SolutionBase: Diagnose common LCD monitor issues with these tips

LCD flat-screen monitors are all the rage, but they do present new challenges. Many of the tricks you learned supporting CRT monitors won't help. This article shows some of the things you can do to troubleshoot issues with LCDs.

Although flat panel monitors have been around for several years, their prices have only recently fallen to the point that they are being adopted by the masses. However, LCD monitors are so mechanically different from CRT monitors that diagnostic techniques that we have all been using for years are now obsolete. Many of the problems that plagued CRT monitors simply aren't an issue for LCD panels. At the same time though, LCD panels are subject to some rather strange issues that you would never encounter with a CRT monitor. That being the case, this article will explain what some common issues are with LCD monitors and how you can resolve them.


Before I get into the troubleshooting section, I want to take a moment and explore the anatomy of an LCD panel and how it functions in comparison to a CRT monitor. After all, it's tough to troubleshoot a hardware problem if you don't understand how the device works.

CRT monitors rely on a special type of vacuum tube called a cathode ray tube (hence the abbreviation CRT). The tube is narrow at one end and wide at the other. The wide end is the surface where the image is displayed. The narrow end of the tube contains an electron gun that fires electrons at the end of the tube that's used for viewing. The large end of the CRT tube is coated in phosphorous. Whenever an electron from the electron gun strikes the phosphorous, it releases a burst of light, and this is how an image is created.

The phosphorous coating is actually made up of three different colors of phosphorous; red, green, and blue. If the electron strikes red phosphorous, then the burst of light that is released is red. A device called a shadow mask separates the various phosphors from each other. Without a shadow mask, an electron might strike a red phosphor, but some of the energy would bleed over into the blue and green phosphors, causing the incorrect color to be displayed. The shadow mask insures that only the correct phosphor is illuminated at a given moment.

The other thing that you need to know about phosphors is that they release the burst of light extremely quickly. However, phosphorous has a quality called persistence that allows it to keep glowing for a little while after the light burst has been released. This is how monitors are able to give the illusion of a solid picture.

The picture is composed as the electron gun is turned off, aimed, and turned back on. Of course the whole process happens much too quickly to actually reposition the gun. Instead, a device called a yoke uses magnets to pull the electron from the gun in the desired direction. The entire screen is redrawn many thousands of times each second. The number of times that the screen is redrawn per second is the monitor's refresh rate.

An LCD monitor is based on the idea that you can re-align a liquid crystal's molecular structure by applying a small amount of electricity to the crystal. When the electric charge is removed, the liquid crystals return to their original state. This concept has been in use for decades in calculators. A calculator screen appears solid gray until an electric current causes black numbers to appear.

In a calculator, the liquid crystals actually form the numbers that are being displayed. In an LCD monitor though, electricity is applied to a crystal to turn it black, not to display an image, but rather to block out light. An LCD monitor has a cold cathode backlight that illuminates the entire display surface. The liquid crystals are sandwiched between the backlight and a thin film containing millions of translucent red, green, and blue dots. Red, green, and blue dots are arranged into triangle patterns called triads. Each triad represents a single pixel.

Suppose that a computer told a specific pixel to display as blue. The LCD panel would energize the crystals behind the red and green portions of the triad. In doing so, the now black crystals would prevent any light from passing through the red or green holes, leaving only blue illuminated. The result is that the pixel displays as blue.

The process sounds simple enough, but think about how many pixels are present on a monitor. A monitor with a resolution of 1024 x 768 contains 786,432 pixels. Being that there are three separate elements that make up each pixel, the monitor would have to manage 2,359,296 separate crystals.

In early LCD monitors, this was a huge problem. The monitor simply couldn't refresh itself quickly enough to produce a decent picture. If you moved your mouse pointer across the screen, a ghosted trail would follow. These early LCD monitors worked great for word processing and things like that, but their slow refresh rate made them a poor choice for things like gaming and video editing.

The problem of slow response time was eventually solved by adding a transistor to every single crystal on the entire monitor. The transistors made it possible to create LCD monitors with decent refresh rates. In fact, most of the LCD monitors in use today are of the TFT variety. TFT stands for Thin Film Transistor.

Dead pixels

Now that I have described how an LCD monitor works and how it differs from a CRT, let's talk about some common issues with LCD monitors. The first issue that I want to discuss is dead pixels. A dead pixel is a pixel that always displays as either black or white, regardless of the image that is on the screen.

This problem goes back to the way that an LCD monitor works. The problem is related to either a manufacturing defect or to a set of blown transistors. What's happening is that the liquid crystals within that particular pixel are either always receiving electricity (black) or are never receiving electricity (white). There have also been cases in which only part of a triad is damaged. For example, red and green might work, but blue is stuck either on or off. This produces a pixel that is partially responsive but that often displays an incorrect color.

