Q73 is the transistor for the right pop bumper. You can short the metal tab briefly to ground to see if the pop fires. But this doesn't test the transistor. It's just testing the emitter leg, trace, connector and wiring to the coil. If the coil doesn't fire, it could either be no power at the coil, or a bad connector/wire.

I would start first at the coil. With a VMM to VDC, black lead to ground strap, red lead to lug that has the same color wire as the pops next to it. This is the daisy chained power feed. All the pop coils have constant voltage and the CPU and Switch supply the ground to fire the coil. The pops in HS are called specials and are both MPU controlled as well as direct switch controlled. If you have voltage here, you can briefly with a jumper wire tied to ground, touch the coil lug that has the striped wire on it. According to the manual it's blue/red. If the coil fires, then the coil is ok, and you have power.

Next, ground the pin IJ19-3. That's the ground wire to the coil. Should also be blue/red. If you ground it at the connector and the coil fires, then the wiring and power are good.

At this point, you're going to need a logic probe. Both the pop bumper switch and the MPU feed a predriver transistor Q72 which actually turns on Q73 and supplies the ground path. It's not uncommon for a bad coil to take out Q72 and/or Q73. With a logic probe, we can see where the logic isn't working. But try the first tests initially and see if you see any activity.

Note: Please follow the manual and verify the wire colors. The Manual isn't actually consistent. One page shows Q73 as the special solenoid on J19-3, but another page shows a different Q on J19-3.

Follow up: Here's another thread that documents exactly what I just described. It also confirms the manual is in error. It begins to get into the logic side of things towards the end, but let's get past the first part before we worry about that: https://pinside.com/pinball/forum/topic/how-to-troubleshoot-special-switchessolenoids

The next step is to test Q73:

1) Turn game off
2) Remove connector J19 from the CPU board
3) Set DMM set to diode function
4) Black DMM lead on the metal transistor tab.
5) Red DMM Lead on each outside leg of Q73
6) DMM should show .4 to .6. If good, proceed to 7
7) Red DMM lead on the metal transistor tab.
8) Black DMM Lead on each outside leg of Q73
9) DMM should show OL or Infinity, NOT zero or some reading

10) If step 6 or 9 fails, replace transistor.

11) If step 6 and 9 passes, you'll really need a logic probe to continue, or you can replace Q72, the pre-driver transistor and cross your fingers.

Do you have a logic probe?

Also, did you try to test the coil from the menu? In HS you can fire the coil from the tset menu....do it with the coin door closed to enable the coils once you select it. That would point to a switch issue or board issue.

So, to summarize, neither the MPU nor the Pop Bumper Skirt Switch are able to fire the coil, but grounding Q73 tab, does fire the coil.

To me, that points to either U45 or Q72. The Ground from the Pop Bumper Switch and the Data signal from the MPU go through a NOR gate (U45). That signal then turns on Q72, which in turn, turns on Q73. Without a logic probe, it's difficult to know for certain. A logic probe will show you the brief pulse from the MPU in coil test mode, that will be difficult to see on a meter, because the signal is rapid, and by the time the meter responds, it's gone. However, we may be able to use the fact that these coils are specials to our advantage. Because the MPU changes the signal rapidly, BUT, the pop bumper skirt switch will be there as long as you hold it in. Normally, in game play, the switch activates, the coil stays on as long as the switch is made, but the pumper pushes the ball off the switch rapidly, so the coil is released.

Here's what you can do, but I don't know if you can get your physical leads of your meter in place well enough to test.

I've attached a diagram of the pre-driver transistor for reference.

With the game on and in a state that the OTHER pop bumpers work (we're going to use them as our reference), do the following. First test, that the working pop bumper works by pressing down the bumper skirt. Then remove Fuse F2 (power to the coil), and retest, making sure the pop bumper doesn't work now (going by memory here, you might have to experiment to find the actual fuse. Also, opening the coin door might do the same, but I don't recall on HS). What we are after, is that the logic works, but we remove the power to the coil so it doesn't fire while testing. Any fuse removal, do with game powered off.

