Ericsson Master Clock

30 June 2024

This is a 1950s Ericsson master clock (yes, that Ericsson!) with a Swiss Moser-Baer movement. It has connections for a bipolar secondary clock circuit and a bell circuit, which can be programmed up to every five minutes with separate programs for weekdays and weekends.

The clock arrived from eBay Germany from a collector’s estate. Though it was missing the pendulum and has quite a few signs of wear, I was drawn to it instantly for its clever mechanism, carefully-arranged wiring, shadow-box-like case, and the lovely etched glass. The original dial is also great in a utilitarian sort of way, but from the beginning I had no intention of using it, since it hides all the best bits.

As it arrived.

The pair of single-pole, dual-throw mercury switches, mounted one in front of the other. They form an H-bridge so that, as each of them is actuated every other minute, the clock will send out alternating positive and negative impulses to the secondary clocks. Below them is a pair of contacts that close every five minutes for ringing bells.
The clock’s own winding motor, a Lavet-type stepper driven by the secondary clock circuit.
A switch for manually advancing the clock circuit. In the foreground, a momentary “mute” switch that disconnects the external clock circuit, so the advancing switch can be used to wind the clock without advancing the secondary clocks.
The original capacitor and resistor for the clock circuit.
The bell program dial.
Wiring diagram.

New pendulum

I decided to fabricate a new pendulum first, to seek signs of life. The suspension spring was bent but present; enough to get on with. I used a 1/4” dowel for the rod, an upside-down furniture leg for the bob, and a 3D-printed hook. The clock accepted it eagerly and, even without a service, began ticking away happily.

Disassembly and service

Disassembly demanded a fair bit of patience and penetrating oil, as most of the fasteners had oxidized in place.

It demanded careful organization, too – especially for the switches, which are composed of several layers of contacts and insulators and must be reassembled in exactly the right order.

Electrical safety

In the meantime, I sought the advice of experts on the NAWCC forums on how to handle the electricals (as well as whether there might be asbestos present – which there isn’t). One gentleman kindly advised me the paper capacitor was probably bad. Since I have an LCR meter, I was able to indulge his interest – and we determined that, indeed, it was liable to go explody if powered up.

Fortunately it’s inert if left unpowered – so I decided to leave it (and its matching 50Ω wire-wound resistor) in place for originality, and attach modern components to the side. I couldn’t find a black Euro-style terminal block to match the clock’s existing ones, so I bought a white one and transplanted its hardware into a 3D-printed black one.

Restored electricals in place.

New dial

To showcase the clock’s mechanism, I decided to fabricate a transparent skeleton dial – and rather than copy the original dial exactly, I decided to imitate the pilot dial from a matching Ericsson punch clock from the same period – no less utilitarian, but with a bit more visual interest (and there is precedent for a similar design in other Ericsson master clocks).

I settled on a design made of two pieces of acrylic and 3D-printed legends, standoffs, and hands, assembled with superglue and M3 screws. The double-level design adds more visual interest and improves readability since it brings the tick marks closer to the minute hand. The acrylic was cut by SendCutSend (who included – fittingly enough – Swedish Fish).

The new dial offers a front-row seat to watch the switches. Here we see the bell circuit contacts in action: the top contact falls into the bottom one to close the bell circuit, then the bottom one falls away to open it. The lever behind the switch (behind the top of the ‘4’) fine-tunes the pivot point of the lower contact, which changes its position relative to the snail cam on the minute shaft, affecting how early or late it falls – thus adjusting the length of the bell pulses. A similar lever, at top of frame, tweaks the pivot point of the mercury switches, to adjust the length of the secondary clock pulses.

All in all, a fortuitous piece for my first vintage master clock. It will appear again on this site, since I plan to use it as a test bed for some other development projects.