Analogue gamma adjustment

[Rikrok]

I'm not sure if this is the appropriate place to ask but...

I have a screen (WG D9400) where the the midtones (or really anything darker than white) are too dark. I have played with all the screen menu adjustments to no avail. I fed it a signal via an Extron device that has a peak control that can boost the level of the input, and while it does help to lighten the image overall I'm also sending whites past 0.7V which might not be good for the circuitry and it makes the whites on an image bleed blue to the right.

What I'd like is to have a gamma control to lift those midtones up. My searches for analogue colour correction, rgb colour adjust, component colour grading etc. give me nothing, so either I don't know the terminology or such a device doesn't exist.

Does anyone know of such a thing, or if it's not super complicated, how I could make one? I'm comfortable with soldering and basic circuitry.

[old_tv_nut]

What exactly is a "screen" WG D9400? What is an "Extron device?" What signal are you driving it with, a legitimate video program signal or something else?
If this is an analog monitor, its gamma will be determined by the picture tube gun characteristics and should be correct - no adjustment available.

[old_tv_nut]

I looked up what it is - CRT monitor.

are the brightness control and CRT bias (G2) controls set correctly? Is it clipping blacks and therefore suppressing dark colors?

[ARC Tech-109]

I'm running into the same problem with my Panasonic TAU series widescreen CRT set (CT34WX54), I'm coming to the conclusion that the "digital" gamma is linear now with the HiDef movement. I've spent hours playing with the various settings regarding the gun drives and bias with no improvements to the mid-tones yet everything looks perfect when I drive the set with my vintage Sony DXC-M3a camera using plain old composite.

[old_tv_nut]

Asking Rikrok once more for details please - is the input from a game, a computer, a video source? Is it a digital input or an analog input?

[ARC Tech-109]

doubled post on refresh, please delete this one

[ARC Tech-109]

Found this poking around, I'm having the same issue with my panasonic TAU series 34" and came to the conclusion it's not the incoming signal gamma. Reason it looked "perfect" driving it with my camera was a lack of 75 ohm termination, forgot I pulled the resistors on the input board.

Here's the what I found.
"The amount of electrons generated by the cathodes is related to their surface area. A cathode with more surface area creates more electrons, in a larger electron cloud, which makes focusing the electron cloud into an electron beam more difficult. Normally, only a part of the cathode emits electrons unless the CRT displays images with parts that are at full image brightness; only the parts at full brightness cause all of the cathode to emit electrons. The area of the cathode that emits electrons grows from the center outwards as brightness increases, so cathode wear may be uneven. When only the center of the cathode is worn, the CRT may light brightly those parts of images that have full image brightness but not show darker parts of images at all, in such a case the CRT displays a poor gamma characteristic."

Doubt I'm going to find another pix tube for this monster or another someone to help carry it.

[old_tv_nut]

When a CRT gets that worn, the image is often described as "milky." I suppose this comes from turning up the "brightness" (DC bias) to get some light in the shadows, while there is little reserve current left for the highlights.

[ARC Tech-109]

Saturation point. I recall seeing this on weak tubes where the capacitance between elements would hold enough voltage to "lag" or smear the highlights to the right when G2 was pushed too high during the peaks.
As a young teen I got this early 70's Panasonic "Quintracolor" modular set that had no red gun. After several hits of my uncles 467 it began to show life but never really recovered, I was always looking for the "why" answers to the point of becoming an obsession. It also had a bad power supply module and being 12-13 at the time my understanding of the SCR regulator was all but non-existent and I ended up with a 13" hybrid Panasonic "swivel" set. Decades later the CRT is still an obsession and I find myself spending more keeping the tubes alive than the cost of a high end plasma or LCD based flatscreen.

[old_tv_nut]

The smear to the right is not because of gun capacitance. It occurs in sets where the video output transistors drive the cathodes, and if the transistor is driven into saturation it then develops stored charge which has to be discharged before it comes out of saturation and the collector voltage can rise.