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My Preston motor keeps "freezing"


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#1 Eli Aronoff

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Posted 27 February 2010 - 12:56 AM

I've had my Preston remote focus system for over a year. It's been getting a lot of use on the TV show that I am a focus puller on. I use Panavision Primo lenses. I use a "clip on" matte box for steadicam and hand held shots (because it's lighter); and a "studio" matte box when the camera is on a dolly or remote head. This is common practice for most AC's.
For the past couple of months the Preston motor began to "feeze" or "jam up" at randem times when I am using the "clip on" matte box. The clip on matte box adds more stiffness to the lens.
I've spoken to almost every engineer at Preston and we've finally decided that it is freezing because the MDR's firmware has a safety limit of power allowed to the motor which shuts it down if the motor needs to draw too much power to move a particularly stiff lens. This is to prevent the motor from burning out.
From my experience, I don't feel that these lenses are particularly stiff but maybe thats what it is.

Has anyone else experienced this problem?
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#2 Jens Piotrowski SOC

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Posted 27 February 2010 - 01:47 AM

yes, especially the 50mm Primo, always calibrate without clip on first, the attach. in cold weather you have to help the motors calibrate by hand. I love the S4"s........
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#3 RonBaldwin

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Posted 27 February 2010 - 03:04 AM

Are the motors in question the dm-2's?
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#4 Eli Aronoff

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Posted 27 February 2010 - 12:26 PM

it actually happens with a dm1x!! I'm able to calebrate fine but in the middle of the shot when the focus doesn't have to move for about 10 seconds it freezes sometimes on some of the lenses.
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#5 Eli Aronoff

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Posted 27 February 2010 - 12:57 PM

It's actually several lenses (35mm, 40mm, 100mm). I'm using the super strong DM1X motor so I shoulnd't have any problems. My 50mm is good, although I do agree that 50mm's are usually the stiffer lenses.. My lenses aren't that stiff, thats one of the qualities I look for in a lens at the camera check out.
Also the lenses are calabrating just fine. the freezing happens in the middle of the shot when the focus is static for 5-10 seconds. Then when I have to adjust it freezes. This doesn't happen every time but enough that it's a problem.
Preston says that it's the safety feature in the MDR that stops the motor if it draws a certain amount of amps to move a stiff lens. This is to prevent the motor from burning out.
The only thing is that my Preston has worked just fine with these same lenses for months even in the bitter cold weather, and all of the sudden now I am experiencing this problem on a regular basis even in a warm studio.
I'm not doing anything out of the ordinary.
Preston is going to adjust the limit on that safety feature a little bit to give me a little more leeway. Maybe that'll do the trick.
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#6 Richard W. Davis

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Posted 27 February 2010 - 01:23 PM

Clip on matte boxes can sometimes bind the elements inside from the external squeezing... Try lightening the pressure on the set screw holding the mattebox on... a delicate balance of keeping enough pressure to keep it on, but not binding the elements inside.

Rich Davis
LA
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#7 Eli Aronoff

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Posted 27 February 2010 - 03:12 PM

I actually do keep the clip on matte box as loose as possible, like you said, but still having this problem. I'll see if I can replace the lenses that I have trouble with and maybe have some luck with that.
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#8 Robert Starling SOC

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Posted 27 February 2010 - 04:09 PM

This an overly simplified suggestion / comment but I was running into some binding with my DM1X that I thought was a stiff lens on a cold day then it happened several times with easy lenses. What I found was that the rosettes on my motor / Hill bracket and the motor mount rods were allowing the motor mount rod to get just close enough to the gear to apply an extra amount of pressure when we tightened it down. It wasn't readily visible just looking at it. I'm sure you're on top of this but it did have me scratching my head a few times until we figured it out.

Good luck!

Robert
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#9 Eli Aronoff

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Posted 27 February 2010 - 08:21 PM

Robert,
Are you saying that some how the motor was being pushed onto the lens tighter than you had set it before it got locked in?
I'll look into that on monday.
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#10 Erwin Landau

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Posted 27 February 2010 - 10:02 PM

It's hard to do a diagnoses over the phone or over the net without seeing the set up first hand... I had people call me about a steadicam failure and trying to explain and trouble shoot over the phone... after i showed up on set and saw the set up it took me about a minute to trouble shoot in that instance they had a modified sled and one of the switches was in the wrong position...

Just out of experience with the Preston... If you have difficulties to rotate the barrel by hand, the Preston will not pull it.
I had Zeiss Superspeed bind on me the moment I put the LMB-5 back on. I had dm-2's burn out (3 times) and the DM-1 just go bonkers on me...

I had one AC recalibrate the lens with out the mattbox and the moment he put the box back on he lost 2/3 of the throw... I guess you know where I am going with that... Shit happens and stuff changes.

See if anything changes during the the pull. Is a cable getting in the way. It's a Primo so it's not breathing... no related problems there... It happens with different lenses so the lens isn't the problem... Did you use different Motor cables? I had faulty cables in the past that got pinched and had intermittent trouble... Did the MDR dropped or did it go through a big shock... was it shipped since the last time... is anything rattling inside... Did you try several different motor, cable, lens configurations? Different MDR, hand-unit set up... running out of ideas...

if it's a software problem send it back in... eliminate all possibilities and at the end you should left with the Gremlin in your site...

That was an utterly useless post, Sorry.


Good Luck,

Erwin
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#11 Robert Starling SOC

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Posted 28 February 2010 - 09:38 AM

Robert,
Are you saying that some how the motor was being pushed onto the lens tighter than you had set it before it got locked in?
I'll look into that on monday.


No, I'm saying that with the new DM1x motor, I found early on that with certain gears we had inadvertently allowed the VERTICAL motor mounting rod on my Hill motor bracket to get close enough to the gear to slightly touch and cause enough friction on the motor gear that it would begin to bind.

Robert
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