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September 4, 2010

How to test LCD screen inverter in a laptop

In this post I explain how I test the LCD screen inverter board in a laptop computer.

The screen inverter failure is very similar to the backlight lamp failure. In both cases the screen gets very dark and the image on the screen becomes very faint, barely visible under a bright light.

If you suspect the inverter board failure, I know only one reliable way to test that. It’s either replacing the inverter board with a known good one and see if it works, or connecting a known good backlight lamp and see if your presumably bad inverter lights it up.

In most cases I go with the second method – testing the laptop with a known good backlight lamp. Why? Because backlight lamps are pretty much universal. The same backlight lamp will work with many different inverters as long as they have matching connectors. I’ll talk about these connectors later.

The inverter board is located inside the display panel under the LCD screen. In most laptops you can access the inverter board if you remove the LCD screen bezel. The inverter board has connectors on both ends.The left side of the inverter is connected to the LCD cable. The right side of the inverter is connected to the backlight lamp which is mounted inside the LCD screen. Check out this display diagram.

To make sure that inverter board is getting power from the motherboard (via the LCD cable), you can test it with a multimeter. In my case I connected the “+” lead of the multimeter to the pin 1 on the connector and the “-” lead to the ground trace around the screw hole. I got about 19.4V DC on that side of the inverter, so it’s getting power from the motherboard.

WARNING! If you accidentally short something on the inverter while testing it, you can damage the inverter or even the motherboard. Proceed on your own risk! Not sure? Don’t do that!

So, the inverter is getting power from the motherboard, but the screen is still dark. Apparently, it’s either bad inverter or failed backlight. Let’s test it with a known good backlight.

Here’s what I’m going to do:
1. I will unplug the LCD screen from the right side of the inverter. Basically, I’m unplugging the LCD backligth lamp which is located inside the screen.
2. I will plug in my known good backlight lamp which you can see on the picture below. Please notice that my test backlight lamp is shorter than the screen, but for the test purpose that’s OK.

Results I’m expecting:
1. If my test backlight lights up, the backlight lamp inside the screen is bad and there is nothing wrong with the inverter board. If that’s the case, you’ll have to replace the LCD screen or replace the backligth lamp (which is not easy at all).
2. If my known good backlight lamp stays dark after I turn on the laptop, most likely we have a faulty inverter board. If that’s the case, you’ll have to replace the inverter board and it’s relatively easy.

There are two different types of backlight connectors, you can see them on the picture below. The top one (big) is not as common as the bottom one (small). I do most of my test with a backligth lamp which has a small connector.

IMPORTANT! If you decide to buy a new backlight lamp for test, you have to make sure that the connector on the lamp matches the connector on the inverter. Very often backligth lamps are sold without any wires attached. If you plan to use this backlight as a test equipment, you’ll have to find one with wires as I have on the picture 3.

You can buy a cheap backlight lamp with wires here.


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