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Arri Sr Pressure Plate


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#1 Wiki Park

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Posted 21 June 2011 - 07:31 PM

The pressure plate on my Arri SR seems to getting a bit loose. I know that the floating pressure plate is suppose to be freely moving to a certain extent, but it seems to be jiggling up and down and side to side pretty loosely every time I load film. Should I be worried about this? Is there a way in which you are suppose to calibrate it after however much usage?
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#2 Dom Jaeger

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Posted 21 June 2011 - 08:23 PM

The SR pressure plate needs to float to allow it to mate exactly with the camera gate when a mag is attached, but the spring pressure behind the pressure plate is quite precisely set.

The top of the plate should exert around 200 gram-force or 2 Newtons, the bottom half that (100 gram-force). Also important is the centre plate pressure (a separate spring), which should exert 10-15 gf.

If these pressure values are off, the film may not be guided properly through the gate, causing possible steadiness issues, focus breathing or scratching. It sounds like the large spring that controls the entire pressure plate force (top and bottom values) might be slack.

It's an easy adjustment for a tech with proper tools, but if your footage is coming out OK maybe not an urgent repair. If you have other mags, I'd use them for critical footage.
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#3 Wiki Park

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Posted 23 June 2011 - 08:44 PM

One more question, this does not have to do with the pressure plate, but when I run film through the mag, sometimes I find that there is some kind of gunk on the edge of the film. I am thinking that this is from the bottom padding of the feed side that is old and is being scraped off by the film as it runs through. I am not sure of this, but would there be any other explanation? Is this a serious issue when it comes to the image? Any thoughts or possibly solutions would be appreciated.
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#4 Dom Jaeger

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Posted 23 June 2011 - 10:44 PM

I've found with some older SR mags that over time the rubberised paint on the platines (the inner surfaces) and the inside of the doors can become sticky. Humidity and bad storage conditions can make it worse. If the film is loosely wound and cones on the core or the guide roller is not in position the film can rub against these surfaces and pick up residue.

That can then cause issues as the film passes through the gate. Make sure you clean the gate regularly, as that sticky stuff could build up in there. Maybe also clean the guide rollers. If you can have the feed and take-up tensions checked that would help.

I haven't found a solution other than stripping and repainting them, which is more time than the mags are worth. It sometimes helps to leave the mag somewhere warm and dry with the doors open, and let it air.
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#5 Wiki Park

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Posted 24 June 2011 - 12:35 AM

Thanks for your reply! I'll definitely try that out. This is the first time I have seen this problem with my particular mag, but would this cause an issue with the image? I'm a little bit nervous about the footage i sent to the lab that I haven't gotten back yet.
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#6 Dom Jaeger

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Posted 24 June 2011 - 02:47 AM

Depends how much goo got carried through the gate.. how dirty were the gate rails after filming? Anything get caught in the gate aperture? Any emulsion dust as a result of scratching?

Chances are your footage is fine. It's just something to be aware of - air the mag before use, take care loading, check the gate regularly, listen out for scraping sounds.
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