Film-Tech Forum ARCHIVE: Unstable Century Gate Tension

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Posted by Steve Scott (Member # 630) on 07-08-2004, 01:53 PM:

One of my Century SA-TM's has been losing focus when splices run through the gate. The focus problem looks like the left and right areas of the screen are shifting in and out of focus, but often times this goes away when another splice runs through the machine, but does occasionally come back after other splices.

I'm pretty sure that this has to do with the gate tension, as the tension knob has to be turned to the six-o'clock position in order to have any on screen stability... What part of the gate might need to be replaced? Is it something in the tension control or is it something else like the gate rails?

Other info: The turret has been checked and it is stable, all set screws tightened, and lenses are both tight in their rings. There is no penthouse soundhead as this is an analog house. The same problem exists with both flat and scope films.

Posted by John Hawkinson (Member # 1135) on 07-10-2004, 12:11 AM:

Speculating here, but perhaps the spring on the lateral guide above the gate? Can you successfully thread it in the out of focus state if you try? Or make the problem happen with junk film, and then shut down the projector and look and see if the film is misaligned with respect to the focal plane?

--jhawk

Posted by Brad Miller (Member # 2) on 07-10-2004, 12:20 AM:

Try making your upper loop 1-2 perforations larger. The SA curved gate is happiest when the loop "bends backwards" over the top of the gate a little bit. It prevents the film from cupping as it enters the gate, which can knock your sides out of focus.

Assuming your bands are in good condition as is the tension spring adjustment at the top of the trap (doubtful if you have been running it at 6:00), odds are you need to lower that tension back to 12:00 and readjust things so the gate closes a little bit farther into the trap enough to produce a steady image and no more at 12:00. Then fine tune your tension adjustment from there. That'll get you in the ballpark.

Posted by Jack Ondracek (Member # 1466) on 07-10-2004, 12:36 AM:

Has anyone been messing with your studio guides? Getting them too tight can do that.

Vern Klingman used to recommend that the guides be adjusted outward so they weren't used. His thought was that if it was good enough for an XL, a Century ought to work that way, too. I tried it once, but wasn't happy with the image stability & put it back... which makes me wonder how an XL does do it......

Posted by Mark Gulbrandsen (Member # 72) on 07-10-2004, 08:21 AM:

Vern was right! I always removed the studio gudes on a Century. They do absolutely nothing and can make cleaning the trap a pain in the butt. Never had any steadiness problems either as Steve K. will attest to. He owns a pair of my ex- dailies machines. My Centurys have been on a gazillion film jobs projecting dailies and they were always dead steady.

Mark

Posted by Steve Scott (Member # 630) on 07-10-2004, 09:32 AM:

Brad,
I've tried various loop sizes (up to the point of what I'd consider the maximum) and we still get the same effect.

Jack,
Aside from cranking up the tension, nobody has attempted anything with this machine.

I will look at how far the trap closes today, and see about removing the studio guides while I have time.

Posted by Dominic Espinosa (Member # 2122) on 07-10-2004, 11:18 AM:

I had a problem with that on an SAW a while back. If memory serves the preassure straps were just in bad shape. I replaced them and readjusted the gate so that it closes a little further back. Currently the tensioner is at 12 0'clock and all is well.

Posted by Jack Ondracek (Member # 1466) on 07-11-2004, 12:56 AM:

quote: Mark Gulbrandsen
Vern was right! I always removed the studio gudes on a Century.

Thanks for the input, Mark. I'll work with that some more when I find the time. It was an intriguing concept... but I had (have) too much on my plate to really get into it at present. Sounds like the guides are compensating for another problem somewhere.

quote: Steve Scott
Jack,
Aside from cranking up the tension, nobody has attempted anything with this machine.

Then, unless there's some gunk in there you haven't found, don't mess with the guides on my recommendation. Mark backing up what Vern told me adds a lot of credibility to the idea, but unless you have a gauge or metal film strip to put the guides back if you decide to, it might be best to look elsewhere for the problem... especially if it worked OK before. Unless they've been messed with, they won't get tighter on their own. On the other hand, if someone's pulled the trap assembly out... maybe for cleaning?... and jarred or dropped it (loose screws?), it's possible the guides could have been moved accidentally.

Posted by Mark Gulbrandsen (Member # 72) on 07-11-2004, 01:24 AM:

I do have a custom Century gate alignment trap that I use. Its much like the standard Century gauge of years past but was made far more accurately by a die maker friend from tool steel. With this gauge I'm able to achieve steadiness out of a Century that is amazing. Steve K. I'm sure will attest to that. Careful alignment of the lateral guide roller and keeping new bands in it are two key things that have to be right for it to work as flawless as possible. Steel film might be another possibility but the gauge actually refrences the edge of the intermittent sprocket to the fixed edge of the guide roller minus about two ten thousandths of an inch to compensate for acetate lateral shrinkage. Dailies are normally still printed on acetate far as I know. Dick Prather also has one of these gauges..... the only other one like this.

