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A-Minima Shutter


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#1 Patrick McGowan

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Posted 12 December 2006 - 10:27 PM

I was test shooting on my school's A-Minima, and I noticed aproblem of the shutter being out of alignment. I didn't drop the camera or mistreat it, but for some reason part of the shutter was visible in the view finder.

The problem seemed to occur pretty inexplicably, all of a sudden I could see a quarter of the shutter in the frame and when I took off the lens I noticed the shutter had stopped past it's normal position.

Does anybody have any thoughts on what would cause this?
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#2 Chris Keth

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Posted 12 December 2006 - 11:31 PM

I was test shooting on my school's A-Minima, and I noticed aproblem of the shutter being out of alignment. I didn't drop the camera or mistreat it, but for some reason part of the shutter was visible in the view finder.

The problem seemed to occur pretty inexplicably, all of a sudden I could see a quarter of the shutter in the frame and when I took off the lens I noticed the shutter had stopped past it's normal position.

Does anybody have any thoughts on what would cause this?


Sounds like a slipping belt somewhere in the mechanism (if there is a belt), I would suspect. Did it go back out of phase when you phase the camera a couple times and run it again?

Edited by Christopher D. Keth, 12 December 2006 - 11:31 PM.

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#3 Nathan Milford

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Posted 13 December 2006 - 09:58 AM

Correct.

It sounds like the mirror drive belt has become distressed or the tensioner has backed off or the camera jammed hard enough to obviate those issues and brute force the belt to skip a tooth or so.

If you manually inch the camera by turning the feed roller (which, as a factory technician, I should never tell you to do as it'll prematurely wear the intermediary belt under the sprocked rollers) and observe the claw and how it moves you can determine if the claw and shutter are in sync. Do it thus (and note it is a bit of an art to get it just right so you ought to play with it a bit):

First, note which direction the shutter turns when the camera is running forward. Note which side the shutter is entering in from from the back of the gate in accordance with the principle that when the claw is engaged the shutter is covering (entering) the aperture and when it is disengaged (exiting) and resetting the aperture is open.

Inch the mechanism to the point where the claw is at it's 'dead point.' That is where the claw is at it's lowest point in the pull down. As it reaches the dead point it will not rise or lower anymore but only pivot in place as the crankshaft comes around. You're looking for the position when it just starts that pivot and if the claw starts exiting and rising then you've gone to far.

Once you've found the 'dead point' look through the gate and if the (exiting) mirror obscures half (or 2/3rds really) of the frame. If it doesn't then you ought to send it in to me. If it looks fine, shoot a timing test to be sure. Lock off the camera and point it at a piece of cardboard with a grid of holes. Back light the cardboard so light is coming through the holes (make sure they're round and clean holes, mind you) and frame the cardboard up tight. Shoot it at different frame rates too. If the timing is correct, the lit holes will remain round and distinct in the footage, if they smear then you have a timing issue. Typically one corner of the frame will have smeary holes if it is a small timing issue.

have fun, let me know how it goes.

- nathan
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#4 chuck colburn

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Posted 13 December 2006 - 02:24 PM

Correct.

It sounds like the mirror drive belt has become distressed or the tensioner has backed off or the camera jammed hard enough to obviate those issues and brute force the belt to skip a tooth or so.

If you manually inch the camera by turning the feed roller (which, as a factory technician, I should never tell you to do as it'll prematurely wear the intermediary belt under the sprocked rollers) and observe the claw and how it moves you can determine if the claw and shutter are in sync. Do it thus (and note it is a bit of an art to get it just right so you ought to play with it a bit):

First, note which direction the shutter turns when the camera is running forward. Note which side the shutter is entering in from from the back of the gate in accordance with the principle that when the claw is engaged the shutter is covering (entering) the aperture and when it is disengaged (exiting) and resetting the aperture is open.

Inch the mechanism to the point where the claw is at it's 'dead point.' That is where the claw is at it's lowest point in the pull down. As it reaches the dead point it will not rise or lower anymore but only pivot in place as the crankshaft comes around. You're looking for the position when it just starts that pivot and if the claw starts exiting and rising then you've gone to far.

Once you've found the 'dead point' look through the gate and if the (exiting) mirror obscures half (or 2/3rds really) of the frame. If it doesn't then you ought to send it in to me. If it looks fine, shoot a timing test to be sure. Lock off the camera and point it at a piece of cardboard with a grid of holes. Back light the cardboard so light is coming through the holes (make sure they're round and clean holes, mind you) and frame the cardboard up tight. Shoot it at different frame rates too. If the timing is correct, the lit holes will remain round and distinct in the footage, if they smear then you have a timing issue. Typically one corner of the frame will have smeary holes if it is a small timing issue.

have fun, let me know how it goes.

- nathan


Good morning Nathan,

That's very clever, probally more accurret than the old way we use to check shutter timing by looking thru the lens port and drawing an x from corner to corner on an exposed frame of film and rocking the shutter back and forth and looking for any film movement before the shutter (mirror) completly covered the apereture.

An olden days camera tech,
Chuck
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#5 Patrick McGowan

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Posted 15 December 2006 - 08:33 PM

Thank you for the insight into this problem.

It's my school's camera so I don't really have the chance to set up any cardboard related tests.

Nathan, you might be seeing this camera, I think they might send it down to Abel Cine to get serviced, but I'm not sure.
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