No Power In HP LCD Monitor Repaired
This HP LCD Monitor came in for repair with the complaint of no power. As usual, in order to check on the circuit boards inside, one must remove the cover. Make sure that you remove the cover gently and do not use a test pen to peel out otherwise you will leave a marking and your customer may not happy with it. You have to use a very thin and strong metal to peel out the cover start from the below of the cover. Once the bottom cover is opened, you can start to peel out the side cover and finally be able to remove the whole cover.
There are three circuit boards in most of the LCD monitor. It consist of the mainboard, front panel control board and the power/inverter board or I/P board-see the photo below:
The main fuse checked to be good and when I powered it up and checked on the 5 volt output voltage there was no voltage. 0.05 volt was considered too small and negligible. This means the power supply board have problem.
So now I have to start troubleshooting starting from the beginning which is at the AC input area. Since fuse was tested good chances for bridge rectifier to go short circuit will be very rare. When I measured the big fat filter capacitor I noticed voltage was still present in the cap. I discharged the cap so that I can check on the board freely without have to worry about being discharged by the cap. It will give you a nasty shock if you accidentally touched on the capacitor pins/legs.
I just checked on some of the major components in the power primary circuit and all seems to be working fine.
So my next test was the secondary output schottky diodes. There were two schottky diodes and one of them was shorted when I tested it on board.
Since I could not find the actual part for replacement, I used another diode from a TV junk board to replace it.
Guess what? The LCD Monitor came to life again and I had tested this LCD Monitor for many hours burn in test and the power remains good-see the photos below:
This article was prepared for you by Suranga Bandara who owns an Electronics shop in Anuradapura, Sri Lanka.
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Note: You may check out his previous repair article in the below link: