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Monday, 20 July 2015

Clock Problems

We have an outside clock and thermometer that hangs on the wall of the house. It's useful to work out how many hours it has taken to mow the lawn, or trim hedges. The thermometer is a mechanical mechanism and seems to work fairly well. The clock, however, is battery powered, which means that I have to get a ladder out every so often and replace the battery. After having done this once, the clock stopped again and I once more got the step ladder out.

After putting a new battery in the clock I was a bit surprised to see that it didn't start up again. After a few more batteries in case they were flat, I decided that th emechanism wasn't suited to outdoor use and had failed. I put the clock back on the wall in anticipation of hunting for a new mechanism.

A few days later I was asked to look at another clock which had also stopped working. I decided to take this mechanism apart and see if there was an obvious fault. The mechanisms are all similar (although the two I looked at were slightly different internally) and look something like this:

The circuit board has the electronics on it, then there's an electromagnet and a gear train that ends with the hands. Anyway, after a look at the internals, I found that the contacts that connect the battery to the circuit board were corroded. As they just touch pads on the PCB, the connection had become bad and the clock wasn't getting power. After a clean up I re-assembled the mechanism (if you do this, then I recommend taking a photo of the internals before you dismantle it. It makes putting the gear train back much easier and quicker). After re-assembly the mechanism worked. So, one clock fixed. I disassembled our outdoor clock and that too had the same contact corrosion. After a clean up that clock also now works as well.

It looks like any moisture on the contacts then cause some form of galvanic corrosion that then causes the connection from the battery to fail. This will probably happen again, so I may solder the connection in future. It's nice to be able to fix these clocks though, and give them a bit more life.

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