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Old 23rd May 2019, 4:58 pm   #21
af024
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Default Re: Bell and Howell TQII 1652 16mm projector

Hello David, John,

Thank you for your thoughts.

Yes the module is very small - remarkably so. I suspect that it does use the metal as a heatsink as there is heat conductive compound between it and the metal base of the projector.

It would be great to create a modern substitute, but I suspect I'd be out of my depth here. I did look at some ICs and what they might offer, but then I got stuck on whether or not I needed to convert frequency to volts or just use volts direct as the feedback.

I also didn't know whether to go down a pulse with modulation output or not etc.

I wonder what goes wrong with the motors? Mine appears to be ok, at least for the moment, but it's an interesting observation all the same.

Thank you for asking about possible spares.

Regards,

Andy
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Old 26th May 2019, 6:25 pm   #22
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Default Re: Bell and Howell TQII 1652 16mm projector

Taking a look at various LM2901 and LM2917 datasheets, I came across this circuit. Perhaps this would form a suitable building block? Of course there are still some challenges as the project supplies an unregulated nominal 40Vd.c. and this IC is limited to 28Vd.c. I think.

I'm not sure what current the motor will pull on full load.

I don't know whether I need a PWM solution to keep the torque up.

My aim would be to try and create something that would just drop in as a substitute, so I have 7 pins (and existing signals, presets etc) to deal with.

Not sure what to do for R1, C1 and C2.
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Old 30th May 2019, 5:45 am   #23
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Default Re: Bell and Howell TQII 1652 16mm projector

Hi!

The 28V max needed for an LM2917–8 can be provided by an LM317T adjustable regulator or a 7824 with a 3.9V zener in parallel with a 10u 25V electrolytic in its middle ("GND") lead, or, if you can find space in your Projector, a small "LM317 Adjustable PSU Module", which you can buy cheaply online from Chinese supply sources, either ready–built or as a kit of parts with a bare pcb!

A darlington transistor such as the TIP147 or BDX53 should do for the LM2917–8 circuit example suggested in the previous post!

I wouldn't be at all surprised to find B & H have buried a '2917 in that module to begin with – it is, after all a similar speed control arrangement with simple switched potential dividers adjusted for each film speed!

Chris Williams
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Old 30th May 2019, 10:41 am   #24
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Default Re: Bell and Howell TQII 1652 16mm projector

Hello Chris,

I wondered about using a zener or LM317, but also wondered what would happen to the drive in doing so. Perhaps it would still be ok?

After reading the datasheet of the LM2917-8 more closely, I did wonder if it would be ok to run it simply with increased resistance at pin 6 and pin 5? Not sure whether any transient suppression is needed.

The attached (somewhat ragged) sketch is what I was thinking about. I've put the Bell and Howell original module pin numbers in circles.

Of course I have no idea whether the original Bell and Howell present arrangement will work ok and I still don't have much of a clue about R1, C1 and C2.
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Old 30th May 2019, 11:04 am   #25
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Default Re: Bell and Howell TQII 1652 16mm projector

Ah, just noticed that the TIP147 is pnp, perhaps the TIP142?
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Old 30th May 2019, 7:41 pm   #26
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Default Re: Bell and Howell TQII 1652 16mm projector

Actually, perhaps that's not a good idea on my part (increasing the connected resistances at pins 5 and 6). I'm also not sure about the max rating of the LM317T. I mean there's circa 42Vd.c. supply to deal with. I don't know what the IC will pull in terms of current, but how if I used a series 68 ohm resistor off the 42Vd.c. supply and take that to a 1.3W 24V zener diode with, say 470uF strapped across it and use that as the IC supply (then take the other resistor values back to where they were)?

I'm still at a loss when it comes to selecting suitable values for R1, C1 and C2 (even though I've read the datasheet several times). Pages 4 and 8 seem relevant.
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Old 30th May 2019, 8:33 pm   #27
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Default Re: Bell and Howell TQII 1652 16mm projector

Not sure if something like this might help?

https://tinyurl.com/y3pyvqxj

and presumably, you could replace the pot with the tacho circuit?..but I may be talking nonsense?
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Old 31st May 2019, 11:23 am   #28
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Default Re: Bell and Howell TQII 1652 16mm projector

Hello,

I did wonder about those, but they don't use feedback from the tacho. Of course they may still work ok without said feedback, but I'm not sure how stable the speed would be as a consequence.

