Laptop LCD Problems

LCD Failures

Hardware failure - LCD, inverter, cable or backlight

Symptoms:

• The laptop appears to start as usual but the screen is black, if you move it, it turns on and works properly. Failed cable.
• The laptop appears to start as usual but the screen is very dark, although it changes colour a little and you can see a very feint image. Failed inverter or cable 90%/10%.
• Immediately after starting the screen has dark pink/red tint and turns dark after a few seconds. Failed backlight.
• The screen flickers a few times but then turns dark, if you move it or tap it lightly on the sides, it flickers again. Failed cable or LCD 50%/50%.
• Screen is either black or white (no picture) and external monitor works. Failed cable or LCD 30%/70%.
• There are stripes on the screen (either vertical or horizontal) that change when you move the screen or tap it lightly on the sides. Failed cable or LCD 10%/90%.

In all above cases, if you plug in an external monitor, everything shows fine on it (you have to plug it in before starting the laptop and may need to press the “Fn” key together with one of the “F1″ - “F10″ keys to make the laptop recognize the external monitor).

Troubleshooting:

If the screen is black, look for the “lid closed” switch and try pushing it a few times. It’s usually a small switch near the hinges that is pressed when the lid(the screen) is closed. Sometimes these switches get stuck in the “lid closed” position that results in black screen(no backlight) or non-starting laptop. Also some laptops have a magnetic switch (newer Dells, most Macs, etc.) that doesn’t get stuck.

The most common failures are (in this order): inverter, “lid closed” switch, cable, LCD, backlight.

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Any ideas on how to properly test the inverter input and output?

That’s a question I keep asking myself over and over. How can I test an FL inverter board using just a multimeter? Unfortunately, I do not have any straight answer at this time. I believe that you can measure the voltage on the input if you connect the positive probe to the pin 1 on the FL inverter and the negative probe to the laptop chassis. The multimeter should be set to test DC voltage. I think that the voltage on the input could be somewhere between 9-30VDC, just in case set the multimeter to a higher voltage. The pin 1 on the FL inverter board should be marked; I’ve seen it marked before. You should get some voltage on the inverter input. If you get zero volts on the input, then it’s possible that cable is bad or the connection on the system board is bad. Now about the output voltage. I believe that you can get an accurate readings on the FL inverter board output only if it’s loaded properly, in other words if the backlight bulb is connected and it’s in a good working condition. It could be a very high AC voltage. I would set the multimeter to at least 1000VAC. Be careful, it’s a very high voltage and if you do something wrong and short a circuit, you can permanently damage the laptop. Sorry, but I cannot provide more specific information. If you find any info on how to test a failed backlight with a multimeter, please let me know.

Usually, when I have a problem with the backlight, I unplug the screen and connect my test backlight bulb. If it lights up fine, then the backlight in the LCD screen is defective. If it doesn’t light either, then most likely it’s the FL inverter board fault. Video cables do not fail very often.

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BACKLIGHTS

Backlights can be tested by connecting them to a known good inverter. You can use one of the inverters they sell for case moding. Just provide a 12V input for it and connect your backlight to one of the output sockets. If your backlight illuminates then it is OK and the fault lies elsewhere. A pink or red hue to the screen is nearly always a sign of a failing backlight and if this happens it is well worth replacing the backlight as the only other thing that can cause this is a faulty screen.

Remember backlights work on an AC Voltage of around 1000v and although the wattage is very small can give you a nasty shock and could prove fatal if you have a pacemaker or similar.

INVERTERS

We now come to an item that can only be tested by a qualified electronics engineer or simply substituted. There is usually a small fuse located near the input connector. Check this for continuity. Depending on the inverter there can be several inputs but it is worth checking the input voltages. Basically you will have a positive and negative feed plus a switching voltage and a brightness connection. The basic voltage for powering the inverter is usually the same as you laptop input voltage but the switching voltage may be as low as 5 volts DC. The brightness voltage can be anything from 0 to 5 volts but normally full brightness is acheived with 0 volts so don't worry about this. Unfortunately inverters often have up to 20 or more connections and may incorporate circuits which show battery charge status, on/off status and almost anything else you care to name! As wiring diagrams are rarely available its impossible to know which connection does what. It is possible by a process of elimination to work out the main positive and negative terminals and you may often find there are several of these but the rest is pure guesswork. If you have an analogue multimeter, set it to the highest AC reading (usually 1000V) and check for an output from the inverter. Initial start up voltage will probably exceed 1000 volts but normal running voltage should be below 1000 volts depending on the size of your screen. Look for a voltage in the region of 400 - 900 volts. Obviously if you are getting a good voltage here then your backlight has failed. Otherwise I am afraid it is a case of replacing with a known good inverter to see if this cures the problem. (If anyone knows of a foolproof way to test an inverter please enlighten me)

LCD/INVERTER CABLE

Check this cable carefully for damage especially in the hinge area and make sure the connectors have not worked loose. It is possible for the wires to break internally with no obvious signs of damage so check the continuity of the cables. A faulty LCD cable can even result in your laptop not switching on.

MOTHERBOARD

The motherboard sends a signal to the inverter board telling it when to switch the backlight on. If there is a fault on the motherboard then the inverter board will not receive a signal and therefore not switch the backlight on. It may be possible to check the input voltages to the inverter board with a multimeter but if you have eliminated the previous items I have found this to be quite a commom fault. NB If you connect your laptop to an external monitor and get a good picture this only proves that the video card is OK, there could still be a fault on the motherboard which is preventing your screen from illuminating.

SCREEN

It is possible for a faulty screen electronics to result in the backlight failing to illuminate. This is rare but can happen and if you have eradicated the previous items is the only possibility left.

I hope this guide has given you some help in repairing your laptop and if you are unclear about anything please contact me and I will do my best to help you. I am not an electronics engineer but like most of you enjoy fiddling with computers and laptops and its a great feeling when you do manage to repair it yourself!