Film-Tech Forum ARCHIVE: Slight flicker problem

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Posted by Craig Mueller (Member # 4912) on 09-06-2008, 02:54 AM:

Hi to everyone,
I have a problem with a slight/mild flicker in the light output on one of my screens. It is not as severe as I have seen when a leg or diode is out, and the current is where it should be (105 amps for a 3k osram). I have not tested the diodes, but have checked the connections of the diodes, and the contactor as best I could. The equipment is an old christie cc40u rectifier, a christie xenolite lamphouse and simplex 35 projector. The light on screen is not too bright(as compared to brighter less flickery image I've seen elsewhere) and there is little to no ghosting. I do not have any idea on how old the filter capacitors (a bank of four) are and do not know how to test them. I read elsewhere on here that you can wire in another "unit" of similar rating in parallel to test them. Does this mean another single or bank of four capacitors?
I am running on here, but for all that have made it through this post thanks for any advice you can give!
Craig

Posted by Mark Gulbrandsen (Member # 72) on 09-06-2008, 08:46 AM:

The best way to tell what your rectifier is doing is to use a current ripple meter such as the one that was made by ORC a number of years back. High ripple current is not only visible on screen as flicker but it will also shorten lamp life... drastically if it is high. You can also measure current ripple across a calibrated current shunt. Osram used to have one of the high spec current shunts they sent out if there were lots of lamp claims on a given booth ... not sure if they still have this any longer though.

Substituting new capacitors for the old ones would be the prefered way to test the old ones... comparing ripple current with each different bank of caps. Paralleling more caps with the existing ones that are in place will drastically increase inrush current and is also detrimental to the lamp's life.

Lastly, if you have a high AC leg in your building this can also sometimes cause a very slight flicker with some rectifiers.

Hope this helps....

Posted by Andrew Bangs (Member # 4938) on 09-06-2008, 01:43 PM:

If ripple is the problem? How do you fix it or reduce the Ripple? I ask because I have a similar problem.

Posted by Monte L Fullmer (Member # 2797) on 09-06-2008, 04:17 PM:

change out the caps, for if they're getting on the dry side....as what Mark said above...

Posted by Mark Gulbrandsen (Member # 72) on 09-06-2008, 06:11 PM:

Like Monte said... start by changing out the caps. Even hi-temp computer grade caps have at best about a 10 year life. I also forgot to mention in my forst post that badly mis-matched doides can also cause it. Its best to always replace all the diodes at once even when just one fails...

Mark

Posted by Craig Mueller (Member # 4912) on 09-06-2008, 06:54 PM:

Thanks for your fast replies, last time I checked the incoming power, the legs were only a very small fraction off from one another. I am afraid of purchasing a whole new set of four capacitors without knowing for sure that that is the problem as we are running "lean" these days, but it'll surely be definitive. I have had no luck finding proper capacitors on allied or any other site, would you know where would be a good place to look? (I understand if you do not want to divulge). And Mark and Monte, thanks both of you for all the advice you have spilled on this site, many of your other posts have been extremely useful in turning my theater into a pleasant place to see a movie.

Posted by Monte L Fullmer (Member # 2797) on 09-07-2008, 12:26 AM:

quote: Craig Mueller
...many of your other posts have been extremely useful in turning my theater into a pleasant place to see a movie.

That's what all of us "Film-Tech'ers" are here for: - to help those to eventually ensure that there is total customer satisfaction in your theatre with 'perfect presentations', so that you will establish a firm customer base for a successful operation.

If you want to begin to search for elec caps, try motor shops since the majority of large motors have caps attached to the frames. Just record down the uF size and voltage that you need.

Thx anywhos - Monte