Thank you for your thoughts though .. keep them coming.
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Old 31st May 2019, 12:20 pm   #29
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Default Re: Bell and Howell TQII 1652 16mm projector

Speed stability is important for sound projection and I very much doubt that adequate results could be obtained without tacho feedback. Connecting this motor to a constant effective voltage (whether adjusted by PWM or otherwise) would probably result in audiible pitch variation through the length of the film due to change in friction and takeup load, wow at takeup and supply reel rotation rates and sprocket rotation rate, and odd jumps at splices etc.

The only way to obtain a passbly stable drive from a PM motor without tacho feedback, is to size it generously (to minimise the effect of load regulation) and improve the remaining load regulation with I*R compensation - effectively adding a negative dynamic resistance in series equal to the motor's apparent armature resistance, one of the major factors in PM speed regulation. This was used in certain portable devices that did not have centrifugal governors. However, the motor in this projector has to work hard and is subject to sudden load changes at splices etc, so I think this method would be barely adequate. It also requires a regulated supply to achieve correct speed at low load. Since a tacho is available, I would definitely use it.

A commercial PWM motor driver with e.g. a 0-10V input can be driven by a low-power analogue servo block reading the tacho and speed switch setting, and applying PID control to produce the output value.

FWIW, when 35mm sound projectors were first introduced in cinemas in the early 1930s, DC motor drive was often used with magnetic tonewheel speed feedback via valve servo-amplifiers. The cost and (then) complexity of such a scheme are indicative of the importance accorded to speed stability.
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Old 31st May 2019, 4:53 pm   #30
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Default Re: Bell and Howell TQII 1652 16mm projector

Have you tried to de-pot the old module? At this point I guess you don't care about damaging the components in it, you just what to extract the innards physically intact so you can see what is in there and hopefully reverse-engineer the circuit diagram.

I'd try heating it (at least to boiling water temperature) and if that doesn't work try a hold soldering iron on the compound and then common solvents. If all else fails, a burr in a Dremel tool can get into such things.
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Old 31st May 2019, 6:26 pm   #31
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Default Re: Bell and Howell TQII 1652 16mm projector

Thank you for your further comments guys. Was that was some kind of (sine wave?) audible tone recorded into a metal disc off the motor shaft? Wow, I didn't know that. I must confess to being rather sensitive to any kind of variation in pitch. I even graters on me when a record isn't pressed concentrically around its hole! So somehow it seems like I need to press on with a tacho feedback-type solution.

By the way, I was also worried about the 30V peak to peak coming from the tacho, that's why I proposed a potential diver before submitting it to the chip.

I haven't tried to de-pot the original module as yet. I'm quite uneasy about doing so as there's still a niggling doubt (ok, hope) that it might be ok. I guess wouldn't think twice if I had a workable spare. That way we'd all know what was inside the little beggar. I guess I'm going to have to work up to it if I can't with your help, come up with a suitable modern substitute.

Thank you again.

Regards,

Andy
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Old 31st May 2019, 6:35 pm   #32
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Default Re: Bell and Howell TQII 1652 16mm projector

I realise it is human nature to always suspect that the part you can't get is the faulty one. But in this case, I assume you've checked the few components around the module, and we know the tachogenerator is producing a signal. So it pretty much has to be that module.

In which case, I don't think you have anything to lose by trying to get into it.
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Old 31st May 2019, 6:50 pm   #33
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Default Re: Bell and Howell TQII 1652 16mm projector

Quote:
still a niggling doubt (ok, hope) that it might be ok
With the module disconnected, check between the DC input and motor output terminals for evidence that the power transistor is shorted, which would be a common failure mode and the likely reason for the retrofit zeners. If it is indeed shorted, the unit must be U/S but also can possibly be repaired by replacing the transistor, external to the module if more convenient.

Like Tony, I would try to unpot it. If the potting material is hard and cannot be prised out, I would put in in the milling machine and remove the case leaving only what's apparently attached to the transistor. Then it would be easier to chip away around the transistor leads and attach flyleads to a temporary external device, leaving much of the module potted and therefore mechanically intact. Of course, whatever drives the pass transistor may be damaged too, but unlike a full bridge, failure of one transistor does not automatically jeopardise everything around it.
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Old 31st May 2019, 8:33 pm   #34
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Default Re: Bell and Howell TQII 1652 16mm projector

The motor pass transistor is on the negative rail (pin 3). Externally the voltages look OK to me so getting to the internals would be the next step!

