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arriflex 400' magazine question


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#1 Tom Doolittle

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Posted 16 January 2006 - 12:37 PM

Hello,

A few questions about Arri 400' magazines:

When I run the camera, should the torque motor speed vary with camera speed?

Should there be any clutching action (slippage) at the spindles to allow for varying camera speed?

I have two torque motors, but they seem to run at very different speeds. I assume the slower one has been converted to 12V? What does this conversion consist of? An added resistor, perhaps?

I'd like to use these soon. Does anyone know where I might find instructions for these mags?

Thanks. Any help appreciated.

-Tom
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#2 Sam Wells

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Posted 16 January 2006 - 12:45 PM

1. Yes

2. Yes

3. I don't know what the 12v conversion consists of.

Do you have a dummy load -- core load of film you can run in the mag thru the camera with the door off ?
(mag door off)
I highly recommend it with this camera & mag, the 400' mags are the Achilles' heel of the 16 S, S/B.

Torque motor grounding problems are one cause of trouble. It's possibly you're "slower" mag is not running fast enough, also. But you need to run film to see because it's a torque motor, and need to see the clutch is working.

-Sam
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#3 Tom Doolittle

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Posted 16 January 2006 - 01:29 PM

I did run some old negative through, but it wound really tight and tore the sprocket holes (at the drive wheel inside the camera), leading me to believe something was very wrong.

I've heard that grounding was sometimes a problem, but since the motor runs strong (perhaps too strong) I thought this was not a concern in my situation. Am I wrong?

Hate to sound dumb, but how would changing camera speed cause the torque motor speed to change? It seems to me they are electrically isolated from one another. (I have a TCS motor, if that makes any difference.)

-Tom
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#4 Tim Carroll

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Posted 16 January 2006 - 01:36 PM

Tom,

I am certainly no expert on this but it sounds to me that if you have a TCS motor, you are probably running the camera on 12 volts. The torque motors were designed to run on 8 volts, so if you are throwing 12 volts through one I am not surprised that it is tearing through sprocket holes and trying to wind the film really tight. If you send the torque motor to Clive Tobin he can convert it to 12 volts and I think he does a simple clean, lube and adjust when he converts them.

-Tim
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#5 Clive Tobin

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Posted 16 January 2006 - 10:30 PM

3. I don't know what the 12v conversion consists of....


Depending on the generation of torque motor, we may or may not have to add a power resistor, arc suppressor and two interference suppressing capacitors. Charge currently is a flat $49.

I have seen "12 volt conversions" done wrong, where the power resistor is at the wrong point and is shunted by one of the brush springs so it has no effect. No substitute for experience.

The torque motor will eventually burn out on 12 volts if not converted correctly, meanwhile pulling too hard.

By definition a torque motor applies torque and is not regulated as to speed. It just pulls as fast as it can, determined by the running speed of the actual drive motor.

The ground jack is a later development to prevent the unreliable running that happens if the torque motor to camera frame connection is not clean and tight. Unfortunately they use a small nonstandard jack for this that is not available to our knowledge, so we are not able to add this to 12 volt conversions of older motors.
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#6 Sam Wells

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Posted 17 January 2006 - 12:34 AM

By definition a torque motor applies torque and is not regulated as to speed. It just pulls as fast as it can, determined by the running speed of the actual drive motor.

The ground jack is a later development to prevent the unreliable running that happens if the torque motor to camera frame connection is not clean and tight. Unfortunately they use a small nonstandard jack for this that is not available to our knowledge, so we are not able to add this to 12 volt conversions of older motors.


You (and Tim) are right of course. The takeup speed should "vary" but not the motor speed.

I recall one nightmare with a torque motor I could not ground -- so we hot wired the motor right from the battery, my assistant urged to touch the bare wires together *just a beat* after I rolled - DON'T you go first :)

Got the shots...

-Sam
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#7 Tom Doolittle

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Posted 17 January 2006 - 08:04 PM

Thank you, everyone, for the help. I appreciate the input from those who have first hand experience servicing these cameras. On that note, I would like to learn how to service this camera myself. As they become increasingly harder to find parts and service for, I can only assume this would be an invaluable asset to an independant filmmaker like myself. An engineer by trade, I have the mechanical apptitude but no prior experience to go by (other than some work I have done on my Filmo and K-3 cameras. As in other areas of cinematography, I get the feeling there are some proprietary aspects to old camera maintenance that some would prefer not be made public. Are there books, manuals, tools, etc. available to folks with a genuine interest in maintaning their own cameras? Where do the "pros" turn? Or is it all based on experience?

TD
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The Slider

Aerial Filmworks

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Ritter Battery

Opal

Media Blackout - Custom Cables and AKS

Glidecam

Willys Widgets

Tai Audio

Abel Cine

Broadcast Solutions Inc

Wooden Camera

Metropolis Post

CineTape

rebotnix Technologies

CineLab

Rig Wheels Passport

Gamma Ray Digital Inc

Paralinx LLC

Technodolly