Quick transistor operation test for WPC:
I like this:
The basic theory of how pinballs run coils is that the coil always has power to it (when the interlock (if there is one) on the front door is activated). Thus if you take your meter lead, and both sides of the coil read, say, 70v (the 50v circuit reads 70v without a load on it), you know the coil is getting power. (And because the voltage is on the other side of the coil, you know that the coil windings are intact, that the coil or the coil's connections aren't broken, voltage is getting to one side of the coil, and going through the coil to get to the other side.)
The control circuitry fails with the big transistor first (sometimes this is a TIP 36, for most coils it's a TIP 102 replaceable part, whether the number is TIP 102, TIP 120, TIP 122, etc.). First transistor in line with the coil is the usual point of failure.
Sometimes the pre driver to this transistor fails. This is a small transistor like a 2n5401, that drives the TIP 102, and sometimes the resistors and suchlike can be compromised and need repair. Very rarely, the chip that drives the pre-driver transistor that drives the output transistor can fail.
Still, for about 49 of 50 'problem on the board' coil failures, it's the output transistor itself, the transistor first in line with the coil, and nothing else.
So, these transistors can fail, and when they do, they mostly 'lock on' and stay on until they incinerate, where they don't connect any more.
The coil has voltage on it. All the time. Most of the time it has voltage on both wires. It needs a path to 'ground' to complete the circuit. The transistor is what connects ground to the coil.
That's why you can take an alligator clip and put one side of the alligator clip to ground (whether that is the ground terminal in the middle of the WPC Power Driver Board, or a grounded screw holding the board in, or the ground strap at the bottom of the backbox), and briefly touch the other end of the grounded alligator clip to the tab of the coil transistors and activate the coils. Touch it to Q30 (Right Sling), and there will be a spark, and the Right Slingshot coil will pull in. This kind of test should be done very briefly, don't hold the coil in until it incinerates.
You could touch your grounded alligator clip to the tab of Q20, and the shaker motor should activate.
Touching your grounded alligator clip to random things can be risky, so let's look at a test that is unlikely to burn up anything and require further repairs if you do something wrong.
Get a lamp socket (I like the one from Marco above, but as long as you have access to two lamp terminals you can do this). Put a bulb in the socket. Because of the way I hook it to 12V power, it would be best if you have a 161 (12V) bulb, but I always use a 555 and just burn it bright.
Connect an alligator clip to one side of the lamp socket, and the other end of that alligator clip to either TP1 (about 14-16V!, TP2 5V, or TP3 12V, I use TP3 for this). The proper voltage for the 555 bulb is 5 or 6 volts, but if you connect 12V or more to it briefly it won't burn out.
Now we have this lamp in about the same condition as the usual state of a coil. It has power to it, and no connection to ground to allow it to complete the circuit to activate it.
Get another alligator clip, connect it to the other (unconnected) lamp terminal.
Disconnect the plug from J126. Find pin 6, and connect the alligator clip to pin 6.
Go into coil test, get to 22 RIGHT SLING, and the bulb should be blinking.
In a similar way, disconnect J122, find pin 4, connect the alligator clip to pin 4.
Go to the coil test, get to 28 SHAKER MOTER, and the bulb should be blinking.
This verifies the control circuit, that supplies ground to the coil or other device, is capable of turning on, and turning off it's connection to ground. You might want to do this when you have replaced a TIP 102 transistor to verify that the board is now properly repaired.
Fun fact: In coil test, when it is on REPEAT, on a coil, you can press the START button on the front of the game and it will cycle through the wires, connectors, fuses, and eventually the transistors that are associated with the coil under test. Very handy!