 1-4 is solenoid ground, J11 pins 8-10 is +35 volts DC, and J11 pin 2-5 is +70 volts DC. The A/C relay is also mounted on the Auxiliary Power board (formerely mounted under the playfield on earlier games), and turns on and off +35 and +70 volts on the Auxiliary power board at pins J11 1-5 (35 volts) and J12 pins 9-12 (70 volts). This is done in conjunction with the A/C driver transistors which control either a flash lamp or a coil.