That'd be my guess too. There's a lot of math that goes into firing the Ball Count Unit Step Up coil:
Blue Chip Ball Count SU (resized).jpg
Ignoring the details for the moment, the circuit breaks into three basic parts:
1: counting up at the start of a new game
2: counting up on an extra ball due to reaching a certain number of points
3: counting up because of a playfield Special
Fortunately it's easy to disable 2 and 3 by just unplugging the adjustment jones plugs which effectively removes those parts of the circuit. So try unplugging them one at a time to see if you can identify which of the three circuits is causing the problem.

phil-lee is right that these steppers often have some kind of electrical limit switch or mechanical limit like a missing gear tooth. I didn't see a limit switch in this schematic though.

/Mark

The switch at the 6 o'clock position on the gear is a limit switch, but it opens when the Ball Count unit reaches zero. Often there is another limit switch that gets opened at whatever the upper limit is. Apparently in this game the upper limit is enforced some other way - that isn't working at the moment.

The two jones plugs shown on the schematic may be different parts of the same jones plug. How many pins does the jones plug you found have? You could also check the wire colors against the schematic to double check.

Look closely at the wire colors of the two plugs. One is grey-yellow and the other is grey-orange. Those match the wire colors of the plugs in the schematic from reply #3. So the plugs are combined in the game but separated in the schematic.

The good news is that if you unplugged this plug and the problem persists, we can ignore most of the schematic from reply #3 and look into just the bottom section which has just three switches.

So as the Ball Count unit is counting, are the Coin relay and Reset relay stuck on?

The Coin relay is probably keeping the Reset relay on. So you need to figure out what's keeping the Coin relay on.
Blue Chip Coin relay (resized).jpg
If you block the switch in the red box on the Coin relay that connects the red-white (R-W-1) wire to the green-white (G-W) wire with a small piece of paper, does the Coin relay let go? If so, the problem is in the bottom portion of the schematic. Otherwise the problem is in the more complicated top portion.

Ok, let's assume that the Coin relay fires when it should, but doesn't release when it should. The Coin relay switch in the red box above holds the Coin relay active until one of the switches to its left opens and breaks the holding circuit. It's possible that one of those three switches is holding the Coin relay indefinitely. Here are a few things to check:
Blue Chip Coin relay 2 (resized).jpg
- Make sure that the switch on the Game relay that connects the yellow wire to the red-white (R-W-1) wire opens
- If you're set up for 3 ball games, check that the score motor switch 3A can open and that there are no shorts on its solder tabs
- If you're set up for 5 ball games, check that the score motor switch 5B can open and that there are no shorts on its solder tabs

The score motor 3A switch is on the 4th cam counting from the score motor and the 5A switch is on the 6th cam from the score motor.

There still could be a path somehow from the yellow wire to the red-white wire that keeps the Coin relay on once it's been fired. The Coin relay is what keeps the Reset relay on, which keeps the score motor running, which keeps the ball count counting.

If you have a meter, put it on the lowest resistance range, shut off the power to the game and clip the leads to the yellow and red-white wires. You should read about an ohm or so of resistance because there is a path through the score motor 5B switch (if you're still on 5 ball play). Double check that the Game relay switch is open. Then slowly rotate the score motor cams. At some point the 5B switch will open. What resistance reading to you get then?

Ok, then the Coin relay must be held on by the upper part of the circuit:
Blue Chip Coin relay 3 (resized).jpg
There could be a stuck switch in the upper part but before checking them all individually, try to eliminate some of them.
There are three jones plug adjustments, probably on the bottom panel near the coin box. They're the:
- 10 cent adjust
- 5 cent adjust
- 1 coin 1 play/2 coin 3 plays adjust
Try unplugging them one at a time to remove their part of the circuit and see if anything changes. Also try them in combinations.

Sorry, I don't know where the various adjustments are. You could figure that out with the wires colors on the schematic and a multimeter, or maybe asking other Blue Chip owners.

But if you removed the 5 and 10 cent adjustments with no changes we can eliminate those circuits too. That just leaves a few switches you could examine and test in the middle of the schematic from reply #19.

How do you start a game? With the credit button on the cabinet or by tripping a coin mech switch? That might help narrow down which circuits are being used.

Something else you could try might be to dim the lights and look for arcing when the Coin relay fires. If you're lucky you might see a spark where power is getting to the Coin relay. Don't just look at the Coin relay, look all over.

Found the Blue Chip adjustments in the manual on ipdb.org:
Blue Chip Adjustments (resized).jpg

If it behaves the same with all those jones plugs removed we must have missed something. Go back to the Coin relay switch in the red box in repy #9. Start a new game and check if the Coin relay is stuck on. If it is, slip a piece of paper between the contacts of that Coin relay switch. Does the Coin relay let go? If you remove the paper, does the Coin relay reactivate?

Try again only this time when the Coin relay locks on, slip the paper instead between the contacts of the Game relay switch to the left of the Coin relay switch in the schematic. Does the Coin relay let go? If you remove the paper does the Coin relay reactivate?

It could be that the game reset as it should (did it?) and was ready to play and since you had the playfield up one of the playfield switches for a 10,000 point target was stuck closed.

What happens if you remove the jones plug, put down the playfield and start a game? I thought you said earlier that it didn't change anything.

Did you do any of the things I described in replies #25 and #27?

I'm a little confused. We've spent 20+ replies trying to figure out why the Coin relay locks on and now it doesn't? Can you elaborate?

I think it's more likely that the Coin relay which fires very early in the reset sequence is affecting the Ball Count and Bonus units rather than the other way around.Blue Chip Reset Sequence (resized).jpg
The schematic and game manual are on ipdb.org.

I'd go through all the suggestions made so far and methodically see what does and doesn't affect the Coin relay. You may need to do things several times since the failure isn't consistent. Make notes of what affects the failure and what doesn't. That should help narrow in on the problem.

What about the other question on reply #25 about the Game relay switch?

If the Coin relay locks on and opening the switch with a slip of paper as you describe makes it let go, the problem is in the left half of the circuit shown in reply #11. Somehow the circuit between the yellow wire and the red-white wire is closing and staying closed.

That supports the theory that something isn't opening when it should. If you don't see the problem in 3 ball games the problem is likely in the make/break switch at 5B on score motor from reply #11. Somewhere there's likely an intermittent problem with the switch leaves, or the solder tabs, or bare wires or something having to do with that switch.

Another possibility is that there's an intermittent short between the red-orange wire and the white-black wire either on the score motor or at the 3/5 ball adjust jones plug or socket. That would be possible if you're seeing failures in 3 ball games too.

Intermittent shorts can take a fair bit of poking around. You could go back to the experiment in reply #17. Clip in your meter, turn the score motor cam, etc.

If you can get back to the point where opening the appropriate score motor switch doesn't increase the resistance that would tell you that the problem is happening. Leave the score motor where it is with the switch open, watch your meter and start poking around or tugging at the two score motor switches, their solder tabs, the 3/5 ball adjust plugs, etc. to see if anything you do changes the resistance.

Do this for both jones plug settings:
- on the 5 ball setting opening the score motor 5B switch should increase the resistance
- on the 3 ball setting opening the score motor 3A switch should increase the resistance

I suspect there are multiple problems. Next time your game starts and the score motor won't stop, remove the ball, lift the playfield and investigate. Which relays are active? Can you stop the resetting by slipping paper between the contacts of some of the switches we've covered? You should probably also look for someone local to come have a look.