Hi Pinsiders, I’ve had a strange issue develop with my STTNG and I’m wondering if anyone else has encountered this.

The issue occurs when two balls are locked and play is under way with the 3rd ball, meaning the main lower ball trough is empty.

If I start a mode and then drain the ball... the empty trough doesn’t recognise the arrival of the 1st ball. The mode continues counting down and after a few seconds the game goes into ball search and finds and returns a ball into play.

The trough boards have been replaced and the problem remains.

The 16opto board has been replaced and the problem remains.

Switch test shows the trough optos working normally.

The game doesn’t have the same issue if only 1 ball is locked when the mode starts.

The game doesn’t have the same issue if you start multiball (thus emptying the trough) and drain the first ball during ball save... it sees the ball arrive and relaunches normally. It only happens in modes.

Feels like a software bug to me... has anyone else encountered this?

Thanks for the reply Pin-Guy, that’s a good idea, but all of the optos were working normally.

I’ve solved the problem and I decided to reply to myself and describe the situation, in case any other STTNG owners have the same problem in future, because its a really odd one.

Whenever a mode starts and balls drain, without ending the mode, my natural first thought is to check for a faulty opto. In this case, negative, all optos working perfectly and normally after thorough testing - both bottom and top opto troughs.

So my mind wandered to the 16 opto board. Luckily I own a Dirty Harry so I have an identical board I could swap in for a blind test. Fault continued exactly the same.... no change. Not the 16 opto board either. Thats actually good news, but it doesnt solve the problem.

So I filled the upper trough with 4 balls and emptied the bottom trough. Then I checked that all 4 balls were visible on the switch matrix, which they were. Then went into solenoid test, left hand popper, and ejected the first ball only. I did this by using the door switch as the trigger. After the 1st ball was ejected I returned to switch mode and made my first important discovery on the pathway to a solution.

Switch 3 AND switch 4 were now closed, meaning 2 balls were missing, rather than one! This explains why the ball arriving at the lower trough wasnt ending the mode, because its expecting 2 balls.

So why was it missing and why does this only happen in this specific set of circumstances? Why is it only when 4 balls are loaded in the upper trough and 1 is ejected? Because of physics.

To pop the first ball upward, the design of the VUK & trough requires the ball to move past the 2nd queued ball, which is immediately behind it. Because the 1st ball is seated in its popper hole, it is slightly lower than ball 2 and being a sphere, this means it will collide with ball 2 on the way past and upward. The kinetic energy is then distributed like a newtons cradle back through all of the queued balls making them all jiggle out of place and then re-seat..... not a problem when there’s only 2,or 3 balls in the trough, but when its fully loaded with 4 balls, there is enough kinetic energy to knock ball 4 back into the subway towards the ball diverter, permenety out of sight of the opto and thus the trough is suddenly seeing 2 missing balls at once.

The fix was to cut down and modify a nice new trough shim to fit the upper ball trough so that the balls want to sit forward in the V and are less inclined to roll backward. Also this helps ball 1 sit at roughly the same height as ball 2, limiting the energy transferred through the staged balls.

I hope this discovery helps someone in future.

Hi PinGuy,

No, the V Shape is not in the trough or the VUK, its in the shim. Normal balls are in use. The trough is in perfect condition otherwise, very little wear evident, the whole game has had very little use. There are no hacks. This game has a 6 ball trough at the bottom and a 4 ball trough at the upper left (Where the VUK is).

The shim helps reduce the kinetic energy back through the queue of 4 balls, when the upper trough is fully loaded, but I still get occasional issues. The games design doesn’t allow the subway plastic to meet the trough perfectly and there is an overlap between the two where the trough flares out to accept the plastic subway... the 4th ball can bobble backwards in this scenario and sometimes it wont have enough gravity fed momentum to overcome the minor ball traps around the entry to the trough....such as the subway wall cutaway where the diverter plunges through the plastic, or the flared entrance to the trough. So the ball gets parked back in the game away from the optos instead of rolling back into the trough.