KYA1 time switch -- Landis & Gyr

The release of the mechanical back up reserve mechanism in case of a power cut.
In this type KYA 1 clock from Landis & Gyr this happens every 2 hours and 3 minutes.
The synchronous motor drives the dial directly but also winds up a spring in a drum which has an overwind slip clutch in it.
To avoid a mechanism sticking over a long period ( no power cuts ) the back up reserve is forced to release by means of a couple of levers within the clock work mechanism. A brief hum is heard when the magnetic lock is moved sideways and the ticking of the escapement.

This clock is about 35 to 40 years old and still works well, can't be said from those battery operated plastic clocks which are sold now these days

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