Recently, I have been asked how an old slave clock could be controlled, it was saved from being scrapped, when a local train station closed down. It is really big and heavy. Eventually, I checked it out and it can be operated from 12 V, 24 V, or higher voltages by adding a resistor. We decided to operate it on 12 V, simply because one of the slides (this is two-sided slave clock) had already been set to this value.

In order to control the slave clock, we have to send a 1~2 second long voltage pulse, every minute, in alternating polarity. Then the minute hand will advance minute by minute. There is no second hand on this slave clock.

The control board is fairly simple, to test it all out, I soldered it on a piece of prototype board. Main control is by an ATMEGA8 microcontroller. This is using its own clock, from a Kyocera HC1 crystal oscillator to derive a 1 minute time interval, and and auxiliary 1 second output, for calibration purposes.

Ｔhe actual adjustment of the clock is done in software, because the oscillator is pretty stable, and this is just a test circuit. If the slave clock will be a minute off, or two, nobody will mind.

The clock drive is completely separate from the controller, isolated by optocouplers. I used a small transformer, with dual 12 V output (because it is small current only, the output voltage is high enough, but you may use a 15 V transformer if it is handy).

Seems we are about 25 ppm out. After correction, the clock was running well within 1 ppm, fractions of a second per day.

To make the contraption a safe and useful thing, I put it in a little box, with some cables and feed-throughs.
Now it is up for some long term test, let’s see if it needs any modification.

My trusty Siemens master clock, after some years of service without any trouble, it needed repair. The electromagnet coil that charges the weight, it is triggered by contacts that got dirty over time. So I cleaned all well with contact cleaner and some ultrafine abrasive paper.

With these little repairs complete, the clock showed another issue. It just stopped after some random time, and that is no good for a Master Clock. Generally speaking, pendulum clocks that stop oscillation randomly are difficult to fix. It may be dirt in some gears or bearings, it may be incorrect adjustment of the escape wheel, it may be some local disturbance.
Fortunately, the full clockworks can be removed without touching the Invar pendulum.

There are connectors at the top, well large in size, and with some silk spun wire.

Upon closer inspection, one of the main gears, which also drives the minute hand, showed issues. It is not fixed in position, but moved in and out. How can it be? When it gets out too far, there won’t be any reliable force transmitted to the pendulum, so it will eventually stop.

There is a washer, brass, on the back side of the movement, and this is supposed to hold the axle in a fixed position, while allowing it to spin freely.

Somehow, this washer had worn out. So I just rotated it.

Giving the clockworks a good clean and oil (only special clock oil made for medium-heavy clocks supposed to be used!), but without a full disassembly.

Now it is ticking away again, and showing the time, day and night.