(Topic ID: 251987)

STTNG Under-Playfield Diverter Transistor Failing with Tieback Voltage

By Williamson

4 years ago


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#1 4 years ago

Hi, I’m a beginner looking for some help on the commonly-seen STTNG under-playfield diverter issue.

TL:DR - I keep blowing Q16 transistors but the tieback voltage appears (from what I can tell) to be coming to the 8-driver PCB properly.

When I first experienced the problem, I did not recognize what was happening in time and was left with a blown Q16 transistor and burned-up coil.

I replaced both and then after reading about this common problem with the 50V diode tieback line, I verified continuity on the 50V tieback line from the drop target coil to the driver board. Thinking that maybe this was just an old transistor that failed on its own, I tested the solenoids and started playing. One game played fine, but then I noticed the plunger catapult firing weakly and while trying to debug this, I realized that the under-playfield diverter coil had locked on again. Luckily I caught it this time before the coil became damaged (based on a resistance check).

This time, I replaced both the Q16 and Q8 transistors (after seeing it recommended to replace the preamp transistor along with the main transistor). I then powered the machine on and checked the tieback voltage on the diode D16, which read about 70V. At first, I thought this was too high for the “+50VDC” signal, but then got the impression after reading forums that seeing 70V is normal. The presence of 70V made me think there was not a problem with the tieback. Thinking that maybe the replacement of the preamp transistor had fixed the root cause of the problem, I tested all solenoids multiple times, which seemed to be working well. I then played a couple of games, and the same symptoms eventually popped up again: a weak plunger catapult followed by a locked-on diverter coil.

Has anyone seen anything like this? What should my next steps be? Could this be a flaky connector pin on the tieback that is connected when I measure it but intermittently loses connection during gameplay? Or, does the plunger catapult issue indicate another problem? I may be missing something very basic based on my lack of experience.

Thanks in advance for any advice that you can provide.

#2 4 years ago

There is a lot of things left unanswered here in your post which is understandable for a pin repair guy/gal in training.

Quoted from Williamson:

TL:DRok, I have to admit I have no idea what this means...

keep blowing Q16 transistors but the tieback voltage appears (from what I can tell) to be coming to the 8-driver PCB properly.

This would be a difficult measurement for anyone to make as it's hard to differentiate between a voltage measurement taken from the tieback voltage applied to the cathode of D16 (tieback) and the coild voltage applied to the anode of D16. With a broken tieback you will read very close to the same voltage on D16

Quoted from Williamson:

I replaced both and then after reading about this common problem with the 50V diode tieback line, I verified continuity on the 50V tieback line from the drop target coil to the driver board.

The best way to verify this tieback is to disconnect J4 from the 8 Driver and check continuity from Pin 1 of the connector to the 50V test point on the power driver board.

Quoted from Williamson:

I replaced both the Q16 and Q8 transistors (after seeing it recommended to replace the preamp transistor along with the main transistor).

I'm not sure this origin of this advice, nor do I agree with it as the pre-drive (Q8) is a signal transistor that carries no load and is also protected by D8; it's much more likely that a bad ribbon cable would causing Q8 to lock on, and damage Q16 vs a failed transistor.

Quoted from Williamson:

What should my next steps be?

Perform a resistance/continuity test to J4-1 as mentioned above, verify the ribbon cable is connected properly, try swapping the ribbon cable around on BOTH sides red line on the opposite side (J1-26) on both connectors to see if this has any effect.

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#3 4 years ago

Thanks, Pin_guy, for your suggestions (and your patience). I measured continuity via your easier suggested method of disconnecting J4 and measuring from J4 Pin 1 to TP6 50 V on the Power Driver board, and there is good continuity (< 0.5 ohms). I also inspected the J4 connector and pin 1 on the header and didn’t see any obvious issues.

I also made sure the ribbon cable connection was not loose on either side (8-driver board or CPU board), and the header pins and cable itself look ok from a quick inspection of the top of the board.

