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#13 STTNG Tie Back Mod *BEST WAY*

Menace

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Staff member
Nov 14, 2012
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Santiago de Aurora
Alright, for anyone that owns or has owned a STTNG you likely have heard about the dreaded Aux8 PCB "tie back" wire along with all of the issues this game can have surrounding it. For those that have not heard about this particular issue, the coles notes are this...

STTNG has a LOT of parts stuffed into the game, and as such there are not enough transistors on the main driver PCB to control everything. What the designers came up with was a small auxilary driver PCB (which we all refer to as the Aux8 board) that is located in the upper right corner of the back box that gives these games the added transistors they need. You will find these on other widebody games of the same era like TZ and IJ etc, but this mod only pertains to STTNG! The Aux8 pcb must be connected to the 50V coil power at all times, or bad things happen... such as locked on coils and ultimately blown transistors.

The factory decided they would connect the Aux8 PCB to the 50V coil line by running a small wire down from the back box to the PF and attaching it to the 50V side of a coil, essentially "tieing it back" to the Aux8 PCB. Out of all the games that use an Aux8 pcb it is the only one that they chose to wire up this way, and I can only guess it was done this way to save money.

ST Tie Back2.jpg

In this picture we have the underside of the STTNG drop target assembly located at the top of the PF, and I am holding the tie back wire in my fingers. You will notice it's purple with green stripes just like the larger AWG wires attached to the coil in the pic. Normally it is attached from the factory to the same coil lug with the other two purplpe/green wires, as this ensures the Aux8 PCB is connected to the 50V coil power. What ultimately happens is due to all of the vibration this mechanism has to endure, the smaller AWG tie back wire can and does break off of the coil lug, causing the Aux8 PCB to lose it's 50V connection. At this point one of the under PF subway divertors usually locks on because it's associated transistor has been damaged as a result of the failed tie-back. I have had the opportunity to work on at least 5 STTNG's over the years, and I've seen at least 3 of them with this coil literally melted.

Now for those that have done any sort of research on how to resolve this issue, there is no shortage of information out there on the subject. Unfortunately I was never overly satisfied with any of the recommendations, as it usually involved running a thicker extra wire down from the Aux8 PCB to another coil under the PF to double up the tie back in hopes to add some redundancy. What I've come up with below is my recommendation to solve the tie back issue by keeping it super simple and keeping it all contained within the back box.

(PSA: AS WITH ANY MOD LIKE THIS, ANYONE THAT ATTEMPTS THIS DOES SO AT THEIR OWN RISK! I CANNOT BE RESPONSIBLE FOR ANYONE ELSE'S WORK!)


Parts / Tool you will need;
-Wire cutters / Stripper
-Crimping Tool
-1pc trifuricon pin
-3 Position 0.156" Molex Connector

ST Tie Back3.jpg

First, start by disconnecting the tie back wire from the coil under the PF and begin to pull it back out of the harness.




ST Tie Back4.jpg
Continue to pull the tie back wire out of the harness all the way up into the back box. As you can see there is quite a bit of wire.




ST Tie Back5.jpg

Pull the tie back wire up and out of the harness all the way back to the Aux8 pcb. Here I have it all coiled up nicely to show you roughly how much wire there is between the Aux8 PCB and the PF. Think of all the potential places this wire could fail over the years!





ST Tie Back6.jpg

Locate header J108 on the main driver PCB, which is along the top edge about dead center. Now run the tie back wire you've pulled back from the Aux8 PCB to the J108 header on the driver PCB and cut the excess off, making sure you leave at least a few extra inches. Then strip the end of the tie back, and crimp a trifuricon pin to the end.




ST Tie Back7.jpg

Install the freshly crimped pin into the 3 position Molex connector as shown, making sure you install it in the left most position of the connector.





ST Tie Back8.jpg

Then connect the terminated tie back wire/connector to J108 on the driver PCB as shown making sure the wire is on the left most pin. (which is Pin 1 of that connector)


That's it, you're done! By doing this you have essentially done the EXACT same thing the factory has done by running that long wire down to the drop target coil, but this connection is more robust saving your Aux8 and subway coils from self-destructing.

D.
 
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Hammerhead

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Nov 16, 2012
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This sound pretty straightforward; I'm going to give this a try. I've already replaced one melted coil and a few blown transistors on the Aux8 board, so I'd like to avoid doing it again.

You mention the Aux8 board was also used on TZ and IJ; what was done on those games to get the 50V if they didn't use the tieback approach?
 

