Adjusting CRT TV Grey Scale for Discolouration

As a television CRT ages, the green output of the tube tends to become more prominent. The blue and red output from the tube tend to lose efficiency resulting in dark areas of the television picture taking on a green tint (tinge) or discolouration in the grayscale rendition.

Often a green discolouration or the prominence of green can be quite obvious, particularly affecting dark grey areas of the picture close to black level. In these cases, turning the TV colour control down to zero, fails to remove the green tint to the TV picture. Any grey scale adjustment (grayscale for my American freinds) should be made with the television colour control set to zero.

Dark grey and near black areas of the TV picture are close to what is known as the tube "cut off" or "pinch off" point. In other words, the point at which the CRT phosphors cease to fluoresce and turn off.

The grey scale tracking of a television is adjusted at the factory and accurate setting requires colour temperature monitoring equipment which measures the red, green and blue drive levels to the tube to get pure white. TV's are normally set up to have a colour temperature of 6500K (degrees Kelvin).

TV CRT Cut Off Point Adjustment Procedure

TV grey scale adjustment is done with the CRT fully warmed up (after 30 minutes) and consists of adjusting the cut off levels of the green, red and blue CRT guns from the TV service menu.

Download Test Card G to assist in grey scale alignment by clicking here and set your TV colour control to zero before making any adjustment. We've also prepared a 1024 x 768 image which is invaluable for grey scale tracking adjustments on any TV or computer monitor. Download our grayscale test pattern by clicking here.

To compensate for dark areas of the picture having a coloured tint (discolouration), adjustment is carried out using the TV service mode options which are commonly labelled G-CUT, R-CUT, B-CUT or possibly G Cutoff, R Cutoff, B Cutoff. These service mode options are present on nearly all recent televisions. However, in some cheap portable TV's the tube cut off point may not adjustable in its own right with only a red, green and blue bias adjustment being provided. Unfortunately, the CRT bias adjustment affects white tone colour temperature as well as dark grey areas of the TV picture, often making it impossible to get perfect grey scale rendition.

Computer Monitor Calibration & Gamma Correction

In order to calibrate a computer monitor for digital photography purposes, free monitor alignment tools are available like the one at http://quickgamma.de/indexen.html. These tools help to align computer monitor colour and luminance linearity (distribution) to the optimal 2.2 setting.

Common Faults with Old TV's

In very old TV's the grey scale adjustment may be by variable resistor (potentiometer) on the CRT neck printed circuit board (often being labelled G-CUT, R-CUT and B-CUT). In some cases only two CRT guns have adjustable cut off levels, the third having a fixed setting - in which case adjustment is made to the two adjustable guns for the best grayscale alignment. If the greyscale is too green and only red and blue cut off points are adjustable, then the red and blue gun cut off point will need increasing slightly to compensate for the excess green in the picture - until the green tint just disappears, leaving pure grey.

Refer to our TV servicing page or the Internet for details of how to enter TV service mode. Be aware that there is a risk of damaging your TV or rendering it unusable by incorrect operations or key-presses whilst in service mode. Adjustment should be done by trained TV service engineers and all factory settings should be noted before they are adjusted, so that you have a reference point to work from.

Grey Scale Adjustment Examples

The examples below show a correct grayscale versus a greyscale tinted with colour (showing discolouration).

1. Correct TV Grey Scale Rendition

2. Green tinted TV Grey Scale (turn down Green Cut Off)

3. Magenta tinted Grey Scale (turn up Green Cut Off)

4. Blue tinted TV Grey scale (turn down Blue Cut Off)

Note:

Of course, if your CRT computer monitor is out of alignment these example images may not appear tinted, or the correct grey scale image might show discolouration if your monitor is badly setup! However, the images display correctly on most properly adjusted, high resolution flat screen computer monitors. Use them for guidance only.

CRT Drive Levels

The CRT drive levels are accurately set at the factory and should not need adjustment. If the TV picture white tone is obviously tinted with another colour (such as green) even after adjusting the CRT cut off, then the TV tube drive levels may need adjusting. Always turn down the colour on the TV to zero and re-check white tone on a predominantly white picture (raster) for discolouration first. If the tube drive levels are wrong, obvious discolouration will be present with the TV displaying a black and white picture. However, bear in mind that even on a correctly adjusted TV, the 6500K colour temperature set at the factory can make the white tone appear very slightly pink - this is normal.

If white tone colour temperature needs adjusting and you lack the right test equipment, try placing a known good TV (preferably fairly new) next to the problem TV and turn down the colour to zero on both TV's. In a darkened room compare the white tone colour temperature between the TV's and adjust the red, green or blue drive slightly on the problem TV to get identical white tone consistency between the televisions, carrying out adjustments in a semi-darkened room. Use identical brightness and contrast settings to give similar picture renditions on both TV's.

Always allow the TV tube to warm up for at least 30 minutes before adjustment is carried out.

Always note the factory settings for red, green and blue drive (usually a number or a combination of number and letter on Toshiba TV's) before making any adjustments.

If you don't do this you can quickly get into a terrible mess.

The TV's red, green and blue drive adjustments are accessed from the on-screen service menus and are often labelled G-DRV, R-DRV, B-DRV or R-Drive, G-Drive and B-Drive. In some cases they may be R-BIAS, G-BIAS and B-BIAS. Refer to my TV servicing page for help in accessing TV service mode. If your model is not listed then either consult the Radio and TV servicing handbooks held in the reference section of most public libraries, or search the Internet using the Google form below.

The above procedure is also applicable to the grayscale adjustment of US model TV's.