I was going to try your suggestion of rotating the ribbon cable on both sides, but I found out that my last burned Q16 transistor incident also fried the new coil (melted sleeve and measuring ~2 ohms). I had an extra transistor, but no extra coil, so I’ll have to order a new one before I can test out the new setup. Is there an easy way to tell if rotating the ribbon cable improved things without waiting for the transistor to be damaged? Should I also try to repin J4 Pin 1 in case it has an intermittent connection? The connector looks ok, but I’m willing to try it if there’s a chance it could help.

Do you have any other recommendations for things to check or try in the meantime? Does the weak catapult plunger seem like it would be related, or does it seem like a different issue?

On the bright side, I’m getting plenty of experience with soldering and desoldering these through-hole parts.

Thanks again for your help.

P.S. “TL:DR” is short for “Too Long: Didn’t Read” and is shorthand for putting a quick one-line summary at the top of a long, rambling post like my post above.

#4 4 years ago
Quoted from Williamson:

Is there an easy way to tell if rotating the ribbon cable improved things without waiting for the transistor to be damaged?

Actually there is, since the drive transistor is a darlington transistor, the metal tab is going to be connected to the transistors collector, . hen the transistor is on, the collector will be very close to ground potential and you can use this to your advantage fairly easily, all you have to do is follow a few simple steps:

1) disconnect J4 from the 8 driver PCB.
With J4 removed, the highest voltage on this board is going to be +12V.
2) measure the voltage between the 12V test point on the power board and tab of this transistor
If you measure 12V the transistor is locked on, if you measure nothing then its off.

Side note, a common practice for checking drive transistors this way is to use a 12 lamp and socket with one wire connected to the test point and a free wire to probe the transistors; this proves to be much faster than a meter when evaluating a driver board. This is exactly what I do when working on Fliptronics boards, I made a test jig with 8 LEDS on it and just connect it to the flipper drivers so even if there are shorted transistors, I cant blow fuses
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#5 4 years ago
Quoted from Williamson:

Does the weak catapult plunger seem like it would be related, or does it seem like a different issue?

It shouldn't be related, but if you are pulling enough current on the 50V line to melt a coil, you may be seeing a sustained voltage drop to the other coils. I personally wouldn't address this until the known driver issues are resolved.

#6 4 years ago

Ok, thanks again for your helpful responses. I’ll check back in when I have a new coil to try out.

#7 4 years ago

Ok, the new coil arrived today, and I installed it this evening (after rotating the ribbon cable at both board connectors). I also ordered some extra 1N4004 diodes, and I installed those onto the four coils (since I read that it was a good backup for the tieback 50V), paying attention to the direction of each diode.

On power-up, things seemed normal, with the sequenced solenoid test successfully activating all of the coils in the machine through a couple of loops. However, like the previous failures, the plunger was sometimes firing weakly, and I realized the under playfield diverter coil had eventually locked on again. This time, the coil reads 10.5 ohms, so the resistance went up rather than down, but I assume it will still need replacing. Luckily, I ordered another coil, but have not installed it yet.

So, a disappointing result, but hopefully it eliminates some possible causes. What would your recommended next steps be?

Thanks again for walking me through this.

#8 4 years ago
Quoted from Williamson:

So, a disappointing result, but hopefully it eliminates some possible causes. What would your recommended next steps be?

What were your results of this test?

Quoted from Pin_Guy:

1) disconnect J4 from the 8 driver PCB.
With J4 removed, the highest voltage on this board is going to be +12V.
2) measure the voltage between the 12V test point on the power board and tab of this transistor
If you measure 12V the transistor is locked on, if you measure nothing then its off.

#9 4 years ago

Ok, I should have tried your recommended test yesterday before hooking up J4, but I tried it tonight after replacing Q16.

The tab of Q16 is jumping around between 0V to -0.5V (when measured relative to the “12VU” TP1 test point). I assume this indicates that the transistor is not stuck on.

I also went ahead and measured all of the tabs of the other TIP102 parts on that board:
Q9: 13.V (solid)
Q10: 11.6V (solid)
Q11: Jumping between 0V to -0.5V
Q12: 6.8V (solid)
Q13: Jumping between 0V to -0.5V
Q14: Jumping between 0V to -0.5V
Q15: Jumping between 0V to -0.5V
Q16: Jumping between 0V to -0.5V

After these measurements, I started the solenoid test on repeat on the top diverter, leaving it running for a couple of minutes. (Obviously the coil did not actually fire with J4 disconnected.) I then ran through the full solenoid test sequence a time or two before stopping the test and then repeating the measurement on Q16, but it still was still jumping around 0V to -0.5V. So, it seems like the transistor is not getting stuck on with J4 disconnected. What would you recommend that I check/try next?

