How to repair the LCD TVs, No video, No backlight, Line fault, Distorted video, Improper video level, White Balance adjustments
No Video, No Back-light
Video abnormalities can be anything from no video at all, missing lines or sections, unwanted pixel lighting, and severe to subtle distortions.
Video abnormalities can be anything from no video at all, missing lines or sections, unwanted pixel lighting, and severe to subtle distortions.
The different scenarios that might be encountered will be classified and practical approaches applied.
A true “no video” condition assumes that none of the various inputs or tuner sources are displaying a picture. As mentioned earlier, it is important to immediately isolate the cause and determine if it lies within the video process or the panel control circuits. All Sony televisions generate what is known as “On Screen Display” graphics (OSD). Any display of on-screen graphics (such as channel, video input numbers, or customer setup menus) immediately disqualifies the panel and the drive circuits as the cause. On-screen graphics can be a powerful troubleshooting tool but its use is unique to the design of the unit. A flowchart specific c to that chassis/model should be used.
All LCD televisions, whether a direct-view or projection type, require a light source be present to pass through the LCD crystals and out to the viewer. In direct-view units the current choice is multiple fluorescent tube lamps whose light output is spread by a diffuser panel. Projection units utilize a high-intensity lamp. In Sony LCD projection units, the lamp light is split into red, green, and blue light components. These light components are sent to individual LCD panels for pixel control and recombined for projection to a screen.
Since current LCD technology is unable to completely block back-lighting, a small amount of light passes through the LCD crystals and can be seen
as a dark gray raster. Ambient room lighting will determine how easy this is to see. In most cases it is easier to watch the screen as the unit is being turned off to see if the raster becomes slightly darker.
All Sony LCD televisions contain protection circuits to monitor the circuits driving the back-light lamps.
If the ballast control circuits or the lamp(s) fail, the unit will usually shut down and display a diagnostics indication. Verification of back-lighting should always be the first step in isolating a “no video” condition regardless of the presence of protection circuits.
No Video with Back lighting
If back-lighting is confirmed, the next step is to observe for the presence of any on-screen display (OSD) graphics. These are generated by the main microprocessor or video process circuits. The presence of OSD is a clear indicator that the LCD panel and its associated drive circuits are functioning. The focus of attention is now in the video input and process circuits.
Distorted Video
Distortions in the video can be a difficult challenge since it can manifest itself in many ways. Fortunately, many of the distortion issues that are caused by the panel control and driver circuits are unique and usually easy to identify. Distortions can be classified into the following groups:
The unlit or fully lit rows or columns of pixels
Digital distortion across the screen
Improper video level
Dark or colored spots on the screen.
The unlit or fully lit rows or columns of pixels
Digital distortion across the screen
Improper video level
Dark or colored spots on the screen.
Rows or Columns of Pixels Lit or Unlit
Fully lit rows or columns of pixels are more common in LCD panels than ones that are not lit at all. The horizontal and vertical address lines are controlled by drive circuits that are linked to the panel via flexible PC cables. These cables are bonded to the outer edge of the panel. If the bond is lost at one or more of the lines, the control voltage will be lost.
This allows the liquid crystals to twist to their normal position and allow full light to pass through. Since an entire line is affected all of the red, green, or blue pixels will light.
This is a clear indication of a panel failure since very expensive and specialized equipment is needed to repair the bond.
This is a clear indication of a panel failure since very expensive and specialized equipment is needed to repair the bond.
If the unit is under warranty will need to have the panel replaced. If it is out of warranty, owner have to buy and replace it.
The entire unit be exchanged for direct-view models. On larger LCD panels, the replacement of the defective panel in the field.
Digital Distortion Entire Screen
Unless the LCD panel has been damaged in any way, this type of distortion is usually caused by the process circuits for the video signal. The proper step is to isolate the cause to a particular board. The two key circuits for processing video are the initial circuits to perform analog to digital conversion, and another to scale the incoming video data to the panel resolution and allocate that information to the proper pixels. Distortions caused by digital process circuits are unique and, in most cases, easy to identify as to the source. Random points of pixel lighting and loss of detail in the displayed image are examples of distortion caused by the initial video process stages. If this happens, using the OSD functions of the unit can help to verify this. OSD is usually inserted near the end of the digital processing, before it enters the panel scaling circuits.
The figure below illustrates an example of a digital process failure in the front end of the process circuits. Note how the OSD is unaffected. The OSD has made it clear that there is nothing wrong with the panel or the scaling and drive circuits.
Distortions caused by the scaling and panel drive circuits usually generate symmetrical patterns. Erroneous highlight and black level can also occur here since gamma correction is performed at this stage.
Improper Video Level
Video level issues can affect one or all of the primary colors. The panel scaling and control circuits are reliant on the initial video process stages to properly reproduce brightness and contrast levels. If an overall picture level problem occurs with white balance appearing normal, suspect the failure in the initial video stages. It is not likely that a failure in the gamma correction or LCD drive circuits will affect all three colors.
In situations where one color is at a level so as to affect white balance, the problem can be in the initial video stages or in the panel. White balance shifts on direct-LCD units are not common. If the entire range of a particular color has increased or decreased suspect a problem in the initial video stages since this is where these adjustments are located and stored on to non-volatile memory. Gamma shifts or failures usually cause white balance problems at the extreme low or highlight areas of a particular color.
White balance issues with a projection LCD unit are approached in an entirely different manner. Since 3 separate LCD panels are used, it is possible for white balance issues to occur that is not electrical in nature. If drive to one LCD panel were to fail, the symptom would be an extreme white balance shift towards the particular color of the panel that lost drive.
Likewise, damage, dust or aging of polarizing filters could cause a drop in one or more of the primary colors.
