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.
Also, you can read the following posts:
Troubleshooting laptops with backlight failure.
Laptop screen shows strange colors. What could be wrong?
How to troubleshoot and fix laptop video problems.
If you find this article useful, please consider making a donation to the author. Thank you!
June 9th, 2009 at 6:43 am
“”Yes you can but I believe you’ll get positive readings only if both the back light 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 back light lamp?”"
An inverter takes a (relatively) high amperage low voltage signal and turns it into an extremly high (KV range) low amperage (mA range) signal. There are 4 failure points; CCFL, Inverter, Cable, and Mobo. Couple of things to keep in mind:
A: If the CCFL fails it will not load the inverter, so no mag field.
B: If the Cable fails (pretty much it can only fail one way), no voltage = no mag field.
C: If the inverter fails, you may or may not get voltage; if the CCFL is loading the inverter and the inverter is not producing enough voltage you’d probably not get a mag field.
D: If the mobo fails, no voltage or not enough.
Say the unit is DOA and you *THINK* it has a backlight issue, the ampmeter is the 3-second solution to see if you’ve got a functioning inverter. Most laptops go through a power-post procedure similar to a desktop psu and will load the inverter if it passes even if the unit fails to post.
I like the idea of ordering a $15 CCFL and using that to test; I hadn’t thought of that before and come to think of it to test this properly you’d need an assortment of CCFL’s and cable converters. It’d be easy to build a professional soluton from epoxy, wood, 2×3′s, plexiglass, some screws and a label maker.
I do keep a spare 12” LCD on hand, as well as a 20” chassis with an inverter hooked into it. You can load a high-end inverter with a low-end LCD ccfl. The problem with either is if you use a 12” LCD CCFL to test a 16” inverter anbd the 16” inverter is ok producing voltage for as 12” ccfl but not a 16” ccfl you wouldn’t know it until you plugged it into the 16” ccfl.
June 3rd, 2009 at 1:37 am
HI,
I’ve changed a broken screen of my laptop, but i get a dark screen with faded images whenever i turn it on ,i tried to test the inverter input and i got 1.85 v at start then it goes down to 0.48v after a few seconds ,the cables and connectors are all well connected and when i connect my notebook to an external monitor it works fine, can u tell me plz whats the problem and how can i solve it, thank u
June 1st, 2009 at 8:50 pm
JH,
Yep, to me your description sounds like a problem with the LCD screen.
June 1st, 2009 at 5:34 pm
I have a screen thats is really bright and everything is washed out, Mouse pointer is only visible if it stops moving and fades in and out slowly when you stop and start it.
Laptop works fine through a VGA connection.
This a screen problem?
June 1st, 2009 at 8:48 am
To AJ Simkatu (comment 11),
If you still can see a very faint image on the screen even though the backlight is off, most likely your problem is related to the screen inverter. If you replae the inverter but the problem is still there, apparenlty it’s a faulty backlight lamp.
Again, sounds like either bad inverter or backlight.
If you have to guess, I would try replacing the inverter first.
May 25th, 2009 at 12:49 am
I have a Dell Inspiron 1501 laptop where the LCD goes black at apparently random times after being turned on. The computer remains on and apparently continuing whatever functions it was working on, but the monitor is dark. Sometimes it will only stay on for a minute after bootup and other times I can use it for 10-20 minutes before it goes dark. I am pretty confident its not a battery issue and it shouldn’t be a hibernation or sleep issue, since I am always actively using the computer when this happens.
I’ve seen other people on the internet with similar laptop complaints, but I can’t seem to pinpoint a common cause. Could this be a bad LCD inverter?
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.