In other news, the plunger was still firing weakly in the sequenced solenoid test, so that may be a separate problem.

Thanks for your help.

#10 4 years ago

@pin_guy - you're awesome! Thanks for handling this one and sharing your LED test bench - what a great idea!

#11 4 years ago
Quoted from Williamson:

The tab of Q16 is jumping around between 0V to -0.5V (when measured relative to the “12VU” TP1 test point). I assume this indicates that the transistor is not stuck on.

I also went ahead and measured all of the tabs of the other TIP102 parts on that board:
Q9: 13.V (solid)
Q10: 11.6V (solid)

Correct, if the Q16 was stuck on you would have measured 12V, the same as if measuring to ground. The tabs of Q9 and Q10 have 20V on them and you are measuring the difference between your 12V test point and the 20V on the tab, so these measurements seem fine to me. Q12 seems off but is not used in your game so I'm going to just ignore it.

If you are still melting coils and know the driver is not locked on, then the issue has to be in the wiring, perhaps a pinched wire providing a conducting path for the coils return wire (BRN-WHT); the next step is to check the resistance of this wire from the coil to ground (game powered off and J4 disconnected); you should be reading an open circuit here, if you measure anything else, you need to correct it.

#12 4 years ago

Ok, I had a couple of busy evenings but was able to get back around to this issue this evening. After disconnecting J4, I am seeing about 10 kilo-ohms between the return wire and one of the ground lugs on the cabinet. Is this lower than you would expect? I am seeing similar readings to ground on multiple other coils (including the other diverter and the plunger), but not on all of them (e.g. the upper flipper, which is reading megaohms).

Also, when I have the problem, it seems like the transistor is being damaged first, because I often have a damaged transistor and an undamaged coil (although I have damaged a couple of coils so far in addition to the transistor when I didn’t catch the transistor locking on in time). So, I don’t believe the coil is becoming damaged first and then that in turn damages the transistor. (It seems to be the other way around.) If it is the transistor being damaged first, could a path to ground on the coil cause damage to the transistor? I was thinking this would instead damage the coil first, but maybe I am missing something. Also, whatever the cause is, the coil is able to fire and release several times during a solenoid test before eventually locking on.

Thanks again.

#13 4 years ago

Your ground checks seem fine, and if the coil is only locking on after the transistor fails then it essentially eliminates a grounding issue; in fact, your entire post reads just exactly like you are missing your tieback voltage and hammering the drive transistor with the coil flyback voltage making it fail...the problem with this theory is that to my knowledge you are ONLY seeing a problem with this one coil and if you were missing your tieback voltage you would be seeing multiple failures.

So we are back at a board issue with a possible bad flyback diode (D16), or broken connection to it on the 8-Driver PCB. You can verify these connection points fairly easily:
1) Cathode of D16 (striped side) to J4-1
2) Transistor tab to Anode side of D16
3) Transistor tab to J4-2
4) Anode side of D16 to J4-2

5) Diode test:
With J4 disconnected, place your meter on diode test, and with the black lead on the cathode of D16 and red lead on the anode, you should measure around 0.55V.

If all else fails you can always put a diode across the coil windings on the coil itself, but if you do this, you must observe the correct polarity with the diodes cathode (striped side) on the same terminal as the coils source voltage (violet/green) wire.

EDIT: Sorry, I forgot you had already done this

Quoted from Williamson:

Ok, the new coil arrived today, and I installed it this evening (after rotating the ribbon cable at both board connectors). I also ordered some extra 1N4004 diodes, and I installed those onto the four coils (since I read that it was a good backup for the tieback 50V), paying attention to the direction of each diode.

1 year later
#14 3 years ago

Dear @pin_guy,

I'm past the CPU repair, (you helped me with on another thread), and now I'm tackling this poster's exact problem, except it's the "bottom" diverter that locks on my machine.

