Riddle me this:

Going through a game I picked up a while back and looking over all the coils. Not everywhere, but on ALMOST every basic 2 lug coil (non-flippers) I am seeing 2 diodes in parallel. Polarity is the same on each diode. Machine and coils work and fire fine. Solonoid test, no issues.

Why would this be the case? I understand the need for the diode, but 2 across the same lugs? Obviously, one diode appears to have been on the coil from jump, and the second appears to have been added after the fact (see picture).

Is this just previous owner/op half-assed way of replacing the diode (without removing the bad one), or an attempt to just "future proof" to protect from blown transistors in the event a diode fails? Is there any harm or possible reprecussions in having 2 and leaving as-is, even though I'm sure this is not normal? I see this on 90% of the coils in the machine, but even solonoids that do the same thing side by side (like pop bumpers) might be different, some with 1 and some with 2.

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By the way, this is a system 6a game, so diodes SHOULD be on the coils and are not on the boards.

Not sure how familiar you are with the schematics for these old Williams games.....but they... Ahem....arent the best.

This is all you get for Solenoid wiring. Looks like 1 diode to me.

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I have a coil that is sticking in an energized state which is what got me looking at this in the first place. It is the reset coil for a 3 bank of the old horseshoe drop targets. Didn't notice at first, but when you hit one of the targets, it wouldn't drop since the reset bar was being held against them. Eventually, the Solonoid fuse would blow. I removed the Solonoid, and clipped one lead from the diode tight to bobbin and it tested good. So I clipped a lead from the other.... Also tested no problem. However, the Solonoid is now swollen from being on non-stop, so I'll be replacing it anyway.

Tested the transistor (Q21) inline with that Soloniod and it also seems to test good. Even stranger, now that the coil is out of the game, I'm blowing that fuse pretty much every time I power up the machine.

Looking at the driver board, it's apparent 1 or 2 of the transistors were replaced at one point. So my thought on the diodes was that one went bad and blew a transistor at some point and a tech might have just attached second diodes all over the place so that if it happened again, they'd have a backup in place to prevent additional transistor failures..... At least that's the only thing I can come up with......sound at least plausible?

Now.... Any thoughts on where to go from here to troubleshoot my growing fuse issue? Or even to better understand what would be energizing that coil all the time since it's diode(s) and the coil itself seemed to test good (minus the swelling, which I'd assume to be a symptom and not the cause). What actually triggers a horseshoe drop target reset in the first place? Possibly a bad switch (or diode on THAT) of some sort?

Exactly my thoughts on the diode.

Checked Q1 to verify both no continuity to ground and also checked resistances across EBC pins. Looked OK.

Unfortunately, when you ask what is on the line with that fuse, the answer is "every Solonoid in the machine". Good times.

Since my last post, I've been busy...... Did the following (in this order).

1. I replaced both the Solonoid and Lamp bridge rectifiers and threw a couple 8Amp SB Fuses in line with them (in accordance with vid's bulletproofing thread). Both rectifiers actually tested fine, but were pretty gunked up and prior owner/op had also soldered the wire terminals to them with a TON of solder (that bastard!). I had to cut them out and put new connectors on. But now my house won't burn down at least.
2. I electrical taped-off a disconnected Solonoid wire at the coin door and wrapped the bare-metal lockbar handle, just in case I had a short somewhere around there.
3. Disconnected the Solonoid connectors 2J9, 2J11 and 2J12 on the driver board and powered up the game. Fuse did not blow. Reconnected them one at a time. Fuse STILL did not blow. Played a could games without issue (minus the fact that the 3 bank drops don't work since I still have that smoked coil out of the machine.

That's where I am at the moment. In the end didn't really do much that should have made the difference here (except maybe the coin door)....... But then again I did move/disturb a sh*#-ton of wiring in the process of doing that stuff..... So I'll check my wires as well to see if I have any shorted/exposed wires..... I'll report back in once I do that (and replace the coil).

Gotta give props to whoever the operator was that owned this machine at one point.....they came up with some really creative ways to leave future surprises for me.

Yes....that is a backwards coil stop being used as the stop for the plunger arm in my upper left flipper. Grrrr.....

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Well.... In my case, the extra vibration this caused by not having a rubber grommet to dampen it loosened one on the screws in my flipper bushing, which eventually worked its way out enough that it was touching the EOS switch, effectively stopping my flipper completely.

