Passing behind a church-operated thrift store a month or two ago, I saw a black bakelite box in the area where they discard stuff they don’t want, about to get rained on. After seeing that it was a metronome, I rescued it and made it mine. This weekend I tried it out for the first time and ended up repairing the motor. The motor’s workings were unfamiliar to me, but its repair was self-evident.
Oh, and I cleaned it up a bit, too.
Starting on the inside and working my way out, the rotor has a metal band around the inside of the shell, and one end of the band had sprung so it was no longer sitting flush like it is again after the repair. That end was sticking out far enough to catch on the stator, so the motor didn’t turn.
The rotor fits around the stator, which is powered directly by 120VAC. I don’t understand at an electromagnetic level how this motor works. If you do, explanations in the comments are always welcome.
The outside of the rotor is shaped like a Hershey’s Kiss ™, to form half of a variable transmission.
The motor is mounted with the point facing down (in normal operation) and gravity pulls the rotor away from the stator to engage with the rubber roller.
The rubber wheel’s axle has a cam to make and break the contact wires that flash the neon lamp in the top of the metronome, as well as to strike a small clapper weight against the back of the metronome case when the metronome is set to audible. The tempo knob on the front rotates this portion of the transmission to make the rubber wheel contact the rotor on the wide part of the cone for a fast tempo
or on the narrow part for a slow tempo.
The switch turns on the AC supply to the motor. It also presses a wire brake against the rotor when in the off position, presumably so that one doesn’t have to suffer through one or two extra strikes of the metronome when switching it off. Such attention to detail!
The small knob on the left engages and disengages the clapper mechanism from the rubber wheel’s cam.
The case bottom thoughtfully includes an interpretation of tempo markings to beats per minute.
Note the rubber feet.
Instead of relying on adhesive or screws, they’re little cylinders held captive by their own elasticity between the cutout corners of the case bottom and recesses in the corners of the case. I’ve never seen that before and I find it delightful.
Et voila! It works, and appears to keep pretty good time. I’m already using it to practice a Scarlatti sonata, to smooth a section where the fingering is giving me fits. Trés bien!