White Balance
White balance adjustments are provided to vary the output level of the red, green, and blue LCD panels to achieve proper gray-scale of the displayed image. In a direct-view LCD television, the level of each red, green, and blue pixels are varied.
White balance adjustments are provided to vary the output level of the red, green, and blue LCD panels to achieve proper gray-scale of the displayed image. In a direct-view LCD television, the level of each red, green, and blue pixels are varied.
Most Sony televisions have more than one white balance setting. Three are most common. They are: Cool, Neutral, and Warm.
Neutral is a “true” white balance. If a test pattern were to be displayed using a “stair-step” pattern from full white to black, all of the brightness levels of the scale would be true black, white and gray. The “Cool” setting adds a small amount of blue to give the picture a “hot” look. “Warm” contains a small amount of red to soften the intensity of the picture. Adjusting of white balance is only required if the unit has had a board replacement in which the circuits controlling the balance are located. Other situations where white balance will require adjustment include aging of the unit through time, or when someone else has changed the settings.
Neutral is a “true” white balance. If a test pattern were to be displayed using a “stair-step” pattern from full white to black, all of the brightness levels of the scale would be true black, white and gray. The “Cool” setting adds a small amount of blue to give the picture a “hot” look. “Warm” contains a small amount of red to soften the intensity of the picture. Adjusting of white balance is only required if the unit has had a board replacement in which the circuits controlling the balance are located. Other situations where white balance will require adjustment include aging of the unit through time, or when someone else has changed the settings.
Adjusting the White Balance
The proper procedure for adjusting white balance is covered in the service manual for each model. A color analyzer is required to properly set the X and Y values of each of the color temperature settings.
The proper procedure for adjusting white balance is covered in the service manual for each model. A color analyzer is required to properly set the X and Y values of each of the color temperature settings.
The steps required to perform this adjustment varies from model to model. It also varies between a direct-view and projection unit. Because of this, it would be impossible to cover the requirements in a general sense. Each type of unit has a unique procedure for the initial setting of brightness, contrast, and what color temperature to start with. Some direct-view LCD models have an adjustable back-light that must be set to a certain level. The procedure for reading color balance from the screen also varies for a projection or direct-view model.
Many technicians will not have the luxury of carrying a color analyzer. They are expensive ($4000 and up) and require training to use properly.
In cases where color balance must be adjusted and an analyzer is simply not available, there is a procedure that can be performed that will produce satisfactory results. Although not nearly as accurate, it is better that not doing the adjustment at all. This procedure should only be done if it is absolutely necessary and a color analyzer cannot be acquired.
Adjusting the White Balance Without a Color Analyzer
In cases where a color analyzer is not available, white balance can be aligned by eye. Technicians who are experienced with adjusting CRT based displays will be familiar with this procedure. They are similar except that CRT’s require that the G2 grid (screen) be adjusted to the point of stopping the electron emissions from the cathode at reference black.
Drive controls are then adjusted to make each cathode emit the proper level of electrons at high brightness to achieve a white raster.
When adjusting an LCD display, there are no cutoff adjustments. In this particular model, the Sub-Contrast adjustments are used to adjust white balance for the bright areas of the screen. The Sub-Brightness adjustments are set for the low-level brightness areas.
The best test pattern to use when visually adjusting white balance is a monochrome stair-step pattern versus a 100IRE white screen. This provides a view throughout the range of brightness levels the display will generate. Another significant difference in the adjustment procedure is starting with the “Neutral” picture setting. The “Warm” setting shifts the white balance towards the red end of the scale whereas the “Cool” setting shifts towards the blue spectrum. By using “Neutral”, the white balance can be adjusted visually. This provides a reference for the eye so that the “Cool” mode can be set with the Blue Sub-Brightness and Contrast increased slightly to boost the color temperature to around 9300K. The “Warm” setting will have more Red Sub-Brightness and Contrast levels to create a white balance in the 6500K range. “Neutral” should fall into the 8000K level.
Be sure to leave the low-bit data settings for the Green Sub-Brightness and Contrast to zero. The high-bit data should be in the mid range. Adjust the Blue and Red data to achieve white by adding these colors to the green. The following procedure can be used for a unit when the white balance is significantly off:
* With the unit in “Neutral”, set the brightness level to one-third and the picture level to two-thirds. Input a monochrome stair-step pattern with at least 75IRE at the highest white level.
* Check that the Green Sub-Contrast and Brightness levels are set to mid-range for the upper-bit data and zero for the lower-bit data.
* Set the Red upper-bit data for Sub-Contrast and Brightness to mid-range and the lower-bit data to mid-range.
* Set the Blue upper-bit and lower-bit data for Sub-Contrast and Brightness to zero. The picture will now be a yellowish/green.
* Adjust the Red Sub-Contrast Low to generate a yellow color at the brightest portion of the test pattern. If you find that you are reaching the extreme end of the data range (0 – 255), set the Red Sub-Contrast High up or down one number and try again.
* Once the bright video level is closest to yellow as possible, repeat the above procedure for the Red Sub-brightness adjustments.
* When satisfactory yellow is achieved throughout the video range, set the high-bit data for Blue Sub-Contrast and Brightness to mid-range. Adjust the low-bit data for each to change the yellow screen to white. If you are at either of the extreme ends of the data ranges (0 – 255), set the upper-bit data up or down accordingly.
* Once the “Neutral” white balance is satisfactory, adjust the “Warm” white balance in the same way. Add a small amount of extra Red Sub-Contrast and Brightness.
“Cool” is adjusted adding a small amount of extra Blue Sub-Contrast and Brightness data.
Although the above procedure can be somewhat subjective, it has worked rather well for CRT-based consumer televisions for many years. It is a viable alternative when a color analyzer is impossible to acquire.