If your LCD monitor or laptop screen is dark or dim and difficult to see, the problem is probably either the backlight bulb or the LCD inverter board. These days, backlight bulbs seldom fail; therefore, in most cases, the problem will be with the LCD inverter board. The most common problem with LCD inverter boards is a lack of power or a faulty capacitor that prevents sufficient voltage from passing to the backlight bulb. Using a few basic electrical tools, you can check the voltage output from the LCD inverter to the backlight bulb to determine if the board is faulty.
Turn off the LCD monitor or laptop. Disconnect and remove the AC adaptor.
Use an awl or small flathead screwdriver to remove the rubber screw hole covers from the outside edge of the LCD monitor screen. On some LCD monitors, the rubber screw hole covers may be located on the rear face of the monitor screen bezel.
Loosen and remove the retaining screws that secure the front and rear bezel pieces with a Phillips screwdriver. Use the awl to carefully pry apart the two pieces of the monitor screen bezel. Separate the two pieces, and then remove the front piece from the screen casing.
Unplug the backlight bulb wire on the right side of the inverter board. This wire is usually located at either the top or bottom of the LCD screen.
Reconnect the AC adaptor and power on the LCD monitor or laptop.
Set the selector switch or dial on the multimeter to display voltage measurements. Touch the solder pin labelled "P1" or "F1" with the red probe lead, which is positive. The correct pins are usually next to the cable connector that runs from the main circuit board inside the monitor. Touch the ground trace receptacle around the inverter board retaining screws with the black probe lead, which is negative.
View the voltage display reading on the multimeter. Depending on the model of laptop or monitor you own, the voltage from the circuit board to the LCD inverter should be between 12 and 20 volts. If the voltage to inverter board is lower than 12 volts, the main circuit board is not supplying enough voltage to the inverter.
Connect the wiring harness for the extra backlight lamp to the receptacle on the inverter. If the backlight lamp light turns on, the inverter board is functioning correctly. In this case, the backlight lamp is defective. If the backlight lamp does not light up, you must replace the inverter board.
Be very careful when testing the power and voltage for an inverter board. If you touch the wrong solder point, you may short or damage the circuit board for the LCD monitor screen.