The STTNG Drop Target Saga

I'll try to keep this as brief (but informative) as possible.

If you saw my previous post about Star Trek: The Next Generation, you know that I had been contemplating the replacement of the single drop target assembly for some time. I had had the part for over a month before I started getting enough malfunctions that I finally decided to give it a go.

What followed was a three-week adventure that, in the end, didn't need to be nearly as complicated as it played out.

Why Was I Replacing It in the First Place? 
Fair question. After all, the game was playing almost 100% when I got it. But there were two ongoing issues: 
  • The drop target is supposed to automatically drop and raise in certain situations. For example, when you select "Launch Probe" as your skill shot, the target should drop down to let the ball pass, and come back up after the ball passes. Up was working. Down wasn't. And, in the switch test, it was confirmed that Drop Target Down didn't work.
  • I was getting phantom ball locks and extra balls in play from time to time. I assumed (and I should never have done that) that these problems were related to the drop target problem.

First, I Tried to Replace the Drop Target Assembly...

Six solder points. Easy to access. What could go wrong?

I desoldered the old assembly and removed it. I installed the new assembly. I put the right diode on the Drop Target Switch (oriented the right way). I soldered all the connections. They looked good.

When I switched on the machine, the right flippers immediately engaged. That shouldn't happen! I switched the machine off right away assuming (again...) that I had screwed something up royally. There's a thin purple wire that goes from the Drop Target Up coil to the 8-Driver board (a small board that controls the drop target and a bunch of other solenoids in the game). This tie-back wire, if it has a bad connection on the coil, can sometimes damage transistors on the 8-Driver. I assumed (AGAIN...) that this is what I had done.

So, I immediately stopped what I was doing, ordered a replacement 8-Driver board (the new Rottendog one) and called my local pinball tech in to help.

Technician: Day 1

My tech, John, came in thinking that all that was going to need doing was for him to clean up a botched solder job--something he's done for me before--and put in a new board (which I could easily do myself, but he wanted to make sure everything was right before potentially blowing more transistors).

Turned out I hadn't blown anything but a fuse. 

Once everything looked good solder-wise and John had made some other maintenance adjustments (tightening posts, etc.), we switched on the game. 


The drop target went kinda nuts. 

When you'd select Light Lock as a skill shot/launch bonus, the first ball would lock as per normal. Then, the drop target would go up. (Fine so far.) But, as soon as the next ball launched, Lock was automatically lighting--it shouldn't do that. When it did, the drop target tried to go up, then it would go down and try to go down again several times. The ball would pass the target and lock. Ditto with the third ball. So I was getting automatic multiball. This is NOT how it's supposed to work.

  • We double-checked that the drop target switch was working in test and not triggering phantom switches elsewhere. Seemed to be working fine.

  • The physical operation of the drop target mechanism seemed fine. It wasn't hanging on the playfield or getting stuck.

  • We checked the opto switches along the subway under the playfield. They seemed to be working fine.

  • We checked the 12-volt power (regulated and unregulated) and both were within spec (unregulated was just under 14 volts). John called someone to double-check that the voltage was correct, and he was told that it might be low. So, just to make sure the power board wasn't at fault, John replaced the bridge rectifier and filter capacitor on the 12-volt circuit. The voltage remained the same.

  • We swapped out the 8-driver board since we had a new one at hand. No change.

We gave up for the day and arranged for him to come by again the next day.

Technician: Days 2-4

John actually ended up coming back three different days over the next couple of weeks. We had both been doing research, and everybody said one of the same three things regarding the possible cause of the problem:

  1. Physical problem with the drop target. It's binding, catching, the spring's too tight, etc.

  2. The drop target switch.

  3. Faulty optical switches (optos).

We had "ruled out" 1, and 2 didn't seem likely--the switch was brand new and worked fine in test. Here's everything we tried:

  • Replaced the Trough Optos. John noticed an issue with the speed at which balls launched when the game starts up. STTNG places balls in locations under the playfield so it can feed them rapidly into play. On startup, if the balls aren't in their designated locations, the machine kicks three balls out of the launcher and sends them to where they belong. My machine was launching them too fast--it should launch, wait until that ball is in position, launch the next one, etc. Mine were coming out all at once. Turned out that the Trough Up opto (switch 67) was dead. We replaced the sending and receiving trough opto boards with new ones. This didn't fix the drop target problem, but it needed to be done.

