Clockwork

28.03.2016 15:17

Recently one of the clocks in my apartment stopped. It's been here since before I moved in and is probably more than 10 years old. The housing more or less crumbled away as I opened it. On the other hand the movement inside looked like it was still in a good condition, so I had a look if there was anything in it that I could fix.

Back side of the quartz clock movement.

This is a standard 56 mm quartz wall clock movement. It's pretty much the same as in any other cheap clock I've seen. In this case, its makers were quick to dispel any myths about its quality: no jewels in watchmaker's parlance means no quality bearings and I'm guessing unadjusted means that the frequency of its quartz oscillator can't be adjusted.

Circuit board in the clock movement.

As far as electronics is concerned, there's not much to see in there. There's a single integrated circuit, bonded to a tiny PCB and covered with a blob of epoxy. It uses a small tuning-fork quartz resonator to keep time. As the cover promised, there's no sign of a trimmer for adjusting the quartz load capacitance. Two exposed pads on the top press against some metallic strips that connect to the single AA battery. The life time of the battery was probably more than a year since I don't remember the last time I had to change it.

Coil from the clock movement.

The circuit is connected to a coil on the other side of the circuit board. It drives the coil with 30 ms pulses once per second with alternating polarity. The oscilloscope screenshot below shows voltage on the coil terminals.

Voltage waveform on the two coil terminals.

When the mechanism is assembled, there's a small toroidal permanent magnet sitting in the gap in the coil's core with the first plastic gear on top of it. The toroid is laterally magnetized and works as a rotor in a simple stepper motor.

Permanent magnet used as a rotor in clock movement.

The rotor turns half a turn every second and this is what gives off the audible tick-tock sound. I'm a bit puzzled as to what makes it turn only in one direction. I could see nothing that would work as a shaded pole or something like that. The core also looks perfectly symmetrical with no features that would make it prefer one direction of rotation over the other. Maybe the unusual cutouts on the gear for the second hand have something to do with it.

Update: my follow-up post explains what determines direction of rotation.

Top side of movement with gears in place.

This is what the mechanism looks like with gears in place. The whole construction is very finicky and a monument to material cost reduction. There's no way to run it without the cover in place since gears fall over and the impulses in the coil actually eject the rotor if there's nothing on top holding it in place (it's definitely not as well behaved as one in this video). In fact, I see no traces that the rotor magnet has been permanently bonded in any way with the first gear. It seems to just kind of jump around in the magnetic field and drive the mechanism by rubbing against the inside of the gear.

In the end, I couldn't find anything obviously wrong with this thing. The electronics seem to work correctly. The gears also look and turn fine. When I put it back together it would sometimes run, sometimes it would just jump one step back and forth and sometimes it would stand still. Maybe some part wore down mechanically, increasing friction. Or maybe the magnet lost some of its magnetization and no longer produces enough torque to reliably turn the mechanism. In any case, it's going into the scrap box.

Posted by Tomaž | Categories: Life

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