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Old 23rd May 2019, 6:55 am   #19
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 sounds like you don't so much need a replacement servo module as an improved servo module, designed to not blow up.

Anything driving a DC motor will see an inductive load and nasty transients from brush-commutator interruptions and sparking. Without good protection, this will degrade semiconductors leading to eventual failure.

Projector motors take a fair bit of power, and that module looks small. So either it uses the metal it's mounted to as a heatsink, or else the output stage is switch-mode.

The first thing is you need an AC tachogenerator to voltage converter. Have a look at LM2907/LM2917. They're as old as the hills (I used a pair for electronic rev counter and speedo when I rebuilt my old Cooper S in 1979!)

There's an opamp in the chip which will make your loop amplifier... all you need then is a good beefy power stage - well larded with transient protection!

David
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