(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.

#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.

#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.

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#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.

#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.

#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.

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