Pin 1 (39.2/?) = +Power, +Motor
Pin 2 (15.3/15) = +Vcontrol
Pin 3 (0.7/0.3) = -Motor
Pin 4 (0/0) = -Power
Pin 5 (10.8,10.4/10.5) = Control
Pin 6 (9/9) = -Vcontrol
Pin 7 (9/9) = Tacho
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Old 31st May 2019, 9:09 pm   #35
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Default Re: Bell and Howell TQII 1652 16mm projector

Ah, I had forgotten those readings, and agree it looks more like a transistor driven on rather than one with no junctions left. Pity, it might have been the easiest fix.
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Old 1st Jun 2019, 12:51 am   #36
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Default Re: Bell and Howell TQII 1652 16mm projector

I thought I'd got one the same as yours up in the loft. I went up there a couple of hours ago and fetched it down, but it seems to be the earlier model with a mechanical speed change that moves a belt from one pulley size to another. I powered it up and the amplifier, motor and lamp all worked, but there was a distinct lack of anything else going on. I took the cover off and found that the movable belt was slipping due to the worm drive being cracked and solid, so that's the end of that one!
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Old 1st Jun 2019, 9:02 am   #37
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Default Re: Bell and Howell TQII 1652 16mm projector

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Originally Posted by Techman View Post
I took the cover off and found that the movable belt was slipping due to the worm drive being cracked and solid, so that's the end of that one!
That's repairable. Parts are easy to get but it's a dirty job with all that old grease around.
Pity to scrap a repairable vintage item.
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Old 1st Jun 2019, 9:31 am   #38
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Default Re: Bell and Howell TQII 1652 16mm projector

Thank you for your further comments. Yes the worm gear is available (well the standard non-stop motion type seems to be), but boy you need patience to fit one and ensure that everything is properly synchronised up again.

Between pins 3 and 4 of the module (everything disconnected from it), using a standard analogue multimeter, I'm 'seeing' circa 650 ohm (one direction).

I also observe (when everything is connected) 0.7 Vd.c. at pin 3 relative to pin 4 when the motor is running and 0.2 Vd.c. when the motor switch is turned to off. This doesn't agree with the voltages marked on the circuit as it states 14Vd.c. 'at projection'.
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Old 1st Jun 2019, 10:55 am   #39
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Default Re: Bell and Howell TQII 1652 16mm projector

Af024
I am curious to know the shutter's relationship with the take-up spool.
Is it linked mechanically to the one motor? thus although your shutter mech is free running at the moment the take-up spool has also increased speed so it can cope with the faster running film.
Or has the take-up another circuit/motor which say supplies more oomph with a full spool and less with an empty one?
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Old 1st Jun 2019, 11:17 am   #40
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Default Re: Bell and Howell TQII 1652 16mm projector

Quote:
I also observe (when everything is connected) 0.7 Vd.c. at pin 3 relative to pin 4 when the motor is running and 0.2 Vd.c. when the motor switch is turned to off. This doesn't agree with the voltages marked on the circuit as it states 14Vd.c. 'at projection'.
That's good as far as the output transistor is concerned. There is some drop across it, as though it is being driven on hard, rather than completely shorted. With the motor off there's no collector supply. The 14V is probably an average as measured with an analogue meter, when the output is actively controlling the motor voltage. If the supply is 40V and 14V appears across the output transistor, this suggests a typical 26V required at normal speed and torque.

Quote:
Is it linked mechanically to the one motor?
Yes. Most 16mm projectors use a single motor, with a slipping clutch or other slippable coupling for the takeup (and supply, when reverse projection is available) that tries to drive it faster than the smallest spool centre requires. A standard mechanism on B&H projectors is a weight-controlled slipping belt. The takeup spindle drive pulley 'hangs' in a fabric belt, so that the increasing film load increases the friction on the pulley and hence the torque. Even some full size cinema projectors up to 70mm use a slipping clutch for the takeup, while others use a separate torque-motor with variable torque.
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