The BenQ T705 LCD Monitor has a common fault of display shutdown after few you "On" for a few seconds. The cause of the problem was a short circuit is usually one or two of the C5707 push and pull transistors. Besides the shorted transistor, one has also to the tuning capacitor for high ESR value with the help of ESR meter to check. If you do not own one, you can use the digital capacitance meter to check the capacity value. But from my experience, checking only the capacitance value does notGuarantee that the capacitor is good if you confirm it with ESR meter. The reason why I was focusing on the review of this sentiment, because the capacitors capacitors have a high failure rate that could have shorted the C5707 transistor will cause. They have also carefully monitor the high voltage transformer pins. I had come through a variety of causes C5707 transistors shorted by dry joints pins of the high voltage transformer.
In this case, as usual, after I have confirmed that onlyC5707 shorted two transistors, I began to test the tuning capacitors and check the pins of the high-voltage transformer