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July 31st, 2009
LCDs are one of the most common repairs we see in the shop, so we figured we’d help out people a little in diagnosing the issue themselves. There is quite a bit of technical terms here, but I’ll keep it as simple as possible so everyone can understand.
Now, let’s start off with the basic anatomy of an LCD screen. The LCDs that we sell come in an assembly which houses the protective front pane of glass, the liquid crystal component which gives the picture on the screen, and the backlight which shines light through the liquid crystal through the glass in the front to your eyes. There are two separate cables coming from the LCD with one controlling the backlight and one controlling the picture.
Now the main confusion comes with the type of backlight these days. All laptop screens are referred to as LCDs; yes, even one’s with LEDs. Apple’s displays over the past couple of years have been marketing the brighter, more power efficient LED displays. This is just referring to the backlight. Again, all laptop screens are called LCDs, the main difference is if the backlight uses the older CCFL backlight using bulbs or the newer LED backlight using (you guessed it!) LEDs.
There are some other factors that play a role in what LCD you have such as screen size (measured diagonally i.e. from the top left corner down to the bottom right corner), matte or glossy finish on the glass pane, and maximum screen resolution (how many pixels are crammed into the LCD), but those only come into play for when you are ready to buy a replacement.
Now, onto diagnosing your LCD problem. There are generally four problems that occur with LCDs and there a couple of easy tests to judge what part may be causing the problem.
First is the ever popular cracked screen. This occurs when blunt force trauma causes the front glass or the liquid crystal panel to break which can cause the liquid crystal to start oozing out behind the front glass. This usually starts a spider web looking pattern from the impact point and puddles of the liquid crystal to form blocking the picture from the screen. This will require a new LCD and usually stops there if that is all it is doing.
The second biggest issue is the backlight. Typically, this is diagnosed when you look real close to the screen you can still make out a picture, but it is extremely dark and changing the brightness does not help. The backlight has a cable coming from the LCD assembly into a small circuit board called an inverter board (on CCFL-backlit displays) or LED driver board (on LED-backlit displays) which controls the brightness. From the inverter board, you have the inverter cable which goes down from the display through the hinges of the machine, and to the main logic board (or motherboard for PC guys). This means there are four possible causes for backlight issues: the backlight in the LCD itself (which is extremely rare), the inverter board, the inverter cable (which depending on the severity of the damage can be tested by tilting the display back and forth and seeing if the problem gets better or worse since it may have gotten damaged in the hinge), and the logic board which will usually only go bad if there is a short somewhere or there is spill damage.
The third issue is just a solid white or gray screen. If the machine still boots up, but there is no picture, this means your backlight is working fine, but there is something wrong getting the picture up to the LCD. This in most cases is going to be a damaged or disconnected display (LVDS) cable. It may have just come loose from the back of the LCD or from the main logic board or it got damaged somewhere most likely in the hinge.
The last issue is the completely black screen with no picture, no backlight, just nothing. Now that you have a general understanding of the anatomy, you should be able to put it together that if you are not getting a picture or a backlight, those two connections only come together with the LCD itself or the main logic board. It is difficult for both cables to get damaged unless the display is completely ripped off the base. It is rare for the LCD to have both connections go bad, so that leaves us with the logic board which more often than not is the culprit in this situation. The most common cause for this is a dead graphics chip.
I hope this clears things up, and hopefully you’ll have a good starting point on diagnosing your own LCD problem.
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