Eugene is a qualified control/instrumentation engineer Bsc (Eng) and has worked as a developer of electronics & software for SCADA systems.
Cleaning Battery Clocks
Once upon a time most of our clocks were wind-up devices, then we transitioned to electric clocks that plugged into a mains outlet. Nowadays, many clocks are battery powered by a single AA cell. The enemy of all clocks though is dust and grime that can easily stop the mechanism from working. In this guide, I show you which parts you can try cleaning to get your clock back working again.
Analog and Digital Clocks
The clock I'm fixing here has an analog display. However the electronics that drives the display is digital and uses an oscillator and counter chip for frequency division. To understand the somewhat confusing difference between analog and digital which are words that can mean different things depending on context, see my article "Analog vs Digital Signals and Displays — What's the Difference".
How Does a Quartz Clock Work?
The mechanism (also called the movement) or working parts in a quartz battery clock are a bit simpler than in a traditional wind up clock. At the heart of the clock is a quartz oscillator which generates a pulse every second. An oscillator is a device that does something regularly, like a pendulum that swings back and forth, a tuning fork vibrating, a string on a guitar or the air in an organ pipe. All these are mechanical oscillators, but there are also electronic oscillators. An electronic oscillator generates a voltage signal that repeats itself at a set frequency. In the case of the oscillator in a clock, this runs at several thousand hertz or cycles per second. An electronic component called a quartz crystal sets the frequency of the oscillator to about 32768 Hz with a high degree of accuracy. The frequency is divided down and reduced so that it eventually becomes 1Hz or 1 cycle per second. The output of the oscillator drives an electromagnet that acts on a tiny magnetic rotor, flipping it half a turn every second. The rotor has gear teeth which mesh with a train of other gear wheels and this eventually turns the hands of the clock.
The good thing about quartz oscillators is that their frequency is very stable and doesn't change much with temperature, humidity or other ambient conditions. This means that battery clocks keep good time and don't gain or lose minutes like a wind up clock as the spring unwinds or temperature changes.
Why Do Clocks Stop?
- Loose or dirty battery connections
- Low battery
- Battery pips not long enough
- Grime accumulating in the mechanism
Things to Try First if a Clock Doesn't Work
- Change the battery. It may simply be flat. You can check battery condition with a universal battery tester like this one suitable for 1.5 volt AA, AAA, C and D cells and also small, square 9 volt "PP3" (MN1604) style batteries
- Clean the connections and battery terminals. with rubbing alcohol (Isopropyl alcohol or IPA) on a cotton bud. The springy electrical strips can also become oxidised as can the ends of the batteries. This oxidisation sometimes appears as a grey or green coating. You can use fine wire wool to remove it.
- Sometimes the the terminal pips on batteries can be a little short. Try slightly bending the positive terminal strip in the clock slightly. Caution! These can snap if you bend too much.
- Check the hands aren't rubbing against the clear cover over the face of the clock.
If the Clock Still Won't Run....
If the clock still won't run, the cogs inside may have grime on the teeth or axles. Even though there's a cover on the mechanism compartment, in humid, dirty conditions, e.g. in a bathroom, dust and mildew can manage to get in. So you need to do some cleaning.The solenoid in the clock only produces a tiny amount of torque or twisting force to turn the little rotor. Any grime on the pins of this rotor can produce enough friction to stop it turning.
- Remove the cover over the mechanism. This differs from clock to clock. There may be lugs on the mechanism compartment that engage with the cover. If this is the case, push them gently aside with the blade of a screwdriver. Alternatively there may be pins that push into the cover like in the photo below. I was able to push into the hairline gap between the compartment and cover with a small screwdriver and prise up the cover.
- Remove the gears. Take a photo before disassembling so you can put everything back together.
- I have found that furniture polish helps to clean and lubricate the tips of the shafts of the nylon gears where they sit into the case and cover. Spray some polish on a tissue, and wipe any dirt off these ends.
- Gears are made from nylon which has a low coefficient of friction, i.e. it's slippery, so the teeth shouldn't need any lubrication. However remove any obvious dust or fluff.
- If you remove some of the gears, the hands of the clock may fall off, so you may need to remove the clear faceplate to replace them.
Other Things You Can Try
- Check the voltage on the electromagnet. This should change polarity every second.
- If you have a soldering iron, you can reflow the solder on the joints on the PCB. Sometimes a dry solder joint can prevent the circuit from working or make it stop working intermittently.
- On some clocks, the circuit board presses against strips of metal that supply power from the battery. These strips are springy and press on tinned pads on the PCB. In a bathroom, steam can cause corrosion at these points of contact. Remove the PCB and clean the pads and spring contacts with IPA and fine wire wool.
- The axle of the magnetic rotor can tarnish if it's made of brass, making it rough and increasing friction, causing it to stick. More expensive clocks and watches have jewelled bearings (e.g.rubies) in which the tiny axles of the cogs rotate, however inexpensive quartz clock movements just have a nylon plate into which all the axles of the cogs fit. You can try adding a tiny drop of oil, (less than the size of the head of a pin) using the tip of a pin or a pointed awl to the point where the rotor axle fits into this plate.
If the Clock Stilll Won't Start, Replace the Mechanism
It turned out that the electronics of my clock in the photos above was faulty. You can buy a replacement mechanism on eBay or Amazon. These are somewhat non standard, so you need to check that the hands are the correct size and will suit the style of your clock, or alternatively if the are plain rectangular types, they can be cut and shortened. The threaded piece that fits into the hole in the face of the clock sometimes differ in diameter and length. This should be long enough that any rubber washers to stop the mechanism turning and bracket hooks for fixing to a wall can be replaced with the piece projecting sufficiently to fix it in place with the retaining ring. Diameter isn't as critical, but it should be small enough so it fits through the hole. If it's too small, it's a little more tricky to keep the mechanism centered properly.
Using Flat AA Batteries
When a gadget indicates that the batteries are flat, this is often because electronics or software detects that the voltage is below a threshold level sufficient to run the device. This happens with high power devices that need a minimum voltage to operate. However the energy remaining is often adequate to run battery clocks for up to 6 months.
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2020 Eugene Brennan