I have tried all you suggest above, but I'm hoping you have another trick up your sleeve. Here's the blow by blow I went through if I can remember the sequence correctly:

- When I brought the machine home, the "top" coil was totally melted down -- I removed the whole subway system, disassembled everything, cleaned it, the optos, tuned up the two diverters, replaced the melted coil. Reassembled and ran the coil test - nothing on those two or the drop target coils - I realized F103 was blown.

- I decided since the drop target mech was all on the same circuit, I'd pull that out, clean, tighten, etc. I combined the tieback and the two voltage leads and ran a single patch to the coil so they remain connected even if that lug lets go down the road - I have tested continuity back to the 8 Driver repeatedly from F4/pin 1 to the 50V test point, and it's always good.

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- Replaced F103 - now all seems ok except the bottom coil does not fire. Top coil + 2 drop target coils test fine.

- Replaced Q16, Q15 + D15 on 8 Driver board

- Now all coils operate through coil test. I load the balls, and for the first time, the machine is able to successfully stage the three subway balls - yay!

- I start a game and things seem ok - until I hear that tell tale crackling of a locked coil. The smoke stinks up the basement for a good couple days. Q15 is cracked and F103 is blown again. Fuck.

- I check TP6 on the driver board and find that it's reading 90V... I had read on here in a few places that 70V was about right. Ninety is nearly double what is called for - this can't be right. I found that R260 was open. Replaced. TP6 back to a steady 70V. Yay!

- Shamefully replaced Q15 and F103 again.

- All coils pass solenoid tests, so I load the balls again, start another game, this time keeping an eye/ear on that coil - it has quickly locked again. I pull the coin door open prior to the transistor/fuse blowing. I head in to solenoid test and I can measure the test pulse across R24. The signal is not stuck on, but if I shut the coin door, it immediately locks up.

The two coils as they currently stand just for reference:

IMG_1360 (resized).jpgIMG_1360 (resized).jpg

I'm not sure what in the heck else I can check here. 8 Driver resistors? I checked the coil lugs to ground with F4 disconnected - no beeps. I also reversed the ribbon connectors on both boards. Do you have any other suggestions?

Thanks a lot.

Paul

#15 3 years ago

Oh, and hey Williamson - looks like you still have your TNG... did you ever get this resolved??

#16 3 years ago

Sounds like you are all over this issue; to me, this really sounds like your flyback diode isn't in the circuit. I would make sure it is by removing J4 from the 8 driver board and measuring the resistance between the cathode of D15 to J4-1 this should be very close to 0 ohms and it should measure exactly the same as the cathode of D16 to J4-1. If D16 looks like its ever been replaced this would be high suspect as your 50V tieback on J4-1 goes directly to D16 in the topside of the board then feeds through a via at D16 to the bottom side of the board for D13-D15.

#17 3 years ago
Quoted from Pin_Guy:

Sounds like you are all over this issue; to me, this really sounds like your flyback diode isn't in the circuit. I would make sure it is by removing J4 from the 8 driver board and measuring the resistance between the cathode of D15 to J4-1 this should be very close to 0 ohms and it should measure exactly the same as the cathode of D16 to J4-1. If D16 looks like its ever been replaced this would be high suspect as your 50V tieback on J4-1 goes directly to D16 in the topside of the board then feeds through a via at D16 to the bottom side of the board for D13-D15.

All those test points showed 0.000 ohms. I realized Q15 was actually blown again, so I replaced it. Tried the it out again, this time the *top* diverter locked, and now Q16 is blown.

There didn't seem to have been any work done on the board previously. (it is getting uglier the more I fumble around blowing it up over and over though)

IMG_1365 C.jpgIMG_1365 C.jpg

I just received a Rottendog replacement board from LSOG (thanks Dennis!) and it arrived today. I'm a little afraid to just pop it in and immediately blow it, too. I guess I'll do a few more tests to make sure there's nothing grounding somewhere.

#18 3 years ago

Well I popped it in, and it appears to have done the trick. I just played three whole games uninterrupted by smoke or fire. I can finally continue with the restoration!

Seems like maybe the 10K resistor mod might help the old board??

Thanks for your help once again @Pin_Guy.

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