This is an explanation of the workings of a standard ceiling-mounted cuckoo clock music unit.

The music-making parts are the pin drum and the comb. As the pin drum turns, its pins flick the tines of the comb, producing musical notes. The pin drum is turned by the chain pulley. The gear on the end of the pin drum powers the fan fly.

The most ingenious part of the mechanism is a four-pronged lever, all one piece of metal. I have labeled the prongs the spring leg, the fan lever, the drum lever, and the pull lever.

The drum lever rides on the side of the pin drum gear, held against it by the spring attached to the spring leg. The end that touches the gear is a pin or projection sticking out sideways from the lever. The side of the gear has a hole, into which the end of the drum lever can drop.

When the music box is at rest, the drum lever is resting in the hole of the gear, pulled to the left by the spring. That way, the fan lever is pulled to the right, just barely blocking the blade of the fan from turning and preventing the music from playing.

The fan lever blocking the fan is what locks the mechanism at rest, NOT the drum lever.


The clock movement has two levers that interact with the music unit to allow it to play when it is supposed to --right after the cuckoo has announced the hour.

One is the music activating lever, on the same arbor as the lifting lever that's raised by the center shaft cam to activate the cuckoo. The other is the warning wire, on the same arbor as the rack hook. A wire runs from the activating lever to the pull lever on the music unit.

When the lifting lever is raised by the center cam to put the strike into warning, the attached activating lever pulls on the pull lever, which pulls the pin drum lever out of its hole, AND thereby backs the fan lever away from the fan, allowing the fan to turn and allowing the pin drum to start turning.

If it continued to turn, it would play its music at the same time the cuckoo is sounding. But as the strike goes into warning, the rack hook rises to allow the rack to drop. That moves the warning wire attached to the rack hook arbor into the path of the fan fly on the music movement, halting it after it has barely started to turn.

That little bit of a run before the warning wire stops things is important.

At the end of the strike, the rack hook falls beneath the rack and the warning wire moves away from the fan fly. The spring on the spring leg has pulled the pin drum lever back against the side of the gear...but the lever pin doesn't go back into the hole, which would let the fan lever block the mechanism from running. It doesn't go back into the hole because the hole has moved a little bit during the warning run.

Since the pull lever is no longer being pulled, why doesn't the fan lever just drop back into the path of the fan? Because the drum lever, riding on the side of the gear, prevents it from reaching into the path of the fan. It won't be able to reach the fan until the drum lever falls back into the hole in the gear.

When the warning wire falls away, the pin drum turns and the music plays. The hole in the side of the gear is situated at the end of the tune, so that when the last note is played, the spring forces the pin drum lever into the hole, moving the fan lever into the path of the fan and bringing the mechanism to a halt.

Next to the chain pulley is the music man cam —basically a disk with a vee-shaped notch in it.The music man perches on one end of the music man wire. The other end of the wire is bent into a crank, sticking into the notch in the cam. When the pin drum starts to turn and play, the cam turns, the crank rides up the side of the vee and the music man pops toward the door. At the end of the music, the crank falls back into the notch, and the music man retires.

Here is the sequence of events.
1. The center cam raises the lifting lever to put the strike into warning.
2. The activating lever pulls the pull lever, putting the music into a warning run.
3. The rack hook rises to release the rack.
4. The warning wire moves into the path of the fan fly, halting the music unit.
5. The strike train runs, the cuckoo counts the hour.
6. At the end of the strike, the rack hook falls beneath the rack.
7. The warning wire moves away from the fan fly.
8. The music plays.
9. At the end of the tune, the pin drum lever snaps into the hole in the gear.
10. The fan lever moves into the path of the fan fly.
11. The run is over.


There are three adjustments that are crucial to the proper operation of the music. One is the activating lever. Another is the fan lever. The third is the warning wire.

The activating lever has a sliding connection to the pull lever: The connecting wire passes through a hole in the activating lever, and is bent down at a right angle on the end. This allows the activating lever to pull against the bent-down end of the wire, but not to push it. When the activating lever snaps back forward (as the lifting lever drops off the center cam), it just slides along the wire rather than pushing it.

The activating lever must be adjusted to pull the pull lever far enough to release the fan lever. And it must do it as close to the end of the lift as possible, to reduce the time between the release of the fan lever and the lift of the rack hook and activation of the warning wire. Too long a warning run will allow the music to start playing before the warning wire can interrupt it.

The fan lever must be adjusted so that, when the pin drum lever is riding on the side of the gear (not in the hole),the fan lever just barely misses the fan.
This is accomplished by gentle bending. We're talking thousandths of an inch. It's important that the lever miss the fan during the entire run, but that it block the fan the instant the drum lever falls into the hole. Otherwise, the drum can continue to turn just enough for the drum lever to ride back up out of the hole, moving the fan lever back away from the fan, and letting the music continue to run.

The warning wire must be adjusted so that it blocks the fan before the first note of music plays. This too is accomplished by gentle bending.

It must continue to keep it blocked until the strike is completed. This is complicated by the fact that it doesn't stand still while doing its job. As the rack hooks bobs up and down, gathering the teeth of the rack, the warning wire will bob back and forth. Getting it to continue blocking the fan while bobbing back and forth can present a challenge.

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