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Weird color cast when viewing monitor set to 6500K

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Nader Mroueh
Weird color cast when viewing monitor set to 6500K
on Dec 8, 2009 at 9:26:29 am

Hello everyone,

I am experiencing a weird problem with a broadcast monitor (Panasonic BT-H1450Y, about 7 years-old).

When I set the monitor to 6500K and feed it a grayscale, there is a slight color cast (dark magenta), even when the chroma is turned all the way down. Setting the monitor to 9300K however produces a grayscale that looks neutral to my eyes.

I have searched the forum and I did find a post about someone's monitor producing a grayscale "tinted on the amber side" when set to 6500K but that's about it. So, is a grayscale supposed to look noticeably different when viewing it on a monitor set to 6500K vs 9300K?

My second question is, does anyone know what the "CRT DRIVE" and "LOW LIGHT" knobs are for?

Thank you.

-Nader


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Fred Jodry
Re: Weird color cast when viewing monitor set to 6500K
on Dec 8, 2009 at 10:06:12 pm

Bingo. About twice a year, you set the lowlight or background potentiometers to a neutral dark grey representing black with the color knob normal and colorless black video coming in from the source. You set the highlight drive potentiometers or rheostats to an even white with full white video(-One of these adjustments usually minimized and the contrast control on the front pushing pretty high). When accessing "color temperature" adjustments on a monitor, it`s usually best to set the above adjustments on each "temperature" type then settle for the one that lacks grayscale problems the most. Monitors are not supposed to display an out of neutral grayscale in production as receivers are meant to be neutral too. (In other words, discard the discardable alternate temperatures). According to the NTSC, cameras may divert from reference white for artistic reasons but monitors add and subtract each primary display color from luminance gray in composite uses. [Cameras diversion follows]. Real white is around 4500 degrees Kelvin and you can balance your camera to it on an average outdoor scene by stepping in the open area and feeding a reflection off of a styrofoam sheet that`s pouring 4/5 blue skylight and 1/5 yellow sun. If you have a very directional white balance place marked in your viewfinder, try aiming it at the DARK TOP of a cloud in the sky. These are my own average daylight suggestions. A flashlight through a mild blue filter is a common daylight setting for a camera, and no filter on the flashlight for studio tungsten. My RCA TK-76 camera has an easy white balance. I turn the contrast pretty low, the brightness pretty high, then unscrew the viewfinder knob so I can white balance the camera in it`s own viewfinder. Ikegami hotshoe cameras can`t do this. I`ll get off this neutrally set color cameras diversion to get back on monitors. Monitors also have 3 (or only one) screen voltage adjustments. The lowlight, maybe -36 volts (variable!) grid voltage sets "black" or dim level. the screens (+75 volts for a week tube, +450 volts for a strong tube) set middle slightly low gray. The drive controls set plain peak or almost peak white, and a CRT rejuvenator followed by 5 minutes of running recovery sets the overwhites maximums the cathodes of each color will do. Unfortunately, tweaking all these controls means that the focus voltage will have drifted a bit, needing it, and the convergence adjustments to have a going over. If a focus adjustment knocks the colors a bit,you only have to touch the screen voltages to get back on track. My monitor is doing the "opposite" of a crt rejuvenator at present. A rheostat dims the cathodes almost a volt because these electron guns (Gross Bertha Howitzers) are presently shelling the other side to dust. Someday I`ll have to brighten them up to normal. Ikegami monitors have the screens and grids labelled incorrectly as each other, both on the monitor and in the manual. Sony monitors often have predistortion amplifiers that set grayscales and should never be tossed on a crt rejuvenator. Their pathetic cathodes seem to use an emitting layer on metalloid instead of thick kaolin and will immediately bridge to the grid without any advantages. You compensate for their weak guns by getting out the soldering iron and making their screen voltages adjustable to ridiculously low voltages positive of the cathode when they go weak. Did I cover everything? Fred


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Nader Mroueh
Re: Weird color cast when viewing monitor set to 6500K
on Dec 9, 2009 at 9:57:06 pm

Hi Fred, thanks for shedding some light on this matter, I really appreciate it. I gave your suggestions a try but the color cast will just not go away. I'm guessing the phosphors are worn out.


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Fred Jodry
Re: Weird color cast when viewing monitor set to 6500K
on Dec 10, 2009 at 2:20:19 pm

Nader, you described a display that goes purple in the lowlights and stays plain white or bloomed slightly greenish in the highlights. Just take your monitor to the TV repairman in your town. He`ll know how to adjust lowlight grid bias, screen voltages, highlights drive, and how to hit the weak gun with the CRT rejuvenator the same as I do. In fact, if the weak gun is spiralling down, he can over- do it on the CRT rejuvenator on the other two guns to weaken them into balance and you may still get years more use out of the display. The one thing a lot of TV fixers don`t know though is that although you should be experimental, most CRTs need for the screen voltages to be lowered and the grid voltages to be raised as the tube ages. Naturally, watch for a computer monitor to hit the curb with the same type of CRT as yours and don`t throw out the old one until you know better. The problem might be a chassis voltage drifting instead of a bad jug.


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Nader Mroueh
Re: Weird color cast when viewing monitor set to 6500K
on Dec 12, 2009 at 1:08:50 pm

Hi Fred, not being one to give up easily, I tried again "fiddling" with the crt drive and low light and surprise surprise, I actually managed to get rid of the color cast this time! I also checked the monitor against another one I have, everything seems to be properly adjusted. I will still take it to a professional to have it checked out. Thanks a lot for your help. I owe you one!


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