Menace

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Nov 14, 2012
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Santiago de Aurora
You mention the Aux8 board was also used on TZ and IJ; what was done on those games to get the 50V if they didn't use the tieback approach?

To be honest I'm not sure and I would have to look at the cab wiring schematics to see if they indicate how they did it, but my guess is they tied it into the 50V in the head somewhere.

If you wanted to take my mod one step further, you might also want to replace the IDC connector on the right side of the Aux8 PCB with a new molex connector and trifuricon pins to eliminate the crappy IDC pins as well...

D
 

spiroagnew

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Dec 1, 2012
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I've done no online digging to find the answer to this, perhaps you know, Doug:

What was Williams' reasoning for running twenty feet of wire to a vibration-prone coil to tap into the 50V in the first place? The whole thing reeks of sloppyness--more like an operator hack rather than a technique used by the world's foremost pinball manufacturer.
 

Chris Bardon

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Nov 15, 2012
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Mississauga, ON
To be honest I'm not sure and I would have to look at the cab wiring schematics to see if they indicate how they did it, but my guess is they tied it into the 50V in the head somewhere.

I'm pretty sure that in IJ and TZ these drivers were only used for flashers. STTNG is the only game that uses it for coils. The real question is, why not drive the coils from the main driver, and the flashers from the AUX? Better still, why drive the coils that are the absolute hardest to see from this prone to failure board? You can tell that the subway coils are locked on during gameplay, but usually the first sign is the smoke and smell of burning plastic :)

This is a great idea, and something that seems pretty obvious when you think about it. None of the docs out there suggested the same thing though. My solution for this was to do the thicker wire back to the PF, but also to double the leads to tie into 50v from 2 places at once to give it some redundancy. This way would have been much simpler.

Now I kind of want this game back though. Haven't played it in a few years now.
 

Menace

Well-Known Member
Staff member
Nov 14, 2012
2,439
249
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Santiago de Aurora
I've done no online digging to find the answer to this, perhaps you know, Doug:

What was Williams' reasoning for running twenty feet of wire to a vibration-prone coil to tap into the 50V in the first place? The whole thing reeks of sloppyness--more like an operator hack rather than a technique used by the world's foremost pinball manufacturer.

I've often wondered this myself, and I honestly cannot come up with an answer to this other than overall cost? Back in the day it was probably cheaper (parts & labor) to run all that wire and tie it to a coil vs. terminating it to another connector as the wire was probably much cheaper than the connector. What I don't understand is why they chose a coil UNDER the PF when there is a perfectly good coil already in the back box?!?

What is even more odd about this whole setup is why it was done in the first place? I mean it's not like STTNG was the first "superpin" that was released (it was actually the 4th) that used this Aux8 PCB, so I have a difficult time believing this was an afterthought or "Oooops" moment? I guess we will never really know.

D
 

Menace

Well-Known Member
Staff member
Nov 14, 2012
2,439
249
63
Santiago de Aurora
I'm pretty sure that in IJ and TZ these drivers were only used for flashers. STTNG is the only game that uses it for coils. The real question is, why not drive the coils from the main driver, and the flashers from the AUX? Better still, why drive the coils that are the absolute hardest to see from this prone to failure board? You can tell that the subway coils are locked on during gameplay, but usually the first sign is the smoke and smell of burning plastic :)

It's an interesting thought... but where coils and flashers are driven the same way you would still have the issue of locked on flashers if the tie back wire was to fail. And not sure if you've seen some of the damage a locked on flasher can do, but I think I'd rather a locked on coil over a flasher. (melted coils can be replaced easily, melted plastics and inserts not so much)

D
 

sml49

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Feb 25, 2020
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Hi there, I used this tie back way on my STTNG this week, but unfortunatly it didn't work so well for me.
First night after half an hour of play, F103 blew up, and second night, again after a few play, the green drop target coil burned too :(
I'm thinking of getting this little wire from J108-1 back to the playfield cabinet.
Anyone has any idea of what happened ? Only thing i see so far is I used a brand new 3 pin IDC pin connector instead of trifucon...

Thanks for your help.

Sam
 

a.stebel

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Jul 7, 2014
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Well, owner a STTNG for as long as I can remember. Never once knew about this tie back mod.

Done. Super easy, thanks for the post Adam.
 
I recently performed the tieback on my STTNG pinball following the directions provided.
On first start up after the procedure, I am getting a tech message of "gnd. short row 7 wht-vio.
I went back and looked at my soldering job on the switch and it all looks good.
Everything was working up to that point before I did the tieback.
Now it seems like all the switches on row 7 are not registering?

