Bizarre incident with preston wireless follow focus

[James Davis]

So.....I was on a music video job recently, it was a 12 hour day I was Steadicam/B-camera, but for 80/90% of the day I would say we were on the steadicam, the follow focus got worked hard maybe harder than on other recent jobs as there was a lot of rehearsals and a few longer shots, with quite a few focus racks built into some of the shots, so not just small adjustments.

My Preston worked flawlessly all day, no issues, then bizarrely the following happened (at approximately the 11th hour)

 

Instead of the usual slow rotation the motor makes to calibrate to the lens initially it kept wanting to make a fast rotation, flying along to the end stop and then jamming stopped against the end stop of the lens trying to push against the end stop instead of coming back to find the other end of the focus ring.

When dis-engaged the motor simply continued spinning in the same direction at the same speed.

No amount of re-setting, changing of power sources, checking connections, checking lenses etc seemed to rectify this.

What is even stranger, is that I have had it checked by Optical Support (UK Preston dealer) and they said they were unable to replicate the fault and that it worked perfectly.

I noticed that the motor felt unusually warm to the touch on the day, did I cause it to overheat?

Is there some in built protection circuitry that kicked in?

 

It's a completely random experience that has never occurred before, i've used the Preston 2 channel and single channel systems, the bartech and the HoFo and never seen anything like this.

Luckily it happened at the end of the day just as I was about to de-rig and they had an idea for a quick Steadicam shot as a cutaway for a final scene, I was unable to do this but we just went to sticks and handheld (as originally planned) and it worked out fine, but if this would have happened to me earlier it could have really messed up my day!

 

Has this ever happened to any of you before?

 

Any ideas what it could be so I can prevent this issue in the future?

 

The System is a Preston Single Channel Focus/Iris unit with DM2 motor.

 

Any input at all would be appreciated.

 

James "confused" Davis

Edited December 21, 2011 by James Davis

[Eric Fletcher S.O.C.]

Instead of the usual slow rotation the motor makes to calibrate to the lens initially it kept wanting to make a fast rotation, flying along to the end stop and then jamming stopped against the end stop of the lens trying to push against the end stop instead of coming back to find the other end of the focus ring.

When dis-engaged the motor simply continued spinning in the same direction at the same speed.

No amount of re-setting, changing of power sources, checking connections, checking lenses etc seemed to rectify this.

What is even stranger, is that I have had it checked by Optical Support (UK Preston dealer) and they said they were unable to replicate the fault and that it worked perfectly.

I noticed that the motor felt unusually warm to the touch on the day, did I cause it to overheat?

Is there some in built protection circuitry that kicked in?

 

 

 

Sounds like a bad cable

[James Davis]

Have you seen this happen before?

Is it possible that the fault could be intermittent, because apparently the tests they did were quite extensive (the technician ran through every test he did with me on the phone) and it showed no issues.

I would have thought once it fails it fails, or if it was intermittent at least it would show up in one of his tests potentially?

 

Thanks for the input anyway.

[Eric Fletcher S.O.C.]

Have you seen this happen before?

Is it possible that the fault could be intermittent, because apparently the tests they did were quite extensive (the technician ran through every test he did with me on the phone) and it showed no issues.

I would have thought once it fails it fails, or if it was intermittent at least it would show up in one of his tests potentially?

 

 

Yes I have and it's always been a bad motor cable

[James Davis]

Great, thanks for clarifying, i'll get them to swap it out and maybe looking at getting a new one to keep as a back up spare, good to know the cause behind the issue and how to prevent it in the future.

[Ben Rowsell]

As per previous replies, bad cable. Or to be more specific when the motor is still receiving power, but there is a fault in the communication between the mdr and the motor, which is usually the cable but can also be the connector at either end (motor or mdr).

Usually for this to happen there would have been some sort of physical impact/contact occur as the connectors are generally prettly solid.

[Lawrence Karman]

Not sure if you answered this, but did you try the motor with a different cable or did you plug it into the iris channel and have the same result? You've go to narrow down if it's the motor, cable or Lemo plug. I recently had a problem with a zoom motor taking off in the manner which you described. I ultimately narrowed it down to the Lemo connector on the microwave transmitter atop a HU2. wiggle cables and connectors and that will help you find the source of the problem. Also, if a motor cable is not pushed in all the way, the motor will take off spinning fast.