Film-Tech Forum ARCHIVE: Shutter Flicker Question

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Posted by Bernie Anderson Jr (Member # 410) on 05-21-2009, 07:45 PM:

I know there has been discussion here about flicker (too much light stop down the lenses, 3 bladed shutters, etc.)but I noticed something today at the theatre that I thought was odd. Here's the set up: 2 theatres excatly the same (throw and screen size). Same Xetron/Nuemade lamphouse consoles. The projectors are Century SA and the lenses are the same size with the same f stop (integrated scope). Both reflectors are the same and none had any explotion in them. What would make one theatre have a shutter flicker and the other one not have a flicker? I'm getting the same brightness too.

Is it something slowing down the one projector?

Also, is there a relationship to the size of the shutter blade to shutter flicker?

Just was wondering.

Posted by John Hawkinson (Member # 1135) on 05-21-2009, 07:56 PM:

Lamp flicker, or rectifier unevenness (filter caps? diodes?)?

Can you see the lamp pulsing if you look at it without the shutter? Or if you look at the ammeter? [Or if you allow the light from the lamp to fall on a fast photocell and look at it on an oscilloscope...]

Have you tried both projectors without lenses and looked at the screen? That might give a clue as well. [possibly without the shutter running, but that's not a very safe thing to try...]

--jhawk

Posted by Bernie Anderson Jr (Member # 410) on 05-21-2009, 08:09 PM:

Not a diode or cap problem. Lamp is not pulsing with projector off. And if it was an unevenness is the rec, if wouldn't be consistant. And this is not a bad diode, I usually get a strobe effect or a tripped breaker. This is just basic shutter flicker.

Posted by John Hawkinson (Member # 1135) on 05-21-2009, 08:14 PM:

Same size shutters? If they're adjustable, what shutter angle?

Maybe slight differences in the motor speed? If you put some pressure on the motor flywheel with a cloth while running (slow it down just a hair), does the effect change?

Do you see it both with and without film?

Poly-V belt or toothed belt on the motor?

--jhawk

Posted by Bernie Anderson Jr (Member # 410) on 05-21-2009, 08:27 PM:

Interesting idea. Belts? not sure, I'm pretty sure that they are both a tooth belt. The shutters, I believe are the same stardard 90 degree bowtie century shutters.

How much slower than the 1725rpms does the motor need to run to start showing flicker? And if it is a motor issue or overall projector issue, how would you go about telling what speed the projector is actually running at?

Posted by John Hawkinson (Member # 1135) on 05-21-2009, 08:34 PM:

Poly-V belts are prone to slippage, such that the belt tightness affects the speed. That should be much less true of the toothed belts.

You can measure the speed with a strobe, or with a photocell looking at light through the shutter (connect to a DMM that measures frequency, or to an oscilloscope, or a frequency counter), or by playing some film and messing up your A chain such that you play perfs and measuring the frequency of the perfs. Just don't play 1k tone and measure the frequency of that, because typical 1k tone can be ~10% off in frequency.

Another thought: if the projectors aren't on the same phase, is the motor line voltage different? Is one phase sagging?

I don't know what speed you'd need to be down to notice shutter variations. I would tend to think it would be substantial, and that you'd hear audio defects long before you saw shutter problems.

Also: Have you checked the fire shutter? On centuris, the fire shutter can fall slightly into the light path and bobble and look like flicker. This is a lot more obvious if you look at the image on the port glass, rather than on the screen, and you'll notice the flicker is confined to the bottom of the image area. If so, open the driving side and hold the fire shutter lever down (up?) by hand and see if the problem goes away.

--jhawk

Posted by Fred Georges (Member # 505) on 05-21-2009, 09:32 PM:

Check to see if the shutter blade is oscillating slightly fore & aft at the bearing point at the shutter guard. That can cause a flicker with Centurys. [Big Grin]

Posted by Bernie Anderson Jr (Member # 410) on 05-21-2009, 09:35 PM:

What do you mean, Fred?

Posted by Randy Stankey (Member # 64) on 05-21-2009, 09:38 PM:

Is the flicker even or does it appear random?

-.-.-.-.-.-. or --.-.--.-.-...-.-.-..-.-..-.- ?

Posted by Bernie Anderson Jr (Member # 410) on 05-21-2009, 10:15 PM:

semms to be consistant

Posted by Randy Stankey (Member # 64) on 05-22-2009, 12:00 AM:

That seems to rule out problems in the rectifier. A lamp that flickers erratically is a symptom of a problem in the power supply like a lost phase, etc.

How is the focus (alignment) on the lamp?

They say they human eye isn't capable of discerning the flicker of the shutter at 48 per second but that's not entirely true. A trained eye can pick it up. It becomes easier as the picture gets brighter. Even a well-tuned projector can produce flicker if the image is too bright.

If the lamp is not focused correctly and produces a hot spot, it will be easy to see the flicker.

If you take the film out of the gate and project white light onto the screen, you should be able to tell what's going on in a few seconds.

Be careful! Don't burn up your lens with the hot light coming out of the open gate! (Film blocks the heat of the lamp from getting to the lens.) Only do this test for 60 seconds at a time. Give the lens another 60 seconds rest before opening the dowser again.

I don't think this is the only answer. Just one possibility. But it is an easy thing to check. When trying to solve problems, always do the easiest things first. Right? [Wink]

Posted by Tony Bandiera Jr (Member # 2365) on 05-22-2009, 12:25 AM:

Good suggestions so far, want to add:

Make sure the shutter blades on the flickering machine aren't warped or burned with a hot spot..that will cause a flicker.

Posted by Mark Gulbrandsen (Member # 72) on 05-22-2009, 03:32 PM:

Changing the two orange and one silver electrolytic on the top board will fix your problem Bernie. Use the exact same value but it's ok to go higher voltage rating as long as it fits. Also try to aquire 105 deg. C. capacitors if at all possible. One of those caps is the filter on the voltage refrence and when that cap gets leaky you end up with noise on the refrence and then it transmits through to the actual light generated.

Mark

Posted by Steve Guttag (Member # 268) on 05-22-2009, 07:16 PM:

Mark...are you referring to a switching power supply? If so, which one?

Steve

Posted by Mark Gulbrandsen (Member # 72) on 05-22-2009, 08:04 PM:

Nope. I'm referring to the capacitors on the top control circuit board. Bernie knows what I am referring to.

Mark

Posted by Louis Bornwasser (Member # 3063) on 05-22-2009, 09:22 PM:

The RA relay! Louis