How to troubleshoot?

Thanks
John
 

MrMikeman

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Nov 25, 2019
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Ottawa
Why are you looking at a soldering job on a switch? The tie-back mod has nothing to do with switches. It's about removing a power wire from a coil and wiring it directly to the driver board. You therefore worked on the wrong wires??
 
No I have the tieback wire done to the coil as per the instructions, but when I was there I re-soldered a loose wire on the switch for the drop target as well...it was just barely hanging on. So there was a bit of soldering done at both points..the coil for the tie back mod as well as the switch at the drop target. I guess I should have been more clear in my description above.

John
 

MrMikeman

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Nov 25, 2019
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No I have the tieback wire done to the coil as per the instructions, but when I was there I re-soldered a loose wire on the switch for the drop target as well...it was just barely hanging on. So there was a bit of soldering done at both points..the coil for the tie back mod as well as the switch at the drop target. I guess I should have been more clear in my description above.

John

Ok.

1st rule of pinball maintenance: If something was working before you worked on it and now is not working, then you did something wrong! Go over your work.

That's where you need to start troubleshooting. Inspect your soldering job on the switch, make sure no blob of solder is causing a short. Make sure you are soldered to the correct pins and the diode(if there is supposed to be one) is still intact and functional and oriented the correct way. Just look at the other switches. The wires/diode should be in the same layout. However to test the diode you need a multimeter.

As part of the troubleshooting steps you can try removing the wire you re-soldered and see what happens.
 
Ok.

1st rule of pinball maintenance: If something was working before you worked on it and now is not working, then you did something wrong! Go over your work.

That's where you need to start troubleshooting. Inspect your soldering job on the switch, make sure no blob of solder is causing a short. Make sure you are soldered to the correct pins and the diode(if there is supposed to be one) is still intact and functional and oriented the correct way. Just look at the other switches. The wires/diode should be in the same layout. However to test the diode you need a multimeter.

As part of the troubleshooting steps you can try removing the wire you re-soldered and see what happens.
Ok so I double checked the tie back to J108 and it all looked good. I checked at the drop target switch and saw I had two different coloured white wires attached at one terminal on drop target switch. One wire is white with violet and the other is white with blue. I checked the switch wiring diagram in manual and see it’s supposed to be white with violet. I disconnected the white with blue wire and I no longer get the row short message on boot up. But all of the switches on the white with blue row don’t work because that wire is now not connected. I am trying to find a switch in that general area with a white and blue wire missing as it’s a short wire so can’t be far from this coil.
See photos. Any idea?
 

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Top lane left. It's in the manual. Row 6 is white/blue.
Thanks for the replies Mikeman. I checked the top left lane and it already has two white with blue wires on one side of the switch, same as the other two lane switches so I don’t think it is there.
If I jumper the loose wire to a return lug on another switch in the same row will it show switch ooening/ closing as I jumper it so it will tell me which switch it belongs too?

John
 

MrMikeman

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Nov 25, 2019
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With that wire not connected, which switches do not work in switch test? Specifically are any row 6 switches not working?

Are you absolutely sure it's white-blue? Hard to tell for sure on the pic. Seems blue. The left pop bumper (closest to the loose wire) should be white-brown. Compare that color with the one that is loose.

And yes it could be possible to have 3 wires going to 1 lug depending on how the game was wired. I'd avoid just soldering wires anywhere without being sure though. First make sure it's an actual switch wire and not something else or some wire that a previous owner just added..

Mike
 
I had a bit more time to look at this on the weekend.
With that wire not connected, some switches I tested on row 7 are not working.....left gun home switch, top lane centre switch etc.
I checked to see if that wire appeared to be an "added" one by another owner etc. and I would say it is not. Looks like its original.
It "appears" blue, but I think its supposed to be violet as that would make sense for that switch and row number. I found another picture online of someone elses STTNG drop target switch and again the wiring looks like mine where one of the switch wires appears more bluish than violet.
With that wire not connected, I don't get any row short message on startup, and alot of row 7 switches aren't working. But with the wire attached I get the row sahort message as soon as it starts up.
Here is the other photo I found online showing another users STTNG drop target switch.....
1650295748656.png
 

MrMikeman

Well-Known Member
Nov 25, 2019
647
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Ottawa
One of the non-working switches is shorted to ground. So when you connect the wire the short shows up. Check the diodes/wiring on those switches.