Film-Tech Forum ARCHIVE: Flicker on an older simplex

This is topic Flicker on an older simplex in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=002800

Posted by Tom Kroening (Member # 201) on 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

Posted by Brad Miller (Member # 2) on 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.

Posted by Tom Kroening (Member # 201) on 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?

Posted by Brad Miller (Member # 2) on 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.

Posted by John Pytlak (Member # 331) on 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

Posted by Pat Moore (Member # 380) on 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

Posted by Mark Gulbrandsen (Member # 72) on 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

Posted by Christopher Seo (Member # 7) on 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.

Posted by John Pytlak (Member # 331) on 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

Posted by Mark Gulbrandsen (Member # 72) on 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

Posted by Paul G. Thompson (Member # 655) on 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....

Posted by Don Sneed (Member # 955) on 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....

Posted by Paul G. Thompson (Member # 655) on 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

Posted by Don Sneed (Member # 955) on 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....

Posted by Matthew Bailey (Member # 601) on 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.

Posted by Tom Kroening (Member # 201) on 11-14-2001, 07:31 AM:

Well i did have a bit of time tonight but not much. I visibly checked the arc itself and it looks very stable. The reflector is on nice and tight. I'd think that it would be very noticable if a film runs slower in analog, but this house has dobly digital (would it even work at less then 24 fps?). This is on the biggest screen in the building. I understand what you mean about the screen being too bright, but even with the old bulb it produced a noticable flicker. I almost had the diode assembly out but someone took my 7/16th wrench so i couldn't disconnect some of the cables. What kind of precautions should I take when attempting to swap out the diode assembly with another power supply? I did notice a spark when i was putting it all back together even though the breakers to the entire room were flipped off. This machine uses an older kniesley rectifier and a 2000 watt lamp. Before i left i checked to make sure everything worked (and of course it did!) but i also opened the douser for a second while the motor was off and did not notice any flickering or instability of the white light whatsoever. Of course this doesnt mean that its NOT a diode, i guess it could just be indetectable without the shutter having the "strobe light" effect to make the flickering visible to the naked eye.

Posted by John Pytlak (Member # 331) on 11-14-2001, 07:56 AM:

Matthew Bailey said: "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."

To be precise, standard SMPTE 40 specifies: "The recordings shall be made so that the audio records will reproduce properly at 96 perforations (18 in [457mm]) per second, which is 24 frames per second".

Most people use 90 feet per minute or 18 inches per second as the nominal transport speed of 35mm prints during projection. But if the distance between perforations is exactly the aim of 0.1870 inches, the actual transport speed for 24 fps is 89.76 feet per minute or 17.952 inches per second.

Posted by Gordon McLeod (Member # 33) on 11-14-2001, 08:57 AM:

The spark you saw is the filter capacitor discharging
They can hold a charge for a very long time. Especially since most Kniesley Rectifiers didn't have a bleeder resisotr across them.
Before servicing any rectifier after locking out the feed to it and to the lamphouse (some lamphouses provide a seperate 120vac back to the rectifiers contactor) Is to take a heavy wire and short out the capacitor.
Since it appears to be storing a charge I think it can be ruled out as the culprit.
If you swap the stack make sure it is with one from a rectifier of the same size. ALSO MAKE SURE THAT THE D.C. OUTPUT IS RECONNECTED WITH THE CORRECT POLARITY OR THERE WILL BE A BIG BANG AS THE CAP BLOWS AND THEN THE LAMP WILL ALSO BE TOAST

Posted by John Pytlak (Member # 331) on 11-14-2001, 09:33 AM:

Gordon said: "...take a heavy wire and short out the capacitor..."

Is there any risk of damaging the capacitor by discharging it through a direct short? Perhaps it would be preferable to discharge it through a large power resistor, or even through a 100-watt tungsten lamp, to more slowly dissipate the stored energy?

Nonetheless, that capacitor could pack a mighty "wallop" if the current goes through your body!

Posted by John Schulien (Member # 215) on 11-14-2001, 01:03 PM:

The sparking was because the circuit had an undischarged capacitor. Whenever you service a power supply, you should make sure that any large capacitors are discharged before you start working on the circuit. Otherwise you can shock yourself, or damage other parts of the power supply while replacing components.

Large capacitors are usually cylindrical, like a soda can. They'll be well marked with values in microfarads and volts, such as "20,000 uF 25 V." In a typical power supply, the power transformer is connected to the diodes, which are in turn connected to the capacitors.

The correct way to discharge a capacitor is by using a resistor, NOT by shorting across the capacitor. When you short across the capacitor, not only will you create a spark, but you run a good chance of damaging the capacitor or another component. You can actually melt the tip of a screwdriver by using it to (intentionally or accidently) short out a large capacitor.

