lcd monitor backlight problem

How To Repair 19" LCD Monitor Dim Display Problem That Are Using 4 CCFL Backlight

The backlight inverter takes a DC input and converts it to a high voltage, alternating current source to supply power to the florescent lamps. The four lamps are required to view the LCD. If the inverter have problem it will cause dim display problem. Below is a backlight inverter that is inoperable.

To test the inverter, connect four lamps and the Monitor Inverter Supply as shown.

lcdbacklightproblem

Turn the supply on and the lamps should light. The lamps did not come on with this inverter. Below is a close-up of the switching circuit that feeds the two MOSFET switches, N2+P2 and N1+P1. These two transistors supply the signal to the two transformers primaries. The first component I check is the power fuse, F1. It was reading open. A common failure in these inverters is shorted switching transistors. There is a lamp current monitor circuit designed into the unit. This monitors the lamp current and shuts down the BIT2023 controller to protect the transformers if no current is detected, so we are limited to static resistance reading for troubleshooting. Once the fuse was checked, I started measurements on the switching transistors and found Q4 to be shorted drain to source.

lcdmonitorbacklightproblem

Remove F1 and Q4. An easy way to remove most components is with a product called CHIPQUIK. It’s a solder that has a low melting point. Apply a little to the leads and the CHIPQUIK stays molten long enough for you to heat all sides of a component. Once the components are removed, use a desoldering braid to remove the excess solder. Clean the solder pads with alcohol and a q-tip.

lcdmonitorproblemininverter