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.
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August 11th, 2009 at 5:54 pm
I want to know if a bad inverter or floresent tube can give low
brightness (about 20% to 25%) I can adjust the brightness with
color settings and get it passable but the laptop (QOSMIO F25)
should be quite bright!
I have orderd a new inverter but I just wanted to pick you brain.
August 11th, 2009 at 5:02 pm
cj2600
thanks for responding to my query it looks like i misread the article above as your advice seems to be the same as was mentioned. Apologies!! I would also be interested to know how to trouble shoot problems with the incoming power i.e. i have seen laptops with faulty dc jacks which i have resoldered and still did not work is it possible to test the board at a given point and determine where the fault may lie or would you need a board schema?? Perhaps this could be a subject for a future article??
Regards
Paul
August 8th, 2009 at 1:55 pm
PaulQ,
You can test the incoming voltage with a multimeter. When you take a look at the inverter side oriented to the cable (not LCD backlight), you’ll see a few a few connectors.
One of them could be marked pin 1. You’ll have to measure voltage between pin 1 and ground. It could be somewhere between 10-20VDC.
Be very careful, do not short the inverter with the voltmeter.
August 4th, 2009 at 3:58 pm
Hi, found this site a number of months ago and regularly use it as a reference point, keep up the good work. I would be interested to know how i can test if the inverter is getting the required voltage from the motherboard? I am sure this must be possible but i am inexperienced in testing electronic components.
July 30th, 2009 at 10:20 am
Hey, thanks for the GREAT info. Please dont ever get rid of this site.
July 30th, 2009 at 7:02 am
i have a Fujitsu Siemens laptop, water poured on it i was able to dry it up and have it working but the voltage to the LCD inverter is 1.7 which i know should not be so, this has made my back light not to work, i will like to know what the possible back light voltages are so if i can tap from a point in the laptop to have it work. i am running a 14.4v batt.
July 28th, 2009 at 7:27 am
Hi,
Nice site to stumble upon.
I have a Laptop here with a strange problem.
its a Toshiba Equium m50-164 and the screen brightness seems to be backwards.
the Fn key for brightness works backwards so up is for lower and down is for higher.
now the problem that creates is that the screen brightness will not go up full (as the button is meant for darker and wont darken enough to fully turn off the inverter)
any ideas??
July 28th, 2009 at 3:09 am
Dell model no:pp01s lobtop power on and off when tested power 18.5v shows be in components all matherboard.doide 330j12 has no power.
July 25th, 2009 at 3:30 pm
This is great info. I’m a PC tech who does a lot of work for a lot of clients (sometimes I think too many).
I often left the LCD repair up to guess-work (whether it was the inverter or the LCD itself). For the most part I guessed (or deduced it correctly), but those ones I guessed wrong on were costly and time consuming mistakes I know I could have avoided.
I used to work with a group of technicians who would hoard their knowledge and never share what they knew with other techs. Needless to say nothing got done very well in that shop, I’ve since struck out on my own and have been doing very well.
Your little blog was very well written, informative and extremely helpful. I thank you for sharing your knowledge.
July 25th, 2009 at 10:18 am
I have a Dell Inspiron 1720 with Vista. If I just turn it on, I get a message to press any key to boot from floppy. It will only boot upcorrectly when I press the F8 key as soon as the DELL logo appears. When I ran diagnostics it gave error code 0322 and said the error was in accessing the LCD inverter. As long as I boot from the F8 key my LCD screen is fine, sharp and clear. It does not fit the description of a bad lcd inverter. Does anyone have any suggestions?