How Coils are Energized (a Technical Introduction).
The basis behind pinball coils is that there is coil voltage at all coils at all times. Coil power is "daisy chained" from one coil to another under the playfield. Each coil is energized by completing its path to ground momentarily using two transistors on the solenoid driver board (a large SE9302/TIP102 style transistor, and a smaller pre-driver transistor contained in a CA3081 chip), allowing the coil to fire. That's why a DMM can be used to check for 43 volts on either coil lug when the game is on and in attract mode (not being played). If coil voltage is missing, either a fuse is blown or the power wire "up stream" has broken.
The following introduction text is thanks to Steve Kulpa and his web page at geocities.com/stevekulpa/bally_sole.htm. He did a really nice job of describing how the system works, so his text is transplanted here with some modifications. This information applies to the AS2518-22 and AS2518-17 solenoid driver boards, used in most Bally 1977 to 1985 games.
The solenoid driver board actually has two functions: The first obviously is to drive the solenoid and relay coils of the pinball, and the second is a voltage regulator. This provides regulated voltages to the other boards, plus high voltage (190v) to the score display driver boards. The voltage regulator stuff won't be discussed here, just the solenoid driver parts. Since the Solenoid Driver board contains the high voltage circuitry for the score displays, there is 190 volts DC on the board. Be careful as this is high voltage, and a shock from 190 volts DC will hurt.
We'll be discussing things from two circuit boards: The MPU board (AS2517-35 or AS2517-17) and the Solenoid Driver board (AS2518-22 or AS2518-17). The solenoid driver gets signals from the MPU board. These signals tell the solenoid driver which solenoid to fire. Up to 16 momentary and four continuous solenoids including the flippers (via the flipper relay) can be controlled by the solenoid driver board.
Driver board Continous solenoid circuit PB4-PB7 and transistors for a playfield coil.
Notice the lack of a 74154 decoder chip here.
The four continuous solenoids are not decoded signals. They are "continuous" because these are for devices that "stay on". This includes the coin door lockout coil (which stay energized as long as the game has not reached maximum credits), and flipper relay (which stays energized during the entire game). Continous solenoid signals are also used for playfield gates and other devices which stay "on". These signals (Pb4-Pb7) come directly from the MPU board's U11 PIA chip (and MPU connector J4 pins 5-8), and go to the SDB connector J4 pins 8-11. Again these signals are not decoded. All Bally games use Pb5 for the coin lockout and Pb6 for the flipper relay. Pb4 and Pb7 can be used for continously energized playfield devices, like a lane return gate or some other continously "on" device. When looking at the list of solenoid self-test numbers in the manual, the last solenoid numbers will be the continuous solenoids including the coin door lockout coil and flipper relay.
Driver board Momentary solenoid circuit PB0-PB3 decoder/transistors for a playfield coil.
A 74154 decoder chip is used in this circuit to activate the correct coil.
There can be up to 16 momentary solenoids. These are solenoids that are only energized for a very short period of time, say 50 milliseconds. These four signals come from the MPU board and are called Pb0-Pb3, which are decoded by the SDB. There is also a Bank Select signal called Cb2 (which tells the SDB that the CPU driven Pb0-Pb3 signals are for the solenoids and not the lamps, since Pb0-Pb3 is also used to toggle CPU controlled lamps.) The four PBx signals come from the MPU's U11 PIA chip and travel out from the MPU board connector J4 pins 5-8 to the Solenoid Driver board connector J4 pins 3-7 (Cb2 comes from J4 pin 10). These four signals Pb0-Pb3 tell the Solenoid Driver board (SDB) which solenoid to fire. This is accomplished by using a driver board mounted 74154 decoder chip that takes the binary pattern of the four signals (16 different patterns) and decodes (or demultiplexes) them into one of sixteen different outputs. The four signals are applied to the decoder then the decoder is strobed. Normally, all sixteen of the decoder output lines are held high (+5 vdc). When strobed, the decoder lowers one of it's sixteen output lines, depending on the pattern of the four input signals.
With no input supplied (strobe is high), the output lines of the 74154 decoder are high (+5 vdc). This puts a voltage at the base of Q1 (this transistor is one of seven in the CA3081 chip). This turns Q1 "on" and the voltage supplied to it's collector via resistor R1 passes through the transistor to ground. At this point, little or no voltage is present at the base of the large SE9302/TIP102 driver transistor Q2, and hence it is "off". With the SE9302/TIP102 driver transistor off, the 43 vdc at the coil has no place to go, and the coil remains de-energized.
When the MPU board supplies the proper input signals (A-B-C-D) to the decoder, and the decoder is strobed (signal drops to low), the proper output signal will go low, which turns the CA3081's predriver Q1 "off" (notice one of the two strobe lines goes to ground, so it's always low). This allows the +5 vdc at the Q1 CA3081's collector to flow through the diode instead of Q1 on it's way to ground via resistor R3. This also puts a voltage at the base of Q2 and turns this transistor "on". When the TIP102 (Q2) turns on, the 43 vdc at the solenoid now has a path to ground through Q1 and current flows through the coil, thereby energizing it. Then the strobe to the decoder is released, the decoder output goes high again, and everything is back to normal.
A diode, resistor, and capacitor work to slow the speed at which the TIP102 and the solenoid are able to turn off. This is important to prevent the "inductive kick" voltage that builds up when a solenoid is turned off quickly. A solenoid coil can build up hundreds of volts if it is switched off quickly. For example, the spark in the sparkplug of a car is generated from this inductive kick when the ignition coil is turned off quickly. In this case, the diode allows the TIP102 and the solenoid to turn ON quickly (which is OK), because the current that used to be flowing through the CA3081's pre-driver transistor can now flow forward through the diode and turn on the TIP102 on quickly. However, when the decoder output goes back to high and the CA3081's pre-driver transistor turns back on, the diode prevents the charge from the base of the TIP102 driver transistor from being sucked down the CA3081's pre-driver transistor. The charge on the capacitor must drain off (slowly) through the resistor and the base of the TIP102. This takes awhile and slows the turn-off of the TIP102 and the solenoid coil, thus reducing the kick. Also, as the solenoid turns off and the voltage on the collector of the TIP102 starts to rise, this voltage is "fed back" by the capacitor to the base of the TIP102 and tends to keep it on a little longer, slowing the turn-off of the solenoid even more. Another diode across the solenoid works to absorb the solenoid's turn-off kick by conducting when the voltage on the collector of the TIP102 is greater than about 43 volts.
