We're having an issue with a Mustang Pro, where sometimes a ball gets stuck in the through, and the pinball registers it as missing.

The issue is that the through jam opto, which works in the test menu, never engages the VUK when the ball gets stuck, never clearing the jam and leaving the pinball stuck.

The optos themselves work fine, as they don't show any issues when placed in another machine. In the test menu, all switches and optos in the through register fine.

Other Stern machines we've seen try to engage the VUK as soon as you interrupt the opto, this one doesn't. We thought it could be a software issue, so we updated from 1.20 to 1.45, but no changes there.

Does anybody have any suggestion?

Thank you kindly for your suggestion! Unfortunately, the plunger doesn't touch under the pinball at all, so I don't quite get how loosening it would free up the ball...

The issue seems to be that the ball on top closes the opto, but the one on the bottom doesn't, and the kicker doesn't engage. If we push the ball in their proper positions (so that the bottom one closes the opto at the same time as the top one), then it kicks it out. Honestly, I was fully convinced that the jam/ball stuck opto would engage the coil by itself regardless of the other switches, but I was mistaken on that.

The only other Stern pinball we currently have in the shop is a Terminator 3, and it seems like it behaves the same way (but with only 4 balls, it much less likely to get stuck like that).

Any ideas?

IMG_20180522_094637 (resized).jpg

When the balls are in that position, the game registers a single pinball in the through (because the conga line is shifted forward just enough to miss all the switches, and the last ball on the bottom doesn't quite reach the bottom opto), so it never clears the game for play...
Removing one or two balls from the through seemed to make the issue disappear, as with less pressure the top ball didn't quite manage to get stuck in that position (reason why the Terminator 3 we had for comparison never did behave like that, given that in that machine, even if we stacked the ball manually to trigger the issue, as soon as it engaged a coil on the field the vibration would knock the ball in the through loose and clear the jam).

Unfortunately, "remove balls from play" is not a solution, so we left it as it were, and we're hoping for the best... A bit ashamed of that, to be honest.

To be fair, it seemed to be pretty uncommon while in the shop for the plunger to fail to get a ball in the through AND get the ball stuck like that without us clogging the shooting lane intentionally to reproduce the issue ( "uncommon" as in, we never saw it do it by itself), but we had to bring it back into the shop because they complained on location, so... Dunno.