The bad news is that if you have a dead or a partially dead pixel, there is nothing that you can do about it other than buying a new monitor. The good news is that the problem isn't as common as it used to be. A couple of years ago, it was common for most LCD monitors to have at least one or two dead pixels because the manufacturing process had not yet been perfected. Today though, monitors hardly ever ship with dead pixels (bargain basement brands aside). It is possible for pixels to die over time though.

Poor picture quality

By far the most common problem with LCD monitors is poor picture quality. There are several different things that can cause the image on the screen to appear blurry or distorted, but there are relatively easy fixes for most of these problems.

One thing that causes poor image quality is the screen resolution that you've selected. As you probably know, CRT monitors have a maximum resolution, but any resolution at or below the maximum resolution will display clearly. The reason for this is because CRT monitors do not have pre-defined pixels. If you tell Windows to use a resolution that's lower than the maximum, then the monitor's yoke simply causes more phosphors to be illuminated for each pixel. This is done in a way that allows the lower resolution to take advantage of the full size of the screen.

LCD monitors work a little bit differently though. The number of pixels that the monitor contains is known as the monitor's native resolution. The monitor can display resolutions below its native resolution, but doing so usually results in a poor quality image because the monitor has to stretch the lower resolution image over a high number of pixels. Think about it for a moment. If you were to try to display an 800 x 600 image on a monitor with a native resolution of 1024 x 768, the monitor does not have enough pixels to assign two hardware pixels to every pixel of the image. Instead, some of the images pixels might be displayed across two triads while other pixels are displayed across one. The resulting image is usable, but it isn't pretty.

There are some situations in which displaying lower resolution images does not cause distortion though. I used to have an old Toshiba monitor that avoided the pixel stretching issue all together. If you selected a lower resolution, it would display the lower resolution image in the middle of the screen and black out the unused pixels around the edges. Another technique that avoids pixel stretching is to lower the image quality in an evenly divisible increment. For example, if a monitor has a native resolution of 1600 x 1200, then it will have no trouble displaying an 800 x 600 image clearly because 1600 is evenly divisible by 800 and 1200 is evenly divisible by 600. The best solution however is to simply use your monitor at the native resolution.

Text is difficult to read

Although LCD monitors are usually considered to have an image that is more crisp than CRT monitors, sometimes, especially on lower end monitors, text can be difficult to read, even if the monitor is running at its native resolution. There are a number of different things that can cause this, such as a poor contrast ratio, a low native resolution, or poor color reproduction. You can sometimes make things better by tinkering with your monitor's settings. However, Microsoft realizes that text is sometimes tough to read on LCD monitors and has built a feature into Windows XP called ClearType.

ClearType adds shading to text in an effort to make it more legible. You can enable ClearType by right clicking on an empty area of the Windows desktop and selecting the Properties command from the resulting shortcut menu. When you do, you will see the Display Properties sheet. Now, select the properties sheet's Appearance tab and click the Effects button. When you do, Windows will display the Effects dialog box. Select the Use the Following Method to Smooth Edges of Screen Fonts check box and then select the Clear Text option from the drop down list. Click OK twice and ClearType will be active.

Blank screen

Another common problem is that the screen fails to display an image at all. There are several things that can cause this problem. The trick to diagnosing the problem is to determine whether or not an image is displayed when you first turn on your computer.

If you switch on your computer and you see an image up until Windows starts loading, then the issue is most likely related to your video card driver (especially if you are using a DVI connector). To fix this problem, use another computer to download the latest driver from your video card manufacturer's Web site. Some monitor manufacturers also provide monitor drivers, so it's worth checking to see if a driver is available for your monitor while you are at it.

Once you have the drivers, boot your machine into safe mode (Press [F5] just before Windows begins loading). When Windows boots, open the Control Panel and click Performance and Maintenance, followed by System. This will cause Windows to display the System Properties sheet. Now, select the properties sheet's Hardware tab and click the Device Manager button.

Navigate through the Device Manager until you locate your video card. You'll find it in the Display Adapters section. Right click on the listing for your vide card and select the Update Driver command from the shortcut menu. Now follow the prompts to load the new driver that you downloaded. If you have a monitor driver, then you will want to locate the monitor listing and update its driver as well.

If you attempt to boot the system and don't get any image at all, then you've got your work cut out for you. Obviously, you will want to make sure that the cables are secure and that the monitor is getting power. You might even try using a spare video cable or try the monitor on a different PC. Keep in mind that some PCs have multiple video outputs, and it could be that the monitor is connected to a video port that isn't active. Look for other ports that you can try connecting your monitor to.

I have heard several reports of blank screens when using a DVI connector. I personally have never had a problem connecting any of my computers to a flat panel monitor through a DVI connector, but apparently, some people have had to flash their BIOS or perform firmware updates to their video cards in order to make a DVI connection work. I have also heard some rather bizarre reports of DVI connections not working until the monitor has had time to warm up.