  • Replaced some of the under-playfield optos. Specifically, we replaced every opto that had anything to do with locking the balls for multiball--31, 32, 35, 36, 41, 42, 43, and 45. Many of these were dirty and were probably original to the game. They have a tendency to fail and cause weird multiball issues. didn't solve the problem.

  • Fixed a sticky solenoid. The Borg Kicker (inside the Borg ship) was sticking. We it now no longer sticks...but that didn't solve the problem.

  • Replaced all the connectors at the bottom of the CPU board. They had evidence of acid damage from the distant past, so it didn't hurt to give it a try. No effect on the problem.

  • Performed a factory reset. I did a factory reset with my NVRAM in place. Then, we pulled the NVRAM from my CPU board, replaced it with a traditional RAM chip, and did several factory resets. On rare occasions, this has apparently cleared some faulty data and fixed weird problems on Williams machines when nothing else did the trick. It did nothing (other than erase five months of high scores).

At this point, it was Hail Mary time. John, John's contacts, and everyone I had been talking to online were baffled. So we started trying some other stuff...

  • Replaced the drop target switch with ANOTHER new one. No effect.

  • Replaced the down-coil on the drop target assembly with a new one. No effect.

  • Tried a new CPU board (with my chips). No effect.

  • Swapped out the ASIC chip with a new one. No effect.

At this point, the problem was actually worse. Not only were we getting the Lock lighting and drop target dropping problem, but we were also getting a problem where, when the drop target went up, it often kept trying to go up over and over again, as if it kept getting an up signal.

At the end of day 4, I had John just disconnect the drop target down coil so I could play the game at partial functionality. We both resolved to do more research. He would come by the next Monday.

How I Actually Managed to Find the Problem on My Own!

Larry, from the Roanoke Pinball Museum, had been helping me online. He was perplexed, too, given all we had attempted up to this point. But he suggested I go back to basics and check all the switches in the row and column of the switch matrix containing the drop target switch. There are really only two switches involved in lighting Lock: the right outer loop switch (58) and the drop target switch (57). But a bad switch anywhere along the line could still cause problems. So, back to the Switch Edge test. 


Next, I decided to drop a ball down the top hole behind the drop target (where it goes when it locks) to see if any switches were being triggered that shouldn't be triggered. At first, this seemed impossible, because the ball triggers switches very fast when it's rolling under the playfield and you can't really see them register on the test. Then, I had a brilliant idea.

HELPFUL TROUBLESHOOTING TIP: Put your smartphone in video camera mode and make a video of the Dot Matrix Display while the ball rolls. That way, you can step through the video and see which switches trigger along the way! (John says he's going to use that trick in the future.)

I did see an errant switch triggering--the center rollover lane (switch 77), which has nothing to do with what's happening under the playfield. The switch was definitely faulty--it triggered when you moved it side to side--so I removed and disconnected it.

That didn't solve the drop target problem, though.

On a whim, I disconnected the Green-Black wire on the right outer loop switch (58). I noted that lock wasn't errantly lighting on launch. I thought I was onto something, so I visually inspected every switch along the Green-Black path and adjusted a few that seemed like they might be shorting on something. I reconnected 58 and fired up the game. The lock problem didn't happen!

Thinking I had found the problem, I reconnected the drop target down coil and and the game played properly. Once. Then, it was back up to its old tricks. I disconnected 58 again...and it was STILL up to its old tricks. What gives?

As I stood there thinking what to try next, I was idly playing with the drop target with my hand. Pushing it up, dropping it down, pushing it up, dropping it down. And I noticed something: the metal plate that pushes the drop target up when the up coil engages was, occasionally, getting above the blade of the drop target switch! That meant that, each time the drop target went up, that plate would sometimes drop and engage the switch again. This would cause the repeated up signals I was getting! 

Comparing the blade on the new switch to the one on the original drop target assembly I had removed, I saw that the old switch was a good half an inch longer. being that long, there was no way for the plate to ever get on top of it.

I realized that this could ALSO be the cause of the drop target down issue. 

I let John know before he came to visit.


John came by and replaced the blade on the switch with the one from the original assembly. We put in a new switch for 77. And we crossed our fingers.

And it worked.

Finally. After three weeks of chasing red herrings in the switches and wiring, it turned out the problem WAS a physical issue with the drop target. The switch was too short. 

Quite an ordeal...but problem solved. And I have a bunch of new optos, too.

Oh. Also...

I examined the parts from the original drop target assembly. Turns out, I could have fixed it just by replacing the down coil. That was the part that was originally bad. I didn't need to replace the whole assembly after all.