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.