>Over the holidays, I worked on about a dozen Smiths tachometers, in
addition
>to the many tachs I've worked on prior to that. I have noticed that a lot
>of them were unable to register above 4 to 5 grand. I surmised that the
>movements were getting dirty and or worn, so I went to my local jeweler to
>tell him my problem and ask him how he lubricates watches. He suggested I
>try sewing machine oil. The good news is it didn't seem to make the
>movement worse, but it sure didn't improve it either. So I gave up on that
>idea.
>
>I solve the problem literally using brute force. I lower the resistance of
>the thermistor/resistor pair to provide more energy to the deflection coil.
>While this works, I'd really rather have a way of effectively lubricating
>these old tachs. The original lubrication is 30 years old.
The Tach movement is a simple d-arsonval movement, not likely to
get hung up a such a consistant point (4000-5000 RPM).
Have you considered that possibly the tach one shots are hitting
full duty cycle at this RPM due to component drift?
This sounds more likely to me then binding jewels.
Perhaps some time with an O-scope is in order.
If you are interested, I have a nice little circuit that
elliminates all that 30+ year old electronics.
Could be better in the long run, and make rebuilding less of an art.
Jarrid gross
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