Once you have the game to a state that the pops would work, but the power is removed, we're going to test the logic circuits.

Meter on VDC, black lead to ground. With the WORKING Pop Bumper (Let's choose upper Left Q77/Q76 pair) not made, put your red lead on each leg of the pre-driver transistor Q76 and record the voltage. You'll see something between millivolts and 5V on each test.

Working Pop Bumper OFF Leg 1 Q76 = X Volts
Working Pop Bumper OFF Leg 2 Q76 = X Volts
Working Pop Bumper OFF Leg 3 Q76 = X Volts

Now, put something on the Pop Bumper skirt to make the switch. Keep it held down for the next tests:

Working Pop Bumper ON Leg 1 Q76 = X Volts
Working Pop Bumper ON Leg 2 Q76 = X Volts
Working Pop Bumper ON Leg 3 Q76 = X Volts

So that's are reference. We're now going to compare to the non working Bumper

Non Working Pop Bumper OFF Leg 1 Q72 = X Volts
Non Working Pop Bumper OFF Leg 2 Q72 = X Volts
Non Working Pop Bumper OFF Leg 3 Q72 = X Volts

Now, put something on the Pop Bumper skirt to make the switch. Keep it held down for the next tests:

Non Working Pop Bumper ON Leg 1 Q72 = X Volts
Non Working Pop Bumper ON Leg 2 Q72 = X Volts
Non Working Pop Bumper ON Leg 3 Q72 = X Volts

You might have to use a paper clip tied to your meter lead to get to the legs....OR, if you are comfortable reading a schematic, there are points that are easier to reach. I've attached the schematic, so for example, to get to leg 2 of Q72, you see that is connected to Resistor R106. One side of R106 is connected to 5V (that's the little triangle), so visually look at the board trace to see which side you should measure on R106 to check leg 2 of Q72. Likewise, leg 1 can be measured at R105, and leg 3 at R107.

2n4401-transistor-pinout-equivalent (resized).png

Schematic (resized).jpg

Anything is possible, but usually the pre driver is more common to fail. You can check U45 by running through the tests I wrote and checking the base voltage of Q72 which is the result of the NOR gate.

https://pinside.com/pinball/forum/topic/terrybs-guide-to-logic-probes

Okay....with the logic probe, we want to check pins 1, 2 and 3 of U45. Pin 2 is the combo switch from the Playfield and command from the MPU to fire the bumper. So the fact that pin 2 is transitioning to low is good. Next you want to check pin 3. It should be held low all the time by the blanking circuit. Finally, check pin 1. If Pin 3 is low, then every time Pin 2 goes low, Pin 1 should go high.

Pin 1 is the trigger to Q72. Once we see it's going high then we can move on to Q72. If Pin 1 isn't toggling with Pin 2, then U45 is bad (assuming again that Pin 3 is okay).

If you want to compare, put the solenoid on Q76 into coil test and check pins 1,2 and 3 of U50. It's the same logic.

Yep, appears U45 is toast, unless somehow Q72 is dragging pin 1 low. Since it's harder to desolder an IC than the transistor, you could first desolder Q72 off the board or clip the legs, and then retest the 7402 IC at U45. Did you ever replace Q72....I read back through, and you said you were ordering some, but I didn't see whether you actually replaced it.

I wouldn't buy from ebay, anyway....buy from Ed at GPE https://www.greatplainselectronics.com/products.asp?cat=12

As for CMOS vs TTL, this link provides a decent description between the two: https://pinballrehab.com/1-articles/solid-state-repair/tutorials/107-electronics-tutorial-integrated-circuits

Can't say definitively...CMOS has a different switch level than TTL as to what it considers high/low. It might work, but given that the original is available easily, why even worry about it. Just buy a replacement 7402.