Film-Tech Forum ARCHIVE: Century C left-side focus flutter

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Posted by John Hawkinson (Member # 1135) on 09-08-2007, 10:40 PM:

Tonight we encountered a bizarre focus problem on one of our century Cs. Any suggestions much-appreciated.

We found that a portion of the left side of the image would intermittently flutter in and out of focus. The effect was quite similar to having the main trap not properly closed. The focus flutter seems to occur on the order of 10Hz or so (hard to tell!). It's much harder to notice in scenes with motion, but the problem seems to come and go, sometimes consistently fluttering and sometimes disappearing for minutes at a time.

The amount of the left image affected varied -- ranging from no visible problem to affecting about 4/7s the width of the image (starting on the left side).

The problem gets substantially worse as tension is increased. With the tension backed off all the way the problem is minimized.

At one point, during one reel, the entire focus of the reel shifted substantially and instantaneously (approx. 1/4 turn of the focus knob). [Supposedly] this did not happen during our previous screening. This problem might be completely unrelated.

We see the problem with two scope lenses and two flat lenses, so it doesn't seem to be in a lens. The trap rails and gate have been cleaned repeatedly, and don't seem to show excess buildup (perhaps a little more than usual). There doesn't seem to be any significant hooking of sprockets, lateral play of sprockets, and the reference guidance roller seems to function properly.

We noticed the problem for real for the first time tonight, during a scope movie, but upon further consideration, realized we had seen it before. Our trailer reel has had some bizarre focus issues the past few shows, but we had attributed them to something specific to those individual trailers (embossing, odd printing, etc.). But on reflection, it's the same problem. Not clear why we didn't notice it on prior features...presumably it was less severe.

It seems to be independent of film stock.

We're running 2k Osram lamps in Xetron XHN2000 lamphouses, and there does not seem to be any hotspotting. The effect is not visible without film. The lamps are being vented with 400CFM and there is no excessive heating of the lamphouses or the projectors.

Any tips/thoughts/suggestions?

At times the problem seems to approach unwatchability.
Throw is 80 foot, lenses in question are all Isco Cinelux ultra-MC (gold). It seems a bit more noticable through the scope lenses (125mm focal length) than the flat lenses (80mm), which seems counterintuitive to me (less magnification with scope).

Thanks again.

--jhawk

Posted by Carl Martin (Member # 1146) on 09-09-2007, 02:03 AM:

is the gate closure set properly? if it doesn't close far enough then the film might be riding up the bevel of the lateral guide roller on one side. that could account for a 1/4 twist of the focus knob. if it vacillates that could be the flutter. increasing tension might further incline the film to ride up in this manner.

if that's the case, the gate tension on this machine is bound to be set higher in general.

we have sa's but i assume the c is similar enough.

Posted by John Wilson (Member # 269) on 09-09-2007, 02:42 AM:

Hi John

Is it every print or just the new one? I'd check the gate rails haven't moved. Move them out as far as they go and try again. Any chance you've got someone in there that likes to fiddle?

John

Posted by Scott Balko (Member # 438) on 09-09-2007, 08:17 AM:

If your machine still has the studio guides, get rid of them. The lateral guide rollers are all it needs. Sounds like it's squeezing the film in the trap, and the more tension you impose just forces it into the already too narrow space and curls the image area.

Posted by Gordon McLeod (Member # 33) on 09-09-2007, 08:17 AM:

Check that the studio guides havent got shifted and the film is too tight of lateral pressure in the trap. Also check the guide rollers are not to tight and snap back and forth freely as the film can climb out of them
Also check for wear on th the gate rails and you could reverse them if the other side isn't worn

Posted by John Hawkinson (Member # 1135) on 09-09-2007, 08:30 AM:

Thanks for all the advice.

Clarification, the major instantaneous focus shift recurred in the same place at the next show, so I think that's just some unrelated print issue.

We'll double-check the studio guides to make sure they're as far out as possible. It seems to be every print (at least it is the two prints we have right now, as well as a smattering of trailers; both prints on 2383, btw).

Hmm. Gate closure...I hadn't realized that was adjustable. Will hunt and check. The film doesn't seem to be riding oddly.

Scott, by "trap rails" I mean the spring-loaded rails, not the studio guides.

Swapping the trap rails from another projector doesn't seem to affect the problem, so I don't think it's a wearing problem on them. The reference guidance roller springs freely as it should.

Thanks!

--jhawk

Posted by Louis Bornwasser (Member # 3063) on 09-09-2007, 10:27 AM:

30 years ago we installed an 8-plex with 6 SA and 2 "C" century. The C 's were straight gates. Every show, the film "popped" fully out-of-focus at the same number of minutes from the start. A curved gate conversion kit stopped the problem instantly.

Has something recently changed in your installation? New reflector? Larger bulb? Louis

Posted by John Hawkinson (Member # 1135) on 09-09-2007, 01:10 PM:

Nope, no recent changes. That's what makes it so weird...

--jhawk

Posted by Mark Gulbrandsen (Member # 72) on 09-09-2007, 06:40 PM:

I agree with Scott... Start by trashing the studio guides... worthless pieces of metal! This holds true for ALL Century projectors! If you are using no larger than a 2kw lamp and have either a good dichroic reflector or good heat filters the straight gate will work fine. Over a 2kw lamp then you really need a curved gate.

Mark

Posted by John Hawkinson (Member # 1135) on 09-09-2007, 07:19 PM:

Well, adjusting the outboard studio guide to its extreme travel seems to have indeed solved the problem (at least, so far...). THANKS!

