Raise the playfield and check all of the switches involved with the subway for broken wires, missing arms, and opto function. The game preloads balls in the subway to make the gameplay smoother so you don't have to wait for the ball to roll down from the top. If it is cycling the balls, then it isn't able to get the balls where they are supposed to be and it will keep trying until the end of time. 99% of the time it is a switch issue, the other 1% is a blown transistor for the diverter. It sounds like the diverter will fire in test mode, so it has to be a switch.

Looks right so far.

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Now put the balls in and check it again.

It will stage three balls.

When it loads the first two balls, you will hear the diverters buzz.

The third ball won't buzz.
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If it launches a fourth ball before releasing one from the vuk, then you know it's your trough opto boards.

Do you still have the original trough boards? Those things are crap, best sttng mod you can do is replace the transmitter board.

If all the switches were working correctly, it wouldn't keep launching balls. Period.

If you are 100% that there are no problems with the subway diverters, vuk's and their corresponding switches then the problem is in the trough.

The transmitter boards get the crud beat out of them because williams attached it to the coil bracket that loads the ball. Try jiggling and bumping the wires while there are no balls in the trough and see if and of the optos register. Because the components on the board are old and huge, they shake a lot when the ball loads and give a false signal. They would work during test, but not when the game is loading balls.

If you want to salvage the transmitter board, pull it and reflow all the traces for the components and pins. While you are at it make sure all the wires are tight in the molex connector.

Is this still happening? If so, then you know what opto isn't working. Fix this first. In case you didn't know, if you look at an opto transmitter with your cell phone camera, you will see it glow if its working. Sometimes the transmitters just get weak.

Opto glow.

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Remind me to never touch the trough on this game. I just spent the morning rebuilding the connectors because the power wire was flakey and each time it flickered it would launch an extra ball into play.

What a pain.

Did you see the picture where all of my optos are working? My game was still launching extra balls because the optos would flicker when the trough coil fires. They work perfectly in test mode.