Well.... Back to square one. Went through the bottom of the playfield with a fine-toothed comb and adjusted any switch lugs, diodes, etc that looked like they were close to anything that could cause a short, tightened up any coils that had play to eliminate excessive vibration and checked the harness (best I could anyway) for any potential wires than were bent, broken or missing insulation. Removed one more coil that looked a little suspect (2 coils out now) for the time being. After that, fired the game back up.....and the Solonoid fuse blew again immediately. However, this time, a couple of the Solonoid fired the second I turned the machine on. That sounds like a blanking signal issue. Looking more and more like a board problem. Time to yank them and check the MPU/Driver 40 pin connector I think...... unless anyone has any better ideas.....

No sure either, but that upper left flipper is the only one that doesn't have those grommets in my machine....totally different EOS Switch and bracket too. So I'm guessing that was a rebuild from whatever was lying around...

Good to hear that I'm barking up the right tree...fingers crossed that does the trick. I'll update after I do it.

No problem. Yeah, BOARDS coming out of the machine before I do anything.

MPU bench testing solo is fine. MPU and Driver Board need to be connected together to bench test the driver board, since the power and signaling is all being supplied by the MPU. Anyway and it'd be pretty useless to try to test the Driver Board under power without the board that provides the logic.....

I think the reference to bad things happening was to leaving the Driver Board connect to the harness and turning the machine on with the MPU either not present or disconnected via the 40 pin interconnect (Although I'm not even sure how you'd pull that second one off based on the way the boards overlap when in the machine).

Well..... 48 days later, I finally got back around to looking into this further. Long story short, I believe I have found my culprit. Long version of the story, I'm not exactly sure...... Hoping for a little feedback. @vid1900 @MrBally

I pulled my cpu and driver board to take a better look at the 40 pin connector in prep to replace it. Much to my surprise, the 40 pin connector had very obviously (once I was able to look at the back of the board, anyway) been replaced sometime in the machine's past. To make matters worse, whoever did this wasn't exactly a soldering expert. Pretty ugly job.... But the joints appear to be sound nonetheless....

So, considering the soldering/desoldering that already took place and my lack of desire to potentially damage the board just attempting to clean it up or replace the connector again, I decided to check out the other basic culprits for the Solonoids (resistors, transistors, pre-drivers).

In doing this, with boards out of the machine, I ran a standard diode test against each Tip122 NPN Solonoid Transistor on the driver board. Everything looked fine and readings were roughly identical for each one. For grins, I set my multimeter to ohms and discovered that the readings for Q17 (front left 3 bank drop target reset) was approximately 10 times the reading of any of the other transistors.... And I was getting these readings regardless of whether I put the red or black lead on the base.....

Figured something didn't smell right there.

I checked Q17's resistors and pre-drivers and those readings looked in-line with all the other sets.... So I figure nothing is wrong with those.

Next, I pulled 2J9's cable (since Q17 pinned out to that) and powered up the machine. No blown fuse. Tested every Solonoid in diagnostic mode that wasn't running over 2J9 and no issues.

Next, I removed the Solonoid for the left front 3 bank target reset (associated with Q17) from the machine and just tied the 2 reds together to allow all the other Solonoids in the chain to still work. Reattached 2J9 and powered up. No blown fuse. Ran full Solonoid diagnostics and then played the machine for an hour straight. No problems at all... Minus the fact that my drop targets won't reset, obviously.

At this point, I'm comfortable saying I've found the Solonoid that was causing my fuse to blow. But does this sound like a transistor problem? If so, I guess I'll have to order a few Tip122 (or Tip102) NPN Darlington transistors, since I don't have any...... But the test I used to "find" the issue isn't normally what I've seen done to determine a failed transistor in the first place, which is what has be a little nervous that the transistor isn't an issue, since the "standard" tests in diode mode look fine....

Obviously, I'll check out the coil and its associated diode to make sure they are good, since if not, that would likely be the root cause (or that killed the transistor in the first place) , but was just hoping for feedback at to whether there is anything else I should check or if replacing the transistor is definitely in order here. Thoughts?

Since I didn't get any feedback on my long-winded last response (sorry for the lack of brevity), I figured I'd bump this one more time looking for feedback. Anyone? Thx

Thanks mbwalker

I think we can safely say the transistor at Q17 needed to be replaced. When I measured (black lead to collector and red lead to either base or emitter), my reading on Q17 was approx 80kohms. But on EVERY other Tip122 in the machine, this reading comes back with 8Mohms....100 times the value for Q17.

I replaced Q17 with a new Tip102, and while I was at it, removed the diode from the coil that transistor powered and tested the coil's resistance....no problems there. So I replaced the diode with a new 1n4004 and put the whole thing back together. So far so good..... my blowing solonoid fuse issue has not reappeared.