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Old 18th May 2019, 4:22 am   #8
Join Date: Jun 2015
Location: Biggin Hill, London, UK.
Posts: 4,020
Default Re: Bell and Howell TQII 1652 16mm projector

Normally these things use the freqeuency of the signal from the motor's tachogenerator as feedback. It's a more reliable indication of the speed than the voltage from said generator.

Looking at the circuit, the tachogenerator seems to be the 2 yellow wires from the motor. With the unit off and at least one of those wires disconnected, you should get some DC resistance between them due to the tachogenerator windings. Not shorted, but not open either.

With the motor running, a 'scope should show an AC voltage (sinusoidal-ish?) between those 2 wires.

Alas the B&H projector I have is an older one with an AC induction motor so I can't be more help.
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