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Topic: Flicker on an older simplex

Tom Kroening
Expert Film Handler

Posts: 214
From: Janesville, WI USA
Registered: Oct 1999

posted 11-13-2001 05:22 AM

I'm having a slight problem with flicker in two of our houses. Each one is equipped with an older simplex projector and Kniesley lamphouse (about 10 yrs old). They each have brand new bulbs and reflectors yet you can see the flicker (especially in brighter scenes). There is no ghosting and the shutters are clean and look to be in good condition. The only thing I can think of is that the projector motor isn't operating at its rated speed... could be slowly dieing?? I guess it could always be the power supply. Would there be a good way to check for that? Anyone else have some ideas? Thanks

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Brad Miller
Administrator

Posts: 17775
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99

posted 11-13-2001 05:25 AM

The picture is actually flickering, not smearing, right? Have you checked the diodes in the rectifier? Odds are one of them is open. Alternatively I suppose it is possible that you have too much illumination on the screen, but I doubt it.

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Tom Kroening
Expert Film Handler

Posts: 214
From: Janesville, WI USA
Registered: Oct 1999

posted 11-13-2001 05:38 AM

No, definately not smearing... its flickering. The picture did this before the new bulb when the image was dim. I've never heard of a diode going out at this theatre before... id have to look around to see where they are located on the power supply. It is correct practice to replace them all if one is toasted right?

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Brad Miller
Administrator

Posts: 17775
From: Plano, TX (36.2 miles NW of Rockwall)
Registered: May 99

posted 11-13-2001 05:52 AM

Some techs agree with that practice, but others say it's a wasted effort. I would personally recommend changing out all 6 and saving the good used ones for repairing the next machine so you're not mixing new diodes with old ones.

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 11-13-2001 08:08 AM

With a two blade shutter, a slight amount of (48 Hz) flicker can normally still be perceived, expecially in really bright areas of the picture or with no film in the projector. A three-blade shutter virtually eliminates perceived flicker by raising the frequency to 72 Hz.

If your projector was operating much below 24fps, the effect on sound would be obvious. It's highly unlikely the projector motor is slow enough to cause the flicker, unless the analog sound varies in pitch or is obviously too low in pitch. If in doubt, measure out exactly 1440 frames of film (24 x 60 = 1440) and verify that it takes exactly 60 seconds to project it when the projector is at "speed".

Excessive current ripple in the direct current supplied to the lamphouse will definitely increase flicker and reduce xenon lamp life. 60Hz power supply ripple "beats" with the 48Hz or 72Hz shutter frequency to produce an annoying and very visible 12Hz flicker component. Diode failure, mismatched voltage on the incoming 3-phase power (worn, overheated, or oxidized contacts?), or deteriorating filter components are the most likely causes.

Instability of the arc plasma or vibration of the lamphouse optical components can produce an irregular "candlelight" flicker. The arc plasma can become unstable as the lamp nears the end of its' life, or is subjected to uneven/excessive ventilation or magnetic fields. Look at the lamp through the sight glass of the lamphouse and observe whether the plasma "arc" is stable, or if moves around erratically like a candle flame.

Osram recommends measuring current ripple by analyzing the voltage drop across a low-induction shunt with an oscilloscope (DANGER: do not attempt this unless you understand how to properly measure high power circuits), or by letting a bit of light directly from the lamp shine on a photocell and analyzing the variation in light output with an oscilloscope. The Osram booklet "XBO Theatre Lamps (Reference-No. 123 W97E 11/97)", available direct from OSRAM Public Relations in Munich or via FOMK-B, outlines the procedure.

Here are links to xenon lamp information:
http://www.osram.com/service_corner/download_center/photo.html#XBO
http://www.cinemaequip.com/tech4.shtml
http://www.xenonbulbs.com/pdf_manuals/xenon_bulb_info/xenon_bulb_failure_analysis.pdf[/UR L]
[URL=http://www.ltilamps.com/Media/xe_tech_article.pdf]http://www.ltilamps.com/Media/xe_tech_article.pdf

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

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Pat Moore
Master Film Handler

Posts: 363

Registered: Mar 2000

posted 11-13-2001 10:19 AM

Hi;

"Flicker" implies a flashing or a pulsing pattern of light on screen, such as would come from diode problems or something with the shutter. "Arc Waver" will cause a moving pattern of light on screen as the plasma ball searches for a "happy spot" at the cathode tip. Waver would also show up if the flame of the arc is moving around on the anode of the lamp.

