Testing the Pendulum on and Electric Clock – fitting a quartz pendulum movement

This is just a quick article to help those people who have a quartz clock with a pendulum.

The way these clocks work is that they have a pulse generator in the electronics that activates a coil based magnet. The switch for this is linked to the pendulum swing (in different ways on different clocks) but all share the same principle. The pendulum bob contains a magnet with north polarity. The electro coil magnet that turn on and off as the pendulum swings has north polarity as well so they repel each other. The coil magnet gets a current at the precise moment the pendulum swings past it and the magnets repel each other pushing the pendulum left or right (depending on which way it is travelling). I have a brilliant Metamec clock that does this on a grand scale and looks like something out of a science experiment from the early 20th century.

The point here is that once you understand what is going on you can test the effectiveness of the coil driving the pendulum. Lets say for instance, your clocks pendulum is siwngs for two minutes and then stops. How do we diagnose the problem?

Well firstly you have to make the assumption that the failed element is either the coil or the fixed magnet, so check the fixed magnet is there and has not fallen off. You have to search with this using another weak magnet or ideally a paper clip. If the paper clip sticks to the pendulum rod at the bottom of its length then you know there is a magnet in there. Once you know the magnet is in the rod then you can check the coil.

Coils rot or break. If you can find the same gauge wire with the same shielding then you can either completely replace the coil wire by re-wrapping it or connect it back up together where it is broken.

A helpful hint on this is to find out just how much power and magnetism the coil is short of in order to drive the pendulum perpetually. You can do this buy running a comparative test. You remove the battery and then swing the pendulum in as near as possible to the original arc you remember it swing back and forth through. This gives you a zero power starting point. You know how long it swings for without any power at all.

You then put the battery in. Get one out of the packet so you are absolutely sure you are not using a dud (Ive done this so many times and it has cost me a lot of confusion time and unnecessary supplier orders). Now swing the pendulum as you did when the battery was not in the unit. Measure how much longer the pendulum swings for. You see in most cases the problem with the coil is a short out. This means only a percentage of the magnetic field is being generated. Because the very low force required of the coil to keep the pendulum swinging you will find that even if the coil is shorted by 90% you will still be able to see this reflected in a longer swinging time than without any power.

This opens up options on what you can do about it because if the coil isn’t getting enough power its highly unlikely this has anything to do with the power source (although there is one thing I will come to later on). It means that the probelm is almost certainly with the coil itself and its electronic joinery. So if the pendulum is at least getting some power you can look at the following options.

  1. Check the solder joints on the end of each wire coil section. On most clocks they are single thread wind so you should only be looking for two contacts. Look carefully at the joints and if they are onto a circuit board then look at the underneath of the circuit board so check of the solder joint has gone on the board as opposed to the end of the wire. If its broken then fix it by re-soldering or establishing a reliable electronic connection in some other way – barrel joining clips can be used on some of he larger coils.
  2. Rewind the coil. You need a micrometer to measure the diameter of the copper wire. There are two diameters to measure – with and without shielding. You then work out the shielding thickness via subtraction and you have a specification for your shielded wire. Go and buy some, rewind the coil with the same length or longer and join it back up to the circuit.
  3. Buy a new movement. This is not as drastic as you might think and should really be the first option for most modern clocks of this type. You just replace the whole thing in the knowledge that your coil or main logic board has failed. Pay attention to dimensions when you buy your integrated movement and pendulum drive unit. The overall depth of your mounted movement is needs to take account of where the pendulum needs to swing. There will only be a certain deviation from is current swinging arc that will work. To explain; lets say the pendulum movement you buy is available with a 21 or 6mm spiggot. On taking your clock apart you find that what you actually need is a 16mm spiggot on the front, otherwise and the glass front wont go back on because the spiggot sticks too far through the clock. At 21mm it was never going to work and at 6mm it was never going to be enough to even get through thickness of your face (you thick face). In this case you need to pad out the mounting on the back and use the 21mm spiggot with only, say, 10mm sticking out of the face . The remaining spiggot length is being taken up by spacers you made up to mount the movement. But heres the problem. By putting he spacers in you have moved the whole movement back by maybe 1cm. If the pendulum hangs down into a glass faced section below the clock you can bet your last quid that it will not fit in this aperture without hitting the back or sides of the clock. The law of sod takes over your project. Your solution is not going to work and never was because you didn’t do your measuring taking pendulum position and length into account. I wont go through what you have to measure; if you understand the principle of what Im saying here you will know what to measure. If you dont then send me the clock to fix and absorb the cost as its not worth you trying and failing and getting terribly frustrated with it. Thats my job, or was a few years ago when I first started doing these. Now I have a method.
  4. If you look at the back of your clock and see that the unit uses two batteries in two separate areas of the clock then you may have an independent pendulum unit with a standard quartz movement in its centre. These are separate components and can be replaced individually. So, if you clock isn’t working but your pendulum is running fine you may just have to replace the clock movement and not the pendulum. This is handy because for a few reasons. Firstly you can buy a new 57mm quartz movement which tend to be standard size for the hand fit. This means you can use your original hands. Secondly the solution is going to be cheaper because there is no need to replace everything. Lastly and most importantly there is an element of standardisation is what these pendulum movements drive and you will probably be able to use your old pendulum with it as they are overpowered in order to drive a wider range of pendulums – completely logical from a design brief and market point of view. Essentially, because its a universal type of product, it is overpowered in my experience which means its more likely to drive whatever pendulum weight or length you have. With the fixed units where you have a truly integrated movement and pendulum driver, you will find that they are only compatible with a range of pendulums. A good bit of advice is to buy a pendulum from the same place at the same time. Its only a few pounds more and allows you a test unit if nothing else. If you get one without buying a guaranteed compatible pendulum how are you going to know if the unit is faulty, badly fitted, or if your pendulum is too heavy. Another area of incompatibility is centre of gravity. The fixings on these units tend to vary from manufacturer to manufacturer so its really easy to get a unit where your existing pendulum cannot be modified to adapt to its new fixing to its armature on the movement. In short, get a back stop position by having a pendulum that you know is compatible before you start work on it. It saves loads of time in the trouble shooting phase.

I do these clocks so if you don’t like the look of all the above give me a ring and I should be able to quote you on the spot after a few questions. Prices vary drastically between £30 and £200 for some of the really really hard fits with only a mm to spare in some areas. There are just not as many types and variants on the integrated pendulum and clock units which means some really imaginative custom fitting on most occasions. While I have my methods, this is one type of job that rarely repeats in exactly the same way and everything is a custom fit in most cases.

Time is money and its never a quick job unless you are lucky to be able to get a exactly the same movement. Even then, through slight design revisions and improvements, the compatible nature of exactly the same movement produced five years apart is far from 100% on most occasions. Its silly things like where they mount the time change button on the back not matching the hole in your clock to gain access to the same. Hanging fixings often change and it means they need to be sawn off. If you saw a quartz movement the vibrations can kill it dead so it means hot wire for cutting and so on and so forth. A lot of work normally and a surprising amount for what should technically be more akin to changing the toner cartridge on a photocopier.

As usual this started as a quick article and has turned into a two hour marathon of cross referencing and developing an understandable narrative. I wouldn’t do it unless I enjoyed it. The other thing is its really quite empowering broadcasting help to such a wide audience. It encourages and motivates me to make what I say understandable. I hope you do.