A question I am often asked is "How can I tell if the backlight or inverter has failed on my laptop/monitor as I can only just make out an image?"

The answer is that there are several causes of a dim display or the failure of a backlight to illuminate the screen. As most people are aware, inverter and backlight failure are the most common. Other causes are:- a faulty or damaged LCD/Inverter cable, Faulty motherboard and a faulty screen.

Firstly try the obvious!

There is a little switch usually located near the hinge which switches the display off when the lid is closed. This can sometimes get jammed and is easily overlooked. It might simply be a question of unjammimg it or making some sort of repair to it.

Most laptops are able to toggle between an external display and the built in screen. Sometimes the laptop can get confused so it is worth toggling between external and built in screen. As most have several different settings it is worth doing this several times. If the laptop screen flashes when you do this it would indicate that the inverter is trying to start the backlight. This means that there is a good chance that either the backlight or inverter has failed but does not totally rule out the other components.

It may also be worth resetting the bios as these settings can sometimes be lost especially if the cmos battery is on its way out or has been disturbed. 

Unfortunately, unless you are a qualified electronics engineer it is very difficult to test the individual boards and components and fault location is basically a case of substitution until the problem is cured or you decide not to spend anymore money on it!

Laptops usually only have 1 backlight so obviously if the backlight fails there is no illumination of the screen but the same result happens if the inverter or one of the other components mentioned above fails!

In the case of monitors, most monitors have 2 or more backlights and it would be unusual for all the backlights to fail at the same time so if you have no illumination of a monitor screen but the power light is green then chances are the inverter has failed. A failed backlight on a monitor will result in one half of the screen appearing brighter than the  other half.

BACKLIGHTS

Backlights can be tested by connecting them to a known good inverter. You can use one of the inverters they sell for case moding. Just provide a 12V input for it and connect your backlight to one of the output sockets. If your backlight illuminates then it is OK and the fault lies elsewhere. A pink or red hue to the screen is nearly always a sign of a failing backlight and if this happens it is well worth replacing the backlight as the only other thing that can cause this is a faulty screen.

Remember backlights work on an AC Voltage of around 1000v and although the wattage is very small can give you a nasty shock and could prove fatal if you have a pacemaker or similar.

INVERTERS

We now come to an item that can only be tested by a qualified electronics engineer or simply substituted. There is usually a small fuse located near the input connector. Check this for continuity. Depending on the inverter there can be several inputs but it is worth checking the input voltages. Basically you will have a positive and negative feed plus a switching voltage and a brightness connection. The basic voltage for powering the inverter is usually the same as you laptop input voltage but the switching voltage may be as low as 5 volts DC. The brightness voltage can be anything from 0 to 5 volts but normally full brightness is acheived with 0 volts so don't worry about this. Unfortunately inverters often have up to 20 or more connections and may incorporate circuits which show battery charge status, on/off status and almost anything else you care to name! As wiring diagrams are rarely available its impossible to know which connection does what. It is possible by a process of elimination to work out the main positive and negative terminals and you may often find there are several of these but the rest is pure guesswork. If you have an analogue multimeter, set it to the highest AC reading (usually 1000V) and check for an output from the inverter. Initial start up voltage will probably exceed 1000 volts but normal running voltage should be below 1000 volts depending on the size of your screen. Look for a voltage in the region of 400 - 900 volts. Obviously if you are getting a good voltage here then your backlight has failed. Otherwise I am afraid it is a case of replacing with a known good inverter to see if this cures the problem. (If anyone knows of a foolproof way to test an inverter please enlighten me)

LCD/INVERTER CABLE

Check this cable carefully for damage especially in the hinge area and make sure the connectors have not worked loose. It is possible for the wires to break internally with no obvious signs of damage so check the continuity of the cables. A faulty LCD cable can even result in your laptop not switching on.

MOTHERBOARD

The motherboard sends a signal to the inverter board telling it when to switch the backlight on. If there is a fault on the motherboard then the inverter board will not receive a signal and therefore not switch the backlight on. It may be possible to check the input voltages to the inverter board with a multimeter but if you have eliminated the previous items I have found this to be quite a commom fault.  NB If you connect your laptop to an external monitor and get a good picture this only proves that the video card is OK, there could still be a fault on the motherboard which is preventing your screen from illuminating.

SCREEN

It is possible for a faulty screen electronics to result in the backlight failing to illuminate. This is rare but can happen and if you have eradicated the previous items is the only possibility left.

I hope this guide has given you some help in repairing your laptop and if you are unclear about anything please contact me and I will do my best to help you. I am not an electronics engineer but like most of you enjoy fiddling with computers and laptops and its a great feeling when you do manage to repair it yourself!

DISCLAIMER

The advice given here is intended purely to help anyone attempting to repair their laptop or monitor and no responsibility can be accepted for any loss or damage wether consequential or not resulting from this advice.