Drag arm stops

drag arms
magic ring
Heavy duty drag arm stops

You need to have the backglass out and lower the panel behind it to see the drag arms. They are in front of the control unit just to the left of the motor.

In the picture above, you can see the pins sticking sideways out of the cams (a) above the two drag arms (1). These pins periodically hit the drag arm stops (2), and stop the cams (a) from rotating temporarily. Eventually the cams the drag arms themselves are resting against push the drag arms away and release the (a) cams to continue rotating. The relationship between the two (a) cams is what changes the length of time things happen in the game, such as the spotting/mixer wipers rotating, and thus provides the randomization.

The problem that can occur is subtle, and to some extent theoretical. The drag arm stops in the above game are made from a heavy metal...as thick as the drag arm itself. On older games, the drag arm stops where much thinner.

drag arm wear
drag arms from early bingo
Drag arm wear
Here we see a side view of a worn drag arm stop. Years of having the pins slide off the stop have taken their toll. What happens on this game is when a pin comes down and hits the drag arm stop, the drag arm is simply pushed away. The cam never changes its position relative to the rest of the control unit. If both drag arm stops are worn, the two cams (a) will always stay in the same phase. In theory, this means that that when a coin/credit is played, the spotting disk/mixers rotate approximately the same amount every time, creating a pattern out of what should be random.

In the worst case, the spotting wipers or mixer wipers would make exactly one revolution, and and it would be impossible to get certain scores/features.

In practice, though, I'm not sure you could ever know the difference. It was considered enough of an issue for bally to make heavier gauge drag arm stops in later machines.

mpeg4 video clip of the dragarms in action if above movie doesn't work. The timer cams are the light brown bits on the left.

Possible solutions

  1. ignore it
  2. file the drag arm stop down so the surface the pin hits is closer to horizontal again.
  3. replace the drag arm stop. Since you can't buy these, you can steal them from a parts game or make your own. It's just a hunk of metal with a right-angle bend and a couple holes to mount it.

A problem you can't ignore

Dennis A. reported a problem I've never seen, and Jeffrey Lawton said he's seen a couple times. On the old games with the thin drag arm stops, the stop may start wearing a notch in the pins on the cams. In dennis' case, the wear created a slot that the stop got wedged in. That jammed the motor and damaged gears in the gearbox.

Jeffrey suggests removing the thin stops, putting them in a vice and reasonably gently whacking them with a hammer to flatten the tops if they are sharp. This also spreads the metal..you are trying to make the edge wider (like pounding on a knife blade edge to dull it).

If you have a slot in a cam pin, you can fill it in with weld or bend the stop to attack a new piece of the pin (make sure you bend it enough that the stop is completely hitting good metal...not the edge of the slot).

Jeffrey recommends the best solution is to try and get the drag arm stops off one of the later games. The metal is softer and thicker. You still need fill in the slot if there is any danger of it wedging the cam.

  1. Motors
  2. Spotting index arm
  3. Search Wipers
  4. Slip ring hub and wipers
  5. Drag arm stops
  6. Mixers