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Old 12th Jun 2019, 10:21 am   #60
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

Looking at the voltages and the schematic, I think the difference that PJL spotted is part of the problem with the original module.

The voltage readings show 9V on pin 6, whereas on the new PCB it is connected to pin 4 which is 0V. This would make more sense; pin 6 is a common terminal serving the bottom end of the speed pot and the tach coil, both of which might sensibly be referenced to ground. Two ground pins would be preferable to one, as the connection to the supply carries the full motor current, so you would not want the inputs sharing any common wiring resistance.

I would not be surprised if the fault in the original module was some kind of open or high-resistance connection in the ground rail that has left pins 4 & 6 unconnected, depriving the speed pot of its proper voltage span and the tach coil somewhat floating. Either or both of these would cause the motor to run at max speed. If it were mine, subject to a bit of checking I would probably try linking 4 & 6 at the module pins.

I might have a go at re-drawing that schematic later...
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