Testing a Laptop Backlight(CCFL) and Inverter


Nov 11, 2018
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We will be concentrating on LCD Inverter and CCFL bulb.
Laptop with black screen problem is commonly caused by:

1. a defective backlight, which is the cold cathode fluorescent lamp(CCFL) bulb
2. defective laptop’s inverter board
3. no video output going to the LCD Screen

It is not an easy job repairing laptops specially if we don’t have parts that we can use to replace to the parts we are suspecting to be the culprit. So now, we will going to try to make our own tools for testing a bad CCFL backlight of a laptop.

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Tools Needed For this Project:

1. a 9volts battery

2. a 9v battery connector

3. a known good CCFL inverter from any laptop or LCD monitor.

Once you have the tools that we needed, we can continue to the next procedure.

About The Video Output:

If the laptop is equipped with an external monitor connector, we can easily check if the problem of having a black screen is caused by a faulty video card or GPU within the laptop or maybe the problem resides inside the LCD which is the inverter and CCFL bulb.

VGA Port on a Laptop
Connect an external monitor to the VGA Port of the laptop and check if there is a video output to the monitor.

There is some model of laptop that requires you to press a FN key plus a key with a picture of a monitor to switch between the LCD laptop to external monitor.

Then now, if you get a video on your external monitor then you now know there is a LCD screen, CCFL bulb and inverter board problem. If the external monitor doesn’t get any video at all, then suspect a video (GPU) or video cable problem.

Checking the CCFL Bulb:

On checking whether your CCFL bulb if it is still functional, we can use our own known good inverter to test the CCFL bulb is still working or in good condition.

Known Good Inverter
Using the Known Good Inverter:

Now, with our known good inverter, connect the CCFL bulb. And next, supplies a power to it using our 9v battery. A laptop inverter is usually powered by 12v and up but with our 9v battery, it is just enough to start the circuit of an inverter.

Connect the black probe from our battery source to the yellow wire, which is usually the 12v supply coming from laptop circuits, but to find more accurately the connection of the power source of an inverter, all you have to do is to find where is the connection of the SMD fuse in the circuit of the inverter. And the black probe to the ground screw hole of the inverter board.If our CCFL bulb lights up like in the picture below, then we now know that our CCFL bulb is good and working and now we can suspect that our inverter board is bad.

Checking Inverter:
Since that our known good inverter have able to light up the LCD Screen, and then connect the inverter of the laptop back to its position. Then we are going to conduct a test to it using the procedure we did to our known good inverter. And if it won’t lights up the CCFL bulb, then we can suspect that our inverter is defective and needs to be replaced to a new one or simply you can check it if the fuse is open. If it is open, try to make a jumper through the fuse using a thin wire to serves as our fuse, then redo the testing again with same procedure as we use on our known good inverter.
Checking the Power Supply Line:
If our known good inverter light up our CCFL bulb and the Laptop’s inverter also lights up using the 9v battery as its power source, then we are going to suspect the 12 volts power supply line from the laptop’s motherboard.

Let us get our multitester/multimeter and set it to DC Volts.
Plugged our AC Adapter and turn on our Laptop.
Put the black probe to the ground screw hole on the motherboard and the red probe to the yellow or the supply line and see if you have a voltage reading on that connection.
If you are can’t read any voltages from the connector then let us suspect a problem on the motherboard itself and this needs a lot of knowledge and experience on repairing Laptop.
At this point, we will have to bring the unit to a technician who have the knowledge of repairing a complicated problem of a Laptop.

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