Film-Tech Forum ARCHIVE: Vicious Flickering Problem

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Posted by Manny Knowles (Member # 1171) on 11-10-2002, 07:30 AM:

Lamp house is a 2kW Christie CC-CH20 (?) and the projector is a Westar/Century.

Originally, the complaint was that the lamp had failed. It was lit and a show was running. The failure occurred in the middle of the presentation.

That lamp was only installed in April or May of this year, so this all seemed very strange to me.

I replaced the lamp with the same model (PerkinElmer 2000HTP). The "pointy" electrode of the spent bulb looked like it had bits missing; not the usual pattern of erosion that I see when I pull old bulbs out of these machines.

The new lamp strikes up immediately but there is a very noticeable flicker. The quality of the light is a nice, bright white, but the flicker is out of control.

We've had problems in the past where one of the three phases drops out. Could this be the cause of this flickering (and the premature lamp failure of the other bulb)?

Posted by John Walsh (Member # 168) on 11-10-2002, 08:08 AM:

I think you would have seen other problems if one other phase had gone out.

Try to do a ripple test; also check the ignitor.

Posted by Michael Gonzalez (Member # 593) on 11-10-2002, 08:55 AM:

Remove all the diodes and test each one. Replace any bad ones. In my experience (though somewhat limited), a flicker is either caused by an old or bad xennon bulb or a bad diode.

Posted by Ken McFall (Member # 845) on 11-10-2002, 09:03 AM:

Also check your ventilation. If the lamp is getting too hot it will cause it to flicker or wander very noticably.

The end caps will change colour if they are getting too hot, starting off as a light straw and then blue depending on how hot they are getting.

You may have an extract or supply fan on its way out.

Posted by Gordon McLeod (Member # 33) on 11-10-2002, 09:38 AM:

Usually a lamp will flicker if it has too much airflow around it and is being overcooled.
Diodes are either dead or working they don't change state back and forth so all lamps would flicker

Check the contactor and make sure that it's contact areas are clean so that all 3 phases are getting through it they can become intermitent. Also if there is any current selector switch check them as well for corrosion and pitting

The last item is check the condition of the filter capacitors.
The y can get oxidised connections and leads that effectivly take them off line when the wire gets hot

Posted by Aaron Sisemore (Member # 145) on 11-10-2002, 02:16 PM:

If it is a steady 12Hz flicker, then as suggested before, either one phase is gone or you have one or more bad diodes. (another indicator of the above trouble would be the current will be about 1/2 to 2/3 of normal)

-Aaron

Posted by Jack Ondracek (Member # 1466) on 11-10-2002, 02:30 PM:

Paraphrasing Gordon & Aaron, to a point...

You can probably eliminate (or confirm) diodes as the problem by starting with your current meter. If it's running at its normal level, diodes are probably not the culprit & you can look elsewhere.

If you have low current, or if you notice the flicker on the screen but not in the light behind the projector, then it's probably a phase loss, which can be anything from the incoming power line, through all of Gordon's list.

While you're checking, be sure to look at the secondary lines from the power transformer & all subsequent connections. These high-current connections can work loose (or burn off), and aren't always easy to see. However, a burned connection will cause the same problem as would an open diode, bad contactor, or missing primary leg.

If you have a power company problem, you should be noticing other dead equipment in the building... unless you have a 3-phase service feeding only that lamp & you were fortunate not to have one of the other 2 legs go out.... but if you have to check your breaker, this would be easy to confirm, one way or the other.

Posted by Paul G. Thompson (Member # 655) on 11-10-2002, 03:28 PM:

If it is a "strobing flicker", then I agree with all of the above. If it is a random flicker, then something else is wrong.

If a doide went south or you lost a phase, the strobing flicker will be accompanied with a bulb current much lower than normal.

What does it look like with the projector not running, shutter open, CO douser open, and hand douser open? If the flicker is still there, it is probably caused by an unstable arc. It really won't be a flicker, just the appearance of the light wandering around on the screen.

