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.
Finally, when you ready to test the laptop, unplug the LCD screen from the right side of the inverter.
Plug in your test backlight lamp and turn on the laptop.
In my case, the backlight lamp lights up, so the inverter board works properly.
Check out my previous post about troubleshooting laptops with backlight failure.
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May 19th, 2009 at 7:39 am
Bodinator,
Yes you can but I believe you’ll get positive readings only if both the backlight lamp and inverter are in a good working condition. The magnetic field will be created only if the inverter is loaded, right? But what if one of the parts is faulty and there is no magnetic field around the inverter? How can you tell if it’s a bad inverter or backlight lamp?
May 19th, 2009 at 6:38 am
You can also use a cheap electricians amp-meter that measures induction via the magnetic field to test these. The coil creates a pretty hefty magnetic field.
May 17th, 2009 at 8:39 am
Israelinkvm,
If the screen has no backlight and replacing the inverter with backlight lamp doesn’t help, apparently there is a problem with the motherboard.
Also, this problem could be related to the video cable, but it’s not a common failure. If you have the lid close switch – a small button which turns off the light on the screen when you close the display, make sure this button moves freely. A dirty lid close button might get stuck inside the case and the laptop “thinks” that the display is closed even when it’s open.
May 17th, 2009 at 8:34 am
inquirer1,
When you test voltage coming to the inverter from the motherboard, you should set your voltmeter to test DC in the range somewhere between 0-50VDC.
I think you should expect readings somewhere between 10VDC-20VDC, it depends on the model.
You cannot use the voltmeter to test the AC output.
May 17th, 2009 at 8:28 am
chakra,
That means the inverter is getting power from the motherboard.
I don’t think that you can test the AC output with a multimeter, at least it never worked for me. I tried testing the AC output on a working laptop, just to find out what readings I should expect but as soon as I connect the multimeter, the backlight shuts off. That’s way I suggest testing a faulty laptop with another known good backlight lamp.
I’m glad it worked for you.
May 17th, 2009 at 3:43 am
Hi,
Thanks a zillion for this article. I had a dim LCD screen laptop. By following some of the instructions I could zero in on the problem. I tested out the DC part of the inverter board, I was getting around 15V, but I tested the AC section of the board (the other end feeding the backlight) on a multi-meter and I did not get a reading. So i could figure out that the inverter board was the problem. Replaced the inverter board and LCD screen lights up. Gave me a lot of satisfaction, kudos mate … great job of writing up the article in a very lucid manner.
May 15th, 2009 at 6:33 pm
hi,what should be the voltage settings for multimeter if I would test the inverter?thanks.
May 12th, 2009 at 4:20 am
Like I said before, I’ve tested with another backlight but it doesn’t light up, the brightness is ok.
May 11th, 2009 at 8:37 pm
Israelinkvm,
Does it look like there is no backlight at all or it looks like the backlight works but the brightness is very low?
May 11th, 2009 at 7:39 pm
Hi CJ.
I love your blog, there’s a lot of tips that i’ve used to repair some laptop, thanks for that.
I have a problem with a laptop, when I turn it on, the screen is very dark, with one very very soft image, at first, I think it was the lcd inverter, I’ve changed it with no success, the screen keeps black, I’ve read your post and followed the steps to test the inverter with another backlight, when I do this nothing happens, the lamp doesn’t turn on, what this could be??, maybe the motherboard isn’t sending power to the inverter to turn on the lamp??please, I need your help. Thanks.
Note: When I plug an external monitor there’s no problem, video is ok.
P.S. Sorry for my english