A simple Seth Thomas round top alarm repair

Seth Thomas 8-day lyre movement with alarm

A distinctive feature of this Seth Thomas round top is the lyre shaped 8-day Plymouth time and strike spring-driven movement with a hour strike on an iron bell. Not all of these models have the alarm but this one does. The movement is die-stamped “S. Thomas, Plymouth Conn.” and fitted with Geneva stops (stop-works) to improve timekeeping. It is a large movement squeezed into a relatively compact space.

I wrote two previous articles, one on first impressions and the other on sprucing up the case and a future article (December 24, 2019) addressing a small veneer issue on the front access door but this post concerns the alarm mechanism.

A simple job, less than half an hour in total and the only difficulty was waiting for the part

The alarm mechanism is located just below the movement and to the left and connected by wire to the alarm adjustment wheel mounted on the centre cannon. Both the alarm and the movement strike on the iron bell.

Unfortunately, the clock came with a broken alarm mainspring. No part of the spring is reusable since it broke in two equal halves.

Using a micrometer to check pivot
A micrometer is an indispensable tool for the clock hobbyist

I ordered an alarm mainspring from a clock supplier but since there is no mainspring specifically marked for American alarms you must measure the old mainspring using a micrometer and come as close as possible to a replacement.

Alarm mechanism with movement removed

As you can see in the above photo the mechanism is quite dirty.

Disassembly took less than a minute and an additional 15 minutes in the ultrasonic produced a shiny alarm mechanism. The pivots were in very good condition telling me that the mechanism was not used often. Once together it was fastened with new taper pins to hold the small plates. One mounting screw which was missing is replaced.

I doubt that I would ever use the alarm and some might even find the alarm annoying as there is no shutoff. It is a simple mechanism with attached key that is kept silent until the preset time. However, If you have a very important appointment and need to be awakened quickly, use it. Then it rings until the power is depleted, or until they’re smashed to bits by an aggravated sleeper.

Be that as it may, it is always a plus to have everything working on an antique clock.

Alarm reinstalled with new mainspring
The alarm works off the same iron bell as the strike
The completed clock

A simple job, less than half an hour in total and the only difficulty was waiting for the part (I typically wait until I have a large enough order to justify the shipping and handling expense from a supplier).

Now everything is working as it should. It is one of, if not, the most attractive shelf clocks in my collection.


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