Repairing Samsung 153V LCD Monitor With Dim Display Problem

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Published: 01st December 2009
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One of the common problems in Samsung 153v LCD Monitor was display dim. If you get closer to the screen and begin to observe the display under dark environment you actually could see the display but it was very dim. If you press the front panel button, the on screen display (OSD) would appeared and this proof that the main board, driver board and LCD panel is working and the problem is in the inverter board. The function of inverter board is to generate an ac wave from dc voltage to power up the lamps which was located at the top and bottom of the LCD Monitor. If the inverter board has problems it would not light up the lamps and what you could see only a dim display.

I still could recall when I first encountered such problems long time ago and because of lack of information, I could not solve it. I did call up my repair friends to asked about the solution but no one able to give me an answer. Even if you surf the internet for "Samsung 153V LCD Monitor dim display repair", you would not find it. Due to this I have to do my own research to find the answer and I've never give up. Winston Churchill once said "Never, never, never give up". I'm releasing the solution only to my loyal members or subscribers only-thanks for your support.

Let's start now! Once the cover and the internal aluminum casing were opened I saw two bulged filter capacitors at the secondary side of switch mode power supply. The value was 470 and 1000 microfarad 25 volts. I saw four more electrolytic capacitors located at the inverter board have changed color and the value was 47 micro farad 25 volts. Browsing through the whole board with my ESR meter I found another capacitor faulty (220 uf 25 volts) which was near the fuse (3amp). Since this capacitor was located at the same line with the fuse, I presumed that the fuse may give way as well. True enough, checking the fuse with an ohmmeter revealed that it has open circuit.

Now, the real problem faced by many electronic repairers who deals with this model was, after replacing all of the components above the LCD Monitor still would not work-it's still dim! First you have to ask yourself why the fuse blown? Could it be caused by the defective capacitors or something shorted beyond the fuse line? The nearest circuit to the fuse was the start circuit that consisted of a pair of NPN and PNP digital transistor (4k7 +4k7 resistor built into the transistor). Some other LCD monitor designed used the combination of C945 and A733 in start circuit. By the way the part number for Samsung 153V start circuit transistors were the A6J (PNP digital transistor) and the A8J (NPN digital transistor) as shown in the picture.

Sometimes only these transistors have problems (shorted) that caused the fuse to blow but in the above case both digital transistors are tested okay. Since both transistors were working, now the concentration have to be in the four ic's that drive the high voltage transformers because the 12 volts voltage was supplying to each of these ic's. These 4 ic's have many part numbers and the most common one that the manufacturer used was 4532m. Some board used the AD4E10 and BBD3N13 part number. At the moment it is quite hard to get the datasheet or technical specification for this number even from the web search.

Do you know what is inside these IC's? It is actually a pair of P-channel and a N-channel mosfet built into it. And do you know that it can be test with a meter? Yes, if you use the Peak atlas component analyzer, it would prompt to you the result. From the result what you get are N channel FET located at pin 1 (source), pin 2 (gate), pin 7 & 8 (drain and both pins joint together internally). As for P channel FET, pin 3 (source), pin 4 (gate) and pin 5 & 6 (drain and both pins joint internally). From the above test I found two of these ic's have shorted. Replacing these two shorted IC with the capacitors and fuse put the LCD Monitor back to life!

One more thing I would like to share on this model was that the inverter PWM IC BIT1305 was quite robust and rarely gave problem.

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