TweetFacebookShareThis

Liquid crystal display (LCD)

Previous Page    Next Page

Computer Studies Liquid crystal displayModern laptop computers use the active matrix display or TFT Thin film transistor and are utilizing widescreen LCD monitors, there are usually AGP ports for connecting external monitors female DB 15 pin connectors coloured blue and DVI digital video interface 24 pin female connectors coloured white for external monitors, there is normally a function key that enables you to toggle between the external monitor these vary from laptop to laptop Fn + F5 etc. there is normally a picture representing a monitor on the function key that toggles between monitors.

LCD monitors on laptops can be repaired but caution is recommended in dismantling them, to change the CCFL backlights can require specialised equipment, removing and replacing inverters is usually relatively easy to do, and replacing the entire LCD panel can be easy to achieve but can also be expensive to do so in some cases so you will need to evaluate to see if the price of a new LCD screen is worth it.

LCD Screen Resolutions
XGA (1024 x 768) Extended graphics array
SXGA (1280 x 1024) Super extended graphics array
SXGA+ (1400 x 1050) Super extended graphics array plus
WSXGA+ (1680 x 1050) Widescreen super extended graphics array plus
UXGA (1600 x 1200) Ultra extended graphics array
WUXGA (1920 x 1200) Widescreen ultra extended graphics array

An LCD is based on a liquid crystal, which can be made opaque or transparent depending on the presence of an applied electric field. LCD displays are flatter, lighter, and more compact. They consume less power, and so can run off batteries. They also last longer than CRT monitors. Most LCD screens in laptop computers are lit from behind the LCD panel - an arrangement known as backlighting. The panel itself is made up of several components, including the polarizer' s, the row and column electrodes, the colour layer, and the liquid crystal layer itself. The intersection of the row and column electrodes forms a single pixel, and the image is produced on the screen by manipulating these electrodes to affect the transparency of the liquid crystal layer. The refresh rate on LCD monitors is set at 60 Hz, but a good LCD monitor has a response rate of 12 milliseconds or less, the brightness of an LCD is measured in nits 1 nit = 1 candela per square meter so a good rating is around 1000 nits. Contrast ratios in LCDs include 500:1, 800:1, and 1000:1. The contrast ratio is the difference in color between the whitest white and the deepest black, for example 500:1. Dot pitch refers to the size in millimeters between pixels.

Aside from the screen, an LCD monitor comprises of three main parts.

1 Backlight bulb
The backlight bulb, also called the cold cathode fluorescent lamp (CCFL) , and is responsible for illuminating the LCD so that the images are visible on the screen. The bulbs require AC power.
2 Fluorescent Lamp inverter board
The inverter converts the laptop's DC power to AC power, which the LCD uses to light the backlight bulb. If the inverter is faulty, the backlight bulb cannot be lit and you will not be able to view any images properly on screen.
3 Video cables
The video cable connects the laptop video adapter, usually integrated into the motherboard, to the LCD. This cable is responsible for carrying video signals from the video adapter to the LCD screen. For more information on LCDs wikipedia.org

Removing LCD screens

Normally you'll find the screws behind the rubber pieces on the screens bezel which will need prising out with a flat blade screwdriver, and after unscrewing all the screws you can gradually pry the bezel away from the screen, you can then remove the inverter cable from the screen now that it is accessible with the bezel clear, then you can remove the screen, there are usually screws that attach the screen to a side rail once these have been removed it's usually easy to separate from the laptops lid, the LCD cable should be taped to the rear of the LCD screen this will need to be removed carefully in order to lift the screen clear of the lid, depending on the model there may be webcams and wireless antennas attached as well which will need care and attention when removing, again you would use the internet in searching the places like E Bay to buy a replacement LCD screen, and you will need to use the serial and model number which is usually on the back of the screen to search for it on the internet. When replacing the bezel if you kept the screws referenced as to where they were removed from, you will find this will help when restoring, as the rubber pieces normally have a glue residue which is left on the screws this helps if they are re-attached to the same screw to assist in sticking them back on the bezel.

Troubleshooting LCDs

With LCDs always check that things like the brightness and contrast controls or the LCD cut off switch which might be stuck or that it might be toggled to an external monitor by the function Fn keys, always check the keys that perform secondary functions, that toggle between LCD / external monitor / both and test using an external monitor by bypassing the LCD but not the video hardware, verify that the backlight is working the Cold Cathode Fluorescent Light (CCFL) may be broken or you may need to replace the LCD inverter or the display itself, check the resolution, native resolution looks best, check the manual or the laptops specifications on the manufacturer's web site. Some laptops have a shared memory module this is where the system shares the video RAM and the system RAM, if the RAM as failed this might well be a reason for the monitor not working. It's possible that there could be a software problem so try booting in safe mode by pressing F8 on start-up. Dead pixels can sometimes be the result of a loose ribbon connection to the motherboard, but if reseating the cable doesn't fix the problem you will need to replace the LCD screen. With blank LCDs it's worth checking the obvious culprits, like a screen saver set to "blank" and poor wake-up behaviour, or the video getting toggled to a non-existent external monitor by a keyboard failure (or an operator failure).

Useful web sites
www.laptoprepair101.com
www.fonerbooks.com
www.laptoprepairvideos.com

Previous Page    Next Page

Recommended Books