The complaint for this DELL E152Fpb LCD Monitor was when switched on has power (LED lit) but no display. LCD Monitor is different from the CRT Monitor where you could feel the static at the front of the picture tube or hear the rushing sound from the high voltage produced by the flyback transformer. In LCD, if no display you won’t be able to know if the inverter board is working or not unless you use an oscilloscope to point near the inverter board and the scope could display a waveform indicating the inverter board is functioning.
A faulty main board may cause no display problem in LCD Monitor. If there is a problem in the main board, it would not send a “on” (enable) signal to the soft start circuit and no voltage will flow to the inverter IC thus no waveform produce by the high voltage transformer.
In the above case, after the cover was removed, the first thing I check was the vcc (supply voltage) to the inverter IC. This model is using the famous TL1451ACN Pulse width modulation (PWM) IC-some preferred to call it as inverter IC. Pin 9 is the VCC supply input and it should have about 12 Volts when the power is “on”. Measuring the pin with my faithful digital multimeter, it reads only about 3 volts which indicates there’s a problem in the supply line. It could be components that are faulty that dragged down the line or it could be the switch mode power supply itself that don’t produce enough power.
In order to find the answer as which section is giving problem, the first clue was that the power LED indicator lit and this prove that the IC is supplying enough power to the circuit. Another clue was that the meter is registering about 16 volts at the input of the 2A fuse. Because of these clear answers, we can conclude that the switch mode power supply is working fine. You can also use a 12 volt light bulb to confirm it. You may read my other post on LCD Monitor repair articles which talks about using 12 volt light bulb for troubleshooting purposes.
I kept on asking myself, why the voltage drop to 3 volts after the fuse. Before the fuse it reads about 16 volt but after the fuse it reads only 3 volts! I didn’t suspect the fuse, because the fuse can only either yes or no and no in between. The first circuit that I checked was the start circuit but all components checked to be okay. I even desoldered the pin 9 of TL1451ACN inverter IC just to make sure it is not this IC that pulled down the voltage.
This problem really challenges my troubleshooting and repairing skill, because almost all suspected components already checked to be working. Even the start circuit transistors (C945 & A733) I direct replaced, afraid they would breakdown when under load. The only component that I didn’t check was the fuse! By using my meter placed across the fuse, instead of getting low ohms or zero ohm reading I got 1.012 Kilo ohms! I‘ve got a surprised and don’t believe that a fuse can actually go into high ohms instead of open circuit. I always thought that a fuse is either close or open circuit. A fuse with zero ohms turned into high ohm thus the 16 volts dropped to 3 volts! I’ve came across lots of this type of fuse but this is the first time a fuse changed into high resistance.
Replacing the fuse only put the DELL E152Fpb LCD Monitor back to life. The lesson learned from here is that don’t assume that a component is always in a good working condition. You have to confirm it with your meter and if you still suspect the component is giving problem, simply replace it with a known good one and retest the equipment. Semiconductors can breakdown when under full operating voltage.
Jestine Yong is a electronic repairer and a writer, for more LCD Monitor repair information, please visit his website at http://www.electronicrepairguide.com
Source http://ezinearticles.com
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