Safely discharging a capacitor through a resistor is easy and convenient. I use a 10K ohm 2 watt resistor with alligator clips soldered to each lead. I clip the resistor across the leads of my voltmeter, then use the voltmeter, set to measure DC voltage, to measure the voltage across any large capacitors in the circuit. When I encounter an undischarged capacitor, the voltage on the voltmeter shoots up, then quickly drops off to under one volt as the capacitor discharges.

In a power supply circuit with multiple capacitors, the smoothing capacitors usually wired together, so discharging one will usually discharge them all.

NOTE: Be especially careful when servicing tube circuits, as they utilize MUCH higher voltages than transistor circuits, and are much more dangerous. In a typical tube power supply, the power transformer will be directly connected to one tube, which will in turn be connected to the capacitor(s). When checking a capacitor in a tube power supply, use the voltmeter alone to check the voltage first. If the voltage is over 100 volts, use a 100K ohm capacitor instead of a 10K ohm capacitor.

Power supplies are supposed to contain a protective component called a "bleeder resistor" that essentially acts as a small power leak, so that when you turn the equipment off, any stored energy is bled off over the course of a minute or two. However, if the bleeder resistor fails, or is not present, the power supply can still contain dangerous voltages even after being powered down for weeks. This is especially true of tube circuits.

Also, as John Pytlak suggested, a light bulb would work just as well in place of the resistor. And yes, they are dangerous. Power supply capacitors are designed to store large amounts of energy, and release it very quickly. You don't want to complete the circuit with your body. That would be unfortunate.

Posted by Gordon McLeod (Member # 33) on 11-14-2001, 09:24 PM:

Large Filter caps have no problem being discharged into a short as in fact the impedance of a xenon lamp is virtually 0 anyway.

Posted by Dave Macaulay (Member # 813) on 11-14-2001, 10:30 PM:

If you have no lens in the projector, you can open the dowser with the motor stopped and the shutter open for a while without melting anything - certainly long enough to see if there is a flicker. Not with a lens in though, there is no way to predict how long a lens would survive - beyond "probably not very long". I would also remove the aperture plate - it's rather thin metal and probably in the hottest part of the beam. Then start the motor and see what it looks like.
If there is no flicker in the light pattern on screen but there is flicker with the motor running, I think the problem must be in the projector not the lamphouse. I know you have checked the shutter but it sounds like there's a problem there. If you see no flicker in the arc, and there isn't flicker with the motor stopped, then there really isn't anything except the shutter that can be causing the flicker. Obviously slow running speed would, but like you say the digital sound is working and that has a pretty narrow speed tolerance. Same with super-bright picture - but you say it flickers with a dim image.

The most common cause is a hole burned right through a shutter blade by someone opening the dowser and changeover with the motor off. This can happen if a motor, belt, or gear fails and there is no film motion failsafe. It should be rather easy to see a holed shutter though...
Anyway, good luck

Posted by Paul G. Thompson (Member # 655) on 11-14-2001, 10:40 PM:

I never heard of any capacitors that were damaged due to shorting them out to discharge them. Nor, have I damaged any, except for a small pock mark on the screw terminal from the initial arc.

Standard safety equipment for discharging capacitors in the broadcasting industry is with a grounding probe suppled and physically attached to the transmitter main frame. (so it does not get lost) It is nothing more than a grounded metal rod with a wooden handle. This probe has to be hung up on a special interlock switch so the switch closes after the probe has been used to discharge capacitors.

When the doors and panels are replaced after servicing, all these switches on the doors, panels, HV probes, etc. will complete the safety interlock so the transmitter can be fired up.

The discharge probes are similar to what we used with some RADAR sets I serviced while in the navy.

Screwdrivers or alligator clips were used for the boob tubes you have sittin' in your front rooms that you use to watch the cheap VHS movies you rented from Blocksucker Video.

In solid state transmitters, many of the capacitors are even larger in electrical value than what you find in the xenon power supply. The discharge probes do not have any series resistance in them whatsoever. They were just a "dead short".

Oh, by the way - how long is a short circuit?

Paul

Posted by John Pytlak (Member # 331) on 11-15-2001, 06:54 AM:

Dave said: "If you see no flicker in the arc, and there isn't flicker with the motor stopped, then there really isn't anything except the shutter that can be causing the flicker."

If there is excessive current ripple from the power supply, you won't necessarily see the 60Hz flicker in the arc with the motor stopped, but the 12Hz flicker "beat frequency" between the running shutter and current ripple WILL be obvious. So I wouldn't rule out excessive current ripple just because it can't be seen without the shutter running.