One other thing to set properly on an SA is the clearance(penetration depth of the gate into the trap). This should be .25 inch measured between the flat plate that the curved runner is mounted to and the outer studio guide. I carry one guide in my tool kit specifically for this purpose. With the 4 gate operating assy screws loose and the gate in the closed position put the .25" drill bit in between and runner plate and the studio guide, then Slide the entire gate assy. back into the trap allowing it to hold the bit in between the plate and guide. Tighten the four screws down while you or someone else holds it this way.

Mark

Posted by John Pytlak (Member # 331) on 07-11-2004, 07:43 AM:

quote: Mark Gulbrandsen
Dailies are normally still printed on acetate far as I know

All color print film from the three major film manufacturers is now polyester base, including that used for dailies and answer prints.

Posted by Mark Gulbrandsen (Member # 72) on 07-11-2004, 09:20 AM:

Thanks John for that clarification. Its been ay least 5 years since I have run any.

Mark

Posted by Steve Scott (Member # 630) on 07-11-2004, 11:01 AM:

After realigning the studio guides, the focus flutter has disappeared. The tension is better, only needing the 4-o'clock position now, but obviously still too much. Another note, when I turn the tension knob to 12 o'clock, the image can be focused, but remains unstable.

We have our tech coming in this week, but failing him I'll work on that clearance as you described, Mark.

Posted by Mark Gulbrandsen (Member # 72) on 07-11-2004, 03:08 PM:

When I get done with an SA it usually runs tension setting at 1 to 2 o'clock max. This is with new bands, a good curved runner and sprocket shoes(PA-195) and the depth set to .25". Really, just jink the studio guides into the rewind bench junk drawer. You'll only need them to set the depth adjustmant.

Mark

Posted by System Notices (Member # 2357) on 07-31-2006, 03:37 AM:

It has been 749 days since the last post.

Posted by Carl Martin (Member # 1146) on 07-31-2006, 03:37 AM:

regarding setting the 1/4" distance between studio guide and the "flat plate"... not all studio guides are equally thick.

comparing gates today i found one of our sa's (s/n 4578) has studio guides 1/4" thick. on our msa, 2 spare sa heads (one of them strong-era) and 2 saw's at the other theater i'm working, the studio guides are all 3/16" thick.

this oddball sa has always had somewhat dodgy focus. i've noticed lately that a splice can send the inboard edge of the film riding up the beveled portion of the spring-loaded flange opposite the lateral guide, coming out of the upper loop. poking my finger behind the trap i can push out the flange to re-seat the film.

the space between the studio guide and the "flat plate" is just under 1/4", and the gate tensioner is at 3 o'clock, significantly higher than any of the other machines mentioned, yet the gap between the curved runner and the band, at the top of the gate, is clearly too big, allowing the film to ride up the flange as described.

i'll mention this to our tech at the next opportunity. it's possible that this was done intentionally, somehow to mitigate the focus problem. i don't want to inadvertently un-fix anything. in addition, there are foil-tape shims under the curved runners.

Posted by Mark Gulbrandsen (Member # 72) on 07-31-2006, 08:39 AM:

Carl,

First off you need to get rid of those stupid "studio guides". I haven't seen a projector that actually didn't improve on its side weave spec. by removing them. They are usless anda nuisance! Really... think about it... there are no two strips of film that are exactly the same identical width... especially with used prints so what function do they actually serve? More likely to get in the way and prevent proper alignment of the film path. You only need keep one or two of the correct size guides and screws kicking around to re-set the gate clearance into the trap when the gate open/close casting is removed and put back.

It might actually be high time to take that head out and have someone with a large granite surface plate do some measuring of its parts and their alignment..... Something is definately amuck in there for sure. Centurys are always sloppy in that regard but they usually do focus very well. Also check the top of the trap... the little attach points where the tops of the gate bands screw to. One of those little attach points might be stuck causing uneven tension of the bands.
Mark

Posted by Carl Martin (Member # 1146) on 07-31-2006, 11:24 PM:

i neglected to mention that the screen's at a bit of an oblique angle which complicates matters. also, it's possible that that trap was dropped once (not by me!).

i'd probably catch hell for removing the studio guides but i'll check how much clearance there is between them. i'll have a look at the gate band attach points too. won't be back in until thursday though.

anyways, i mainly just wanted to point out the discrepancy in the thicknesses of studio guides for those who use them to set the gate closure. maybe it would be better to ignore them entirely and reckon 7/16" between the surface of the trap and the flat plate with the runners?

Posted by John Pytlak (Member # 331) on 08-01-2006, 07:44 AM:

Just move the studio guides out to their widest position. The edge guiding of the film should be done by the fixed and spring-loaded rollers at the top of the gate.

Posted by Louis Bornwasser (Member # 3063) on 08-02-2006, 04:05 PM:

Adding to John P's posting: The 4 screws that hold the large metal block that holds the gate knob asssembly are critical to focus & stability.

We use a suitable sized allen wrench laid on top of the studio guide to assure that the gate is square with the aperture plate. On a Century it is critical not to "over insert" the solid film shoe into the film path. Way too much will "trap" the film up against the aperure plate, resulting in wear and film damage. Too loose is usually not a problem. Lowest possible pressure on gate should be OK, if the intermittent is OK. Louis