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Old 18th May 2019, 4:38 pm   #15
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Bell and Howell TQII 1652 16mm projector

It looks like the tach pick-up is OK. As the motor gets faster, not only does the frequency increase, but the peak-peak voltage will rise proportionately.

With a DC motor, it's probably a transistor circuit in the box, and a common failure mode of power transistors is short-circuit, leaving your motor at full speed ahead.

Whatever is in the little box can be replicated once its nature is found out. It could be working on either the frequency or the amplitude of the tach signal, though the amplitude version would be concidered rather crude... but stranger things are found.

David
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