Posted by John Pytlak (Member # 331) on 11-11-2002, 09:00 AM:

One way to safely measure excessive current ripple is to let some of the leaking light from the lamphouse shine on a silicon photocell loaded with about a 470 ohm resistor, and look at the output using a scope. If you're missing a leg of the three-phase, it will be obvious on the waveform of the cell's output. Bad filtration will simply show up as more than a few percent of AC ripple on top of the DC steady state photocell output.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Research Labs, Building 69, Room 7525A
Rochester, New York, 14650-1922 USA
Tel: +1 585 477 5325 Cell: +1 585 781 4036 Fax: +1 585 722 7243
e-mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion

Posted by Ken McFall (Member # 845) on 11-11-2002, 11:36 AM:

I have to admit I didn't read the post correctly or was enjoying my friends Mr. G & Mr. T.... my post refered to a severe wander of the flame all over the place, which can be caused by overheating. I've seen it twice.

My mistake, sorry guys.

Posted by Manny Knowles (Member # 1171) on 11-12-2002, 05:15 PM:

Michael wrote:

quote:
Remove all the diodes and test each one. Replace any bad ones. In my experience (though somewhat limited), a flicker is either caused by an old or bad xenon bulb or a bad diode

This is exactly what our service company recommended over the phone. There were several bad diodes -- definitely 2 are bad, possibly 4 (out of 6) Thanks for the help.

Below are notes on some of the other suggestions...

Aaron wrote:

quote:
If it is a steady 12Hz flicker, then as suggested before, either one phase is gone or you have one or more bad diodes. (another indicator of the above trouble would be the current will be about 1/2 to 2/3 of normal)

Yes - 12Hz flicker rate.
Yes - Low current (was 80A; dropped below 60A)

Paul and Jack also cued me to check the current on the bulb. Thanks to all for helping.

By the way...it's still not fixed...I'm waiting for the diodes to come in from Miami. I'll follow up as to whether the problem goes away.

How come one of my meters can't give me a reading on the forward diodes? The one from Radio Shack can read them all, but the Fluke one can only test the reverse ones.

Posted by Paul G. Thompson (Member # 655) on 11-12-2002, 07:01 PM:

Manny, the Fluke does read them both way. If the meter says "OL" in one direction, and .XXX in the other direction, the diode is just fine.

Posted by Manny Knowles (Member # 1171) on 11-12-2002, 07:23 PM:

But it says "0L" in both directions of all the forward diodes (even the brand new ones).

Posted by Joe Redifer (Member # 3) on 11-12-2002, 07:39 PM:

What do you have your meter set to? AC? DC? Ohms? Continuity test?

Posted by Steve Guttag (Member # 268) on 11-12-2002, 07:47 PM:

Or even more properly...if it is so equipped..."diode"

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Manny Knowles (Member # 1171) on 11-12-2002, 08:26 PM:

Continuity. The icon looks like an arrow pointing to a perpendicular line.

Posted by Paul G. Thompson (Member # 655) on 11-12-2002, 10:46 PM:

Something is goofy here. The meter is not supposed to know the difference between a forward or reverse diode. Just switch the leads around.

A continuity tester is not the greatest thing to use for checking diodes. Use the diode checker facility of your Fluke.

Posted by Manny Knowles (Member # 1171) on 11-13-2002, 11:43 AM:

Actually, now that I think about it, I probably was using the diode feature...don't people confuse that with "continuity" a lot?

Might be a good idea if someone could take a photo of the "diode" graphic so we can all be clear on what it looks like..? Pleeeease???

Here's the good news: Only two diodes were faulty (not 5) but I changed all six of them anyway and I kept the four "good" ones as emergency spares since there's quite often a shortage around these parts.

The lamp is working fine now. Thanks again to all for helping!

~Manny (still a fat-free snack!)