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Old 14th Jun 2019, 8:21 pm   #61
Lucien Nunes
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Default Re: Bell and Howell TQII 1652 16mm projector

I can't tell now whether that is 9V shown at pin 6 of the original frame wiring dwg and as per your measurement, or 0V which seems more sensible and in agreement with the new module.

Anyway, I've done a hasty redraw of the new module. As PJL says there must be a missing connection on your schematic from the regulator to the part of the rail shown connected to the opamp, which I have added.

THERE MAY BE ERRORS (either yours or mine) as I have not checked it and am now out of time to do so. Please compare back with the board itself..
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Old Yesterday, 11:10 am   #62
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Default Re: Bell and Howell TQII 1652 16mm projector

It's amazing how many different formats/presentations of film came about - thank you for sharing those comments.

Wow Lucien Nunes, not only may you have determined the nature of the failure of the MKI unit (I'll bell the terminals out and see), but you have also spent some considerable time drawing out the circuit for me! Simply fantastic. Thank you! I will do my best to go through the PCB tracks and compare things against your much better presentation.

I can confirm that I missed off a connection and yes is was power to pin 7. A good shout! For completeness I have doctored my sketch (just for the record) and attached it to this thread again.

Exciting stuff this ...

Regards,

Andy
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Old Yesterday, 11:25 am   #63
Lucien Nunes
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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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Old Yesterday, 11:57 am   #64
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Default Re: Bell and Howell TQII 1652 16mm projector

Hello Lucien,

It looks like a '9' on the sheet, but the original resolution isn't great. Funny you were enquiring about this pin 6/pin 4 business, because I was just trying to trace things though again against your fantastic drawings (nothing untoward spotted thus far).

Something is just slightly clipped left of frame on drawing (3) on the ground line however. Would that be pin 4 perhaps?

Looking at the PCB, the bulk of the copper track ends up going to pin 4 with just a thin track joining pin 6 to the main bulk of it.

Is Z1 some kind of varister perhaps?

Interesting. Still not sure how it works though.

Regards and grateful thanks,

Andy
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Old Yesterday, 12:06 pm   #65
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Default Re: Bell and Howell TQII 1652 16mm projector

As regards the original module, pins 4 and 6 aren't solidly linked (or if they were supposed to be, they aren't now). There is a resistance reading in one direction though (interesting in itself).
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Old Yesterday, 2:14 pm   #66
Lucien Nunes
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Default Re: Bell and Howell TQII 1652 16mm projector

Quote:
Would that be pin 4 perhaps?
Yes

Quote:
Is Z1 some kind of varister perhaps
Yes, that's my assumption too

Quote:
Still not sure how it works though.
Square tach pulse from Q2 triggers monostable Q3/4 to produce one identical rectangular pulse per tach coil cycle, ie. duty cycle depends on speed, smoothed by LPF to give actual speed value. U1 compares this to target speed, output flips back and forth increasing mean drive period of Q5 when speed is too low etc. U1 switching and loop constants set by RC's in feedback path.
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Old Yesterday, 3:51 pm   #67
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Default Re: Bell and Howell TQII 1652 16mm projector

Thank you Lucien,

I had to double check the position of D2, but sure enough it's there. If the tacho itself produces something of the order of 30V peak to peak, it seems really weird to short one cycle out. Mind you, I don't really know what that component is for sure. There's just an ordinary diode symbol in the PCB screen printing a that location.
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Old Yesterday, 3:54 pm   #68
Lucien Nunes
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Default Re: Bell and Howell TQII 1652 16mm projector

That's the open-circuit voltage. The source impedance will be high so it's really a current source that delivers a few mA alternately into D2 and Q2, the voltage never rising above 0.7V on either half-cycle.
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Old Yesterday, 4:48 pm   #69
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Default Re: Bell and Howell TQII 1652 16mm projector

Thank you Lucien, understood.

Also, I've been trying to determine exactly what Z12 is. It's probably a TNR9G820k (although the '9' is missing off the lettering on my example). That makes it a general purposes metal oxide varistor (9mm diameter) - 50Vac rms, 65Vdc, 0.2W, 10A, 10% device according to the web.
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