Posted by Matthew Bailey (Member # 601) on 11-15-2001, 06:00 PM:

I have a couple of large wirewound ceramic tube resistors
I salvaged from an old elevator controller. They are of
a high enough wattage & probably of several megohms
& able to discharge large electrolytic capacitors.
These resistors have brackets to mount them on the inside
of the elevator controller cabinet. If you can look up elevators
in the phone directory such as under Otis Elevator company
for example & ask if they have such mentioned resistors
that are used on their elevator controllers & tell them what you need the resistors for.

Posted by John Pytlak (Member # 331) on 11-15-2001, 08:00 PM:

Matthew: "Several megohms" would take an awful long time to discharge a power supply filter capacitor. Wirewound resistors used in power circuits would typically be much lower resistance. If the resistance is not written or coded on the device, use an ohmmeter to measure it.

Posted by Will Kutler (Member # 770) on 11-15-2001, 09:18 PM:

ALWAYS BE CAREFUL ABOUT HOW YOU DISCHARGE A CAP AND FOLLOW RECOMMENDED SAFETY PROCEDURES!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Some of those cap really store a lot of energy and can seriously injure you and possibly kill you not to mention damaging other components and maybe cause a fire!

Some of those caps can let off a nice explosion when discharged--we did that for fun in college - but they can also weld and melt a screwdriver across the contacts!

Posted by Phil Hill (Member # 371) on 11-16-2001, 02:39 AM:

Gosh! Yous! I'm "shocked" to read some of these posts.

A xenon lamp is NOT a short or zero ohms (what? impedance? ...AC?) across the power supply. If it were, there would be no light (or heat) out. A typical 7kw lamp runs about 165 amps at about 42 volts. In order to develop the 42 volts across the lamp, it must have resistance. Thus the power supply "sees" a load around 0.25 ohms. A 15kW lamp would see less than 0.1 ohms at 38 volts and 400 amps. These are small but very important "real" numbers and why humongus size, low-resistance wire must be used for the DC to the lamp.

The "Shorting Stick" or automatic shorting bar that is dropped across the HV caps of high-power transmitters or similar high-energy equipment is not for the normal discharge of the capacitor's stored energy. But rather it is assumed that the caps are already discharged. The stick and other devices are exclusively for safety and extra security for the technician when working on the equipment in the event the normal bleeder resistor or circuit fails. It is ALWAYS prudent to short out big caps before working on high-power equipment. Some caps will hold their high-voltage energy for days and longer...including xenon switching power supplies and DC ignitors. Some caps will even regain a charge after being shorted out. And, yes, the equipment can be damaged by a direct short across the cap. Most high-energy capacitors have a safety blow-out plug that will safely vent the internal gases/liquids in the event of excess heat or pressure build up as from a short.

>>> Phil

Posted by John Anastasio (Member # 607) on 11-16-2001, 05:21 AM:

Well geez, there's not much difference between the humongous wire that connects to the lamp and the screwdriver in my toolkit! By the time you take into account the resistance from all the crud on the blade, you've probably got more on your phillips head than on that lamp. The blowout plugs are there to protect you from a catasrophic explosion in the event of an INTERNAL short in the cap, which leads to a lot of steam. When a power supply is delivering to a large load, those capacitors are charging and discharging 60 times a second! (although admittedly not completely) It's what they're designed to do. Discharging them only once isn't going to make one explode or destroy the dialectric. You can't make them explode by removing energy from the system, but you sure can by putting it in the wrong place....like reversing the polarity and boiling off the dialectric or sticking in enough EMF to arc across the dialectric, causing a "short" internally by punching a hole in it. Believe me, my students do it on a regular basis. They get a big kick out of seeing a little 100mfd 25v cap flying up into the gridwork with a loud "pop". The only "pop" I can ever remember causing was the feed-through capacitor I supernovad on a 50,000 watt FM transmitter one night by overloading it. I thought a bomb had gone off.

Posted by John Pytlak (Member # 331) on 11-16-2001, 10:29 AM:

Phil: I totally agree!

I'm still of the opinion that it is wiser to discharge the capacitors through a resistive load than a direct short. I agree, once the caps are discharged, a shorting jumper is a good idea for operator safety. A "Lockout-Tagout" policy on turning off all power must be followed too.

Posted by Gordon McLeod (Member # 33) on 11-16-2001, 11:00 AM:

According to Christies publication from the late 60's on design of xenon powersupplies They state that since the lamps represents a almost dead short to the powersupply especially during ignition it is nesasarry to design sufficient reactance into the supply to limit current.
During the ignition the filter cap is effecively discharged into the lamp heating the electrodes and establising the current flow through the arc established by the HV ignitor

Posted by Tom Kroening (Member # 201) on 11-16-2001, 08:42 PM:

Well I found replacement parts for the power supply. I guess i'll find out tonight whether it works or not. Ill just shut it off for half an hour and short out the caps after to get rid of any charge. Ill let you guys know how it turns out! Thanks for the advice.