My game has the exact same issue.

And it has HAD this issue since it was shipped from the factory.

I've been able to completely rule out anything in the clock except the motor - it's not Ingo's board, it's not the hour/minute hand looseness.

It's a real low priority for me, since it doesn't affect game play (and like you said, it resets the next time code calls to home the clock..) but here's the two issues I'm thinking it is -

(1) The clock motor is running faster than expected (motor out of tolerance, flasher voltage too high, etc.) and the CPU's scanning of the 9th switch column is slow enough to where the minute hand's breaking of the 12:00 opto is missed, or
(2) The 8-driver PCB has a cap or resistor out of tolerance, and the column driver is occasionally not rising high enough, or
(3) There's a resistor/cap out of tolerance on the MPU, and the quick break of the hour opto isn't dropping low enough for the 339, or high enough for the 240 to raise the 240's output.

My next step, when I got around to it, was going to be replacing the 8-driver PCB.

Edit to add that this 'missing' of the 00 minute opto could - and likely is - occurring on the other optos. The ROM is programmed such that it can handle individual misses (i.e. If the clock is running backwards, and it sees 9:45, then misses 9:30, then 9:15, it won't stop on that 'miss' as an error, it will keep going. In this case you - the player - won't see anything unusual - until the hour opto is the one that's missed, and when it happens when the clock reaches 12:00. Also note that you will NEVER see this in test mode. Why? Because this 'reset' speed has no test in Test Mode - the 'Fast' speed in test mode isn't the same 'fast' used to reset the clock.

I had weird issues a few years ago with mine. Bought a new board and the problem went away but quickly came back. Here was my issue. One of the clock hands could be bent just enough that the tab did not trigger the opto. Or because of the play in the shaft the tab sometimes triggers and sometimes not. I added a small amount of black electrical tape to the tab and no more issues. A few people have had a similar issue and resolved it in this way.

Good luck.

My game is an early sample - I haven't compared motors with a production one close enough to notice - however, there's at least one other user on here (Pinside) that has a TZ that was made a day before mine (literally) and noone else seems to have reported this issue, including him. So I'm leaning towards something just barely out of tolerance causing the MPU to 'miss' it.
On my game, I've replaced the ribbon cable, to no effect. (Not because of this issue, but because they were all just really old.) I have a feeling that if it WAS the ribbon cable, you would experience other issues with the flashers that that board controls as well. (i.e. Barekly firing, or flashers NOT firing when they should..)

This IS a possibility - but not on my game, at least. If I use electrical tape to WIDEN the amount of the interrupter (i.e. make the interrupter wider than the hand), so that it causes a longer break in the opto, yes, that 'fixes' it - causes the issue to not happen. As it is, my interrupter is already too long - if I raise the playfield up, it will 'nick' the bottom of each opto as it spins. I can't make it any longer than that.

I'll also add, a fact that leads me to believe it's something on one of the boards is that it ONLY seems to happen when the MPU is handling a lot of different processes.. Like at the end of a ball when modes are getting canceled, at the end of powerball mania (if you drained before timer runs out).. On lighter times (like exiting from test mode) I don't think I've ever seen it happen.

+1, I had a similar issue on my TZ 7-8 years ago and fixed it using the above described method. I haven't had the issue surface again since using this fix.

Gord

I'm using that as a general term - i.e. Voltage to the motor is too high,. making it spin a little too fast, to the column transistor on the 8-driver PCB not bringing the column low enough (or a pull up resistior on the same board bringing the column too high), and so on.

It's just a matter of ruling everything out. I'm going to be borrowing another game's 8-driver PCB (that controls the switch column driver) this weekend for testing - if I plug this board in, and it still occurs, then I can rule out anything on the 8-driver PCB.

Mine does this too but I decided to try and not let it bother me. I do notice it when it happens but know that it'll fix itself.

Eric

Yeah, it's honestly so far off my radar that I never actively tried fixing it. Now, however, another Pinsider local to me has a TZ, and I can borrow the driver board to test it out. If it turns out NOT to be the driver board, I will likely stop there until I have a chance at a WPC-89 MPU board, etc..

An update -
Like I mentioned, the next step for me was to try a different 8-driver board. With a local pinsider having a TZ in-house that he's working on, I borrowed that game's board, and swapped it into my game.

Unfortunately, the problem remained. For S&Gs, I'm going to borrow the MPU board next, and try that.