Normally for this BenQ FP71G+ LCD Monitor although model of Q7T4 similar to complaint was display appears for a while and then shutdown (cut off) however for this unit the complaint was dim display! Element you don't have the opportunity to see the bright BenQ logo the moment power is 'On'. For look at the screen closely you could actually see the picture other than the display is very dark. If you often see the display, this helps to ensure that the Main board, power board and the LCD Panel are working!
There could be two reasons, it is either the backlights have problem or the inverter board is faulty. Since almost all of the LCD Monitors planet market have top and bottom backlights, chances for the top and the underside backlights failed at liquids time is very thin out. Due to this description, I've eliminated backlights as why it's happening of the problem. So my concentration will get in the inverter board. This assumption came across my head even before the LCD Monitor cover was taken down! It is good to assume and expect which circuit that causes the problem as this will keep you going even more if the assumption is correct. Many suggestions came out at a mind and you have to decide for the best. If the assumption is wrong, then you have request yourself is there anyway you can improve thought. Don't put too much pressure on yourself if the assumption is way faraway from what you have expected, as learning from our own mistakes is the best guide for us!
The moment the metal casing was removed, I could clearly see a burnt mark surrounding a transistor located at the inverter board area. My assumption was right about the inverter circuit fault and upon close inspection; the slightly dark transistor have the part number of FQU11P06. It is a P-channel FET transistor and it can be replaced with part number FU9024N. The area now has darkened and in case you directly install a fabulous transistor in it, I do believe the darkened area could cause intermittent problem or total destruction again to the actual transistor! Why? It is mainly because the darkened area has started to become carbonized and current can flow to other circuits causing the transistor to spend time visiting shorted again. In order to eliminate this problem, one has to scrap off the darkened area and due to the board is so fragile; the only way you are do is to make one hole in will probably have. Clean the area with thinner and install fresh transistor and it should not have problem after your.
Another thing that arises in my mind was is it the transistor itself has problem that goes shorted and burnt the board or other components failure that caused the transistor to go shorted? I began to use the surrounding components possess relation with the burnt transistor and found the tuning capacitor ESR value have increased, one of the push pull transistor (C5707) shorted and the Pico fuse open circuit! Upon close inspection of the capacitor, I could see bulged at one side of the capacitor. The significance of this bad plastic film polyester type capacitor is .22 uf 160 Volts. If you test this capacitor with more recent digital type capacitance meter, it would show about .2 uf which you may think is acceptable. If you didn't replace this capacitor, the newly installed transistor (FQU11P06) would surely burn again. For your information, any non polarity capacitors that have the value of or higher than out.1uf (104), besides test it with digital capacitance and analogue meter, I will also employ ESR meter to look at the ESR value of the capacitors too. In previously mentioned case, the bad cap has 47 ESR Ohm and the good is attempting 7.2 ESR Ohm!
Do you know that, if you owned either the dick smith or the Blue ESR meter by bob parker, besides testing the electrolytic capacitor, should also use it to check on on the non polarity capacitor too? But the non polarity capacitor value must me around can.1uf or higher. The ESR meter can't test any value which is lower than that. If you don't have any ESR meter, I guess this may be the time now to invest one to speed your current repair. With the aid of ESR meter, I have solved lots of intermittent problem caused by capacitors. Some intermittent capacitor problem can't be revealed together with help from applying freezer and heat but surprisingly it can be discovered faulty by using ESR meter- this really amazed me! If you desire to buy the Blue ESR meter, you can please click the link here Blue ESR meter.
Next, checking around all the circuit, I found another electrolytic capacitor have problem too. The faulty capacitor is located at the secondary output side and act as a filter to remove ripple contemplate output line. A good 1000 uf e-cap require ESR less than .1 Ohm but the faulty one read 4.9 ESR Ohm! A new replacement with the Pico fuse, C5707 transistor, FQU11P06 FET, 1000uf 16Volts e-cap and .22uf 250V metallised polyester capacitor brought the LCD Monitor for you to life again.