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Old 15th Jun 2019, 11:25 am   #63
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
Default Re: Bell and Howell TQII 1652 16mm projector

Andy I'm curious as to whether the voltage shown on the general wiring dwg for pin 6 is 0V or 9V. It's hard to tell on the posted image, maybe you can see it more clearly on your copy. My manual doesn't give the voltages. It looks like a 9 and you measured 9V, but evidently with the new module it's at 0V. Maybe the two are interchangeable nevertheless.

Perhaps the potted module used an alternative input stage for the tach coil with bias from pin 6, although it if it's only driving a pulse generator for frequency-to-voltage conversion I can't see much point. The low end-end voltage it puts across the speed pot seems odd though.

I'm working overseas at the moment so can't help by way of digging out a working projector and checking voltages / waveforms.
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