I don't think projector speed is your problem -- it would have to be really slow to be noticeable unless there is a LOT of light on the screen, as John notes above.

One other thing to look at: Some of those lamphouses were a bit notorious for allowing the reflector to vibrate in it's holder. This can cause some really funny light patterns on screen. The vibration is usually caused by a bulb seal blower that's dirty and out of balance. Seems like Kneisley also held the reflector in with spring clips to allow the reflector to expand -- are they secure?

Pat

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 11-13-2001 10:37 AM

This could also be caused by too much light hitting the screen. If this is on one of the smaller screen you have there Tom, that could be the problem. New reflector plus new lamp + lots-o-light! If not focused properly, or running at too high of current. Could also be a vibration or rectifier problem too.
Mark @ GTS

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Christopher Seo
Jedi Master Film Handler

Posts: 530
From: Los Angeles, CA
Registered: Jun 99

posted 11-13-2001 10:39 AM

Is it possible that the shutter could also be responsible for a random, wandering-arc-looking flicker? I seem to notice this effect on one of the Century JJs I run. The counter-rotating shutters definitely appear to be running unevenly, as evidenced by the random jumpiness of a dark band which is visible on the forward shutter, and in line with the middle bar of the shutterbox window. This band is steadier on the other JJs.

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 11-13-2001 10:46 AM

Flicker perception definitely increases as screen luminance increases. That is why standard SMPTE 196M specifies a range of 12 to 22 footlamberts for theatres, with an aim of 16 footlamberts. Any higher than 22 footlamberts risks noticeable shutter flicker with a two-blade shutter at 24fps. If you don't know your screen luminance, MEASURE IT!:
http://www.kodak.com/US/en/motion/newsletters/notes/march2000/pytlak.shtml
http://www.kodak.com/US/en/motion/newsletters/notes/june2000/pytlak.shtml

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Mark Gulbrandsen
Resident Trollmaster

Posts: 16657
From: Music City
Registered: Jun 99

posted 11-13-2001 12:02 PM

Tom,
WITH THE AC POWER OFF AND THE MAIN BREAKER ALSO SHUT OFF AND VERIFIED!!!!
Another thing I forgot to mention is to carefully check all AC wiring in the rectifier. Check all of the connection bolts and especially the AC connecting points on both sides of the contactor itself. Kneisley's are famouns for the connections overheating and burning up. Ditto with the contacts in the rectifiers. With a bright flashlight look for discoloration on any of the bolts and contactor terminals.
The lamp will still light and run fine although with a bit lower current than normal and some flicker. The phase may not be completely dead but a bad connection with high resistance will lower the input voltage of that phase a bit causing flicker.
If this is a console ALSO check the main wiring harness and the back of the three phase breaker for burnt up wiring. This is another common thing that happens with these particuluar systems.
Mark @ GTS

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Paul G. Thompson
The Weenie Man

Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000

posted 11-13-2001 03:10 PM

Tom:

I have to agree with John on this one. Sounds like you may be over-powered on screen brightness. Measure it. Your keyword was on Bright scenes....

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Don Sneed
Master Film Handler

Posts: 451
From: Texas City, TX, USA
Registered: Aug 2001

posted 11-13-2001 05:24 PM

It also could be the filter capacitors, I seen this on Cinema Film Systems consoles & ORC II consoles, I would check the filter capacitors after checking diodes....

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Paul G. Thompson
The Weenie Man

Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000

posted 11-13-2001 08:22 PM

Don, you could also be correct, right along with some others. One more thing to look at: Is the power supply Tom is using operating within parameters, or is being used to power too big a bulb?

Paul

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Don Sneed
Master Film Handler

Posts: 451
From: Texas City, TX, USA
Registered: Aug 2001

posted 11-14-2001 12:54 AM

I think my next question to this problem is, what wattage of xenon bulb being use ?? What is the Amp reading on the lamphouse's volt/amp meter ?? Then take it from there....

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Matthew Bailey
Master Film Handler

Posts: 461
From: Port Arthur,TX
Registered: Sep 2000

posted 11-14-2001 05:44 AM

Mr. Pytlak,I believe there is a difference in film speed which is
inches per second & frames per second. I figured out that if 6 is subtracted from 24 the film speed & not the 24 frames per second speed is 18 inches per second in other words 1 foot,6 inches per second.

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