I'm working on a friend's STTNG, and the bottom under-playfield diverter is giving us problems. At a basic level, the coil won't fire. When we go into the coil test, all the other coils fire but that one. Firing that one causes the machine to reset and a fuse to blow. We've replaced the sketchy-looking auxiliary board with a new Rottendog one, and we're still having issues.

Using my DMM, I can put the red lead on a lug and the other lead on the ground braid and see that there's power going to the coil, as well as all the other working coils. When I connect an alligator clip to ground and touch the side of any other coil in the give with a single wire (which ,as I understand, is the ground return wire), the coil engages. Pop bumpers and kickouts all engage when I try this test. Even the top under-playfield diverter. I tried this test once with the bottom diverter with the previous, sketchy-looking auxiliary board. The spark was much bigger than on the other coils and the machine rebooted. I should note that this coil has but a single wire on each lug, as opposed to the other coils, which have two on one lug and a single on on the other. Maybe this is normal.

Anyway, I had thought the problem was with the original board, but the problem has persisted with the new Rottendog auxiliary board. I feel like coil issues are pretty simple to track down, but I'm really stumped here. The only thing I can think of is that there's a short somewhere that only comes into play when the coil is fired. But that doesn't make any sense based on how I think coils work.

As I understand it, the transistor on these machines sits on the ground wire side of the coil and provides a path to ground, which is why the coils always read as having voltage, even when not firing. Therefore, if there were some kind of short in this coil's circuit, it'd have to be after the transistor, otherwise it would lock on when the machine is first powered on. But a short after a transistor seems odd to me. Power comes from the purple wire, through the coil, out the other wire and into the board - through the transistor. From there, it'd just go to ground, which would be some kind of pin. Unless each coil has its own path to ground after the transistor, then a short there would be shared by everything that shares that common path to ground. SO CONFUSED.

Anyone have any ideas? Thanks!

The problem coil initially had a resistance of 7.5 ohms, but at some point, it dropped to <1. This is the third coil that's been in this spot since we started troubleshooting it. The coil before this one also had 7.5 ohms, and then dropped to <1 during testing. The first coil had problems before I got involved, so I can't really speak to what readings it had.

So we know that nothing blows or locks on when the machine is first turned on with a verified auxiliary power board and new coil in the machine. It isn't until we start trying to fire the coil that things freak out. In one instance, just testing the coil through the menu caused the machine to lock up and a fuse to blow. In another instance, doing the straight-to-ground test that works on all the other coils in the game caused the coil to drop to <1 ohm, thus needing to be replaced.

If there's a problem in the circuit for this coil that only manifests when the coil is grounded (either via the on-board transistor or manually by touching the ground lug to ground), then how do I go about troubleshooting this? I feel like I'm missing something.

The two replacement coils that we've gone through did have diodes on them, but I was under the impression that a diode on the coil wasn't detrimental to the system. I'm not sure how a second diode in the circuit could cause a short.

Let's lay out where we are now.

We have a Rottendog auxiliary board in-place (with possibly damaged components on it) and we have three replacement coils on the way. The Rottendog was tried once with a diode'd coil, the coil looks to have blown. The coil's resistance now reads <1 ohm, where before it read ~7.5 ohms.

Some transistors on the new auxiliary board appear to have been affected as well. Here are my test results. Imagine the auxiliary board flipped over, with the LEDs facing away from you on the lower left. Q13 is the lower left of the transistors with the board in this position. The DMM is on the diode setting.

During these tests, keep in mind that Q15 drives the bottom under-playfield coil, and Q11 doesn't look like it drives anything (according to the manual).

Test 1: Black lead on left pin, red lead on middle pin

Q15: Reads enough voltage to put it out of 0L, but not enough to go over 0.000
Others: 0L

Test 2: Black lead on left pin, red lead on right pin

Q11: 1.2V
Q15: 0L
Others: 0L

Test 3: Black lead on middle pin, red lead on right pin

Q15: Reads enough voltage to put it out of 0L, but not enough to go over 0.000
Others: 0L

Test 4: Red lead on left pin, black lead on middle pin

Q11: 1.85V
Q15: Reads enough voltage to put it out of 0L, but not enough to go over 0.000
Others: 0L

Test 5: Red lead on left pin, black lead on right pin

Q11: 1.85V
Q15: Reads enough voltage to put it out of 0L, but not enough to go over 0.000
Others: 0L

Test 6: Red lead on middle pin, black lead on right pin

Q11: 1.85V
Q15: Reads enough voltage to put it out of 0L, but not enough to go over 0.005
Others: 0L

So, this is weird. Aside from Q15 reporting really small amounts of voltage, there doesn't seem to be anything glaringly wrong with it. Maybe that small voltage is enough to cause things to go wrong.

However, Q11 looks like it's got all kinds of things wrong with it. Whether the board came this way or our experiments have affected this transistor is unknown. It doesn't drive anything, so I don't know how we could have messed it up.

Of the eight transistors on the board, it looks like only six are used. One of the two unused ones is acting weird. Maybe it's a coincidence. If I wanted to replace Q15, could I pull out and use transistor Q12? It appears to be working fine. Otherwise I have to pay ten bucks and wait a week for the replacement to get here. And since Q11 isn't actually used either, could I pull it out of the board completely to minimize unknowns in this whole thing?

I plan on going over the whole thing with a de-lousing comb, but I still have to deal with the possibilty of fried components. Hence the Giant Wall of Text.

Interesting. I was looking at 3-24 and 3-25 which says that J3-4 and 5 (which trace to transistors Q11 and Q12) doesn't connect to anything. I'll admit that I'm not quite sure how this whole thing works. Is this related to the "tieback" thing that everyone's talking about? If so, can someone elaborate on it for me?

Yeah, it's weird. Page 3-24 in the manual says in one spot that J3-4 and 5 don't connect to anything, and that's might be true, but it also looks like there's some kind of special circuit hung off of these two transistors that's not present in the other 6 coil-driving transistors. This could explain why the screw-up in the bottom diverter coil circuit could also be affecting the one transistor that drives switch column 9 (or vice versa).

By all means, please hit me with any more knowledge you'd be willing to give on this. Most of my experience is with System 6 and 11. I haven't messed with these newer Williams machines much.

How does a switch column make use of a MOSFET transistor? That's the part I'm a little unclear on. What does the tieback wire actually do?

I guess I never really thought about what a switch matrix design would look like. When I think of, say, a System 11 board, I think of the coil and lamp drivers as being controlled with transistors. Are there transistors that drive the normal 8 x 8 matrix as well? If so, are they discrete transistors, or are they in some kind of IC form?

I'm starting to understand how the tieback wire fits into the coil driver picture, but I'm still not sure how it all works. Is it called a tieback wire because it ties back into the ground return that the eight transistors are also connected to?

It's also possible that there are two tieback wires and I'm getting them confused.

Oh, man. That page is making my eyes go cross. I understand why you put a diode across a coil in these games, and I understand how transistors work, but trying to actually read the diagram and really grok it is... tough. I'll just have to accept the fact that I have no idea how the electronics of the game actually work.

For what it's worth, I suspect the problem may have been due to the fact that we were trying to use a coil with a diode in a circuit that already had one. Based on absolutely nothing at all, I suspect this was causing a short or something when the coil was grounded by the system (or by me when testing the coil), which would then fry the transistor.

I replaced Q11 and Q15 long with replacing the coil with a no-diode coil, and everything worked just fine. So, yay for me!