(I'm thoroughly mystified why this was never a problem previously and how it suddenly became one...).

Dumb question: what's the minimum disassembly necessary to actually adjust the screws that hold in the studio guides? I managed to sneak a right-angle screwdriver in just enough to loosen the operators's-side guide (the one I needed), but afterward I couldn't get the screws very tight...what's the right
way to get access to those screws?

All this time I had naively assumed the studio guides were at their outer limits, since we'd never had any issues of this sort...live and learn.

Yes, it's an original straight gate with a 2k lamp. We have not had focus issues previously, so I think we're ok on that...

Thanks again, everyone.

--jhawk

Posted by Steve Guttag (Member # 268) on 09-10-2007, 09:13 AM:

Take the trap out and adjust/remove them (I keep them in but ensure they can't touch the film). To take the trap out...you will need to remove the light shield (two screws) and then the trap, three screws (in triangular formation). There are pins so you should get it back in exactly the same spot.

Steve

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-10-2007, 10:54 PM:

quote: Mark Gulbrandsen
Start by trashing the studio guides... worthless pieces of metal! This holds true for ALL Century projectors! If you are using no larger than a 2kw lamp and have either a good dichroic reflector or good heat filters the straight gate will work fine. Over a 2kw lamp then you really need a curved gate.

Hmm, at the drive-in I was considering replacing our Super Lume-X lamphouses running 2.5kW lamps with a pair of X-60Cs and 4kW lamps. Is this not recommended with water cooled straight gate Century CCs?

Posted by Mark Gulbrandsen (Member # 72) on 09-11-2007, 09:23 AM:

I am pretty sure that you'd have alot of film flutter making focus all but impossible. I thought you installed one of the Technilite kits there...

Mark

Posted by Louis Bornwasser (Member # 3063) on 09-11-2007, 10:11 AM:

The Century C's I mentioned above: focus popped with a CXL-10 bulb in CH-10 Christie consoles. I agree with Mark: above 2000 watts in a good/great reflector, you will need curved gates, probably with water.

According to Strong: S. Lumes/2500 watts and X60D with 3000 watts have within 1% the same Lumens. It would logically follow that 4K would only be slightly brighter. Louis

Posted by Daryl C. W. O'Shea (Member # 1303) on 09-11-2007, 06:23 PM:

quote: Mark Gulbrandsen
I am pretty sure that you'd have alot of film flutter making focus all but impossible. I thought you installed one of the Technilite kits there...

Different drive-in. Although 4 years later and I'm still not sure I can see the huge difference I'm supposed to be seeing at the other drive-in. The picture seems to be blue-er though, which is nice, I like blue.

quote: Louis Bornwasser
According to Strong: S. Lumes/2500 watts and X60D with 3000 watts have within 1% the same Lumens. It would logically follow that 4K would only be slightly brighter.

Well, scrap that idea. Thanks guys, saves me wasting any time on that.

Posted by John Hawkinson (Member # 1135) on 09-12-2007, 05:00 PM:

Wow...this is embarassing, evidently I wasn't taught how to clean a Century C sufficiently...even though we clean the gate rails (technically Film Gate Pads, PA-{1,8,9}-E) religiously, somehow I always assumed that there was some magic that meant that the surface they mate with didn't need the same attention.

Removing the trap shows pretty clearly that you can get quite an accumulation on the film trap shoes (SH-15-E) -- right under/around the studio guides. How often is it recommended to remove the trap and clean those shoes (for a non-FilmGuard theatre)?

--jhawk

Posted by Mark Gulbrandsen (Member # 72) on 09-12-2007, 05:32 PM:

Again, if you remove the guides you won't have this problem. There are so many positive reasons I can think of to pull them. I took off three sets today.... lots and lots of gunk under them. They serve absolutely no purpose on a C model head except to accumulate gunk. The image steadiness can actually improve with out them and cleaning the rails is a cinch. Different with an SA as they are used to measure and set the gap when the gate is closed... I still yank em off SA's as well but keep them in the booth for doing the adjustment.

Mark

Posted by Louis Bornwasser (Member # 3063) on 09-12-2007, 06:09 PM:

They help you insert the aperture plate, but (agreed) are otherwise useless. I "back 'em away" like Steve. Louis

Posted by Mark Gulbrandsen (Member # 72) on 09-13-2007, 09:14 AM:

Inserting plates has never been a problem in either a C model or an SA... The main thing here is to remove them so cleaning is much simpler and all the crud is removed.

Mark

Posted by Jack Ondracek (Member # 1466) on 09-13-2007, 09:34 AM:

Is there a chance the film stock might be an off-brand? If John's machines worked fine up to this point, maybe he got a print that wasn't slit right. ?? While I'm all for pulling the rails (done that... it works!), why is it that he's now having troubles with a system that had not been changed or worked on? The only real variables here are the film and any possible gunk between the rails.

Posted by John Wilson (Member # 269) on 09-13-2007, 10:14 AM:

...or someone who may be having an 'adjustment' moment.

Posted by John Hawkinson (Member # 1135) on 09-13-2007, 10:49 AM:

No, it was Kodak 2383 from 2007.

Having actually taken the trap out (to finalize my adjustment of the studio guides), though, I could believe that some of the gunk on the trap shoes was sufficient to cause this problem, and that moving the studio guides was sufficient to dislodge it or cause its effect to be mitigated.

It's not a very compelling theory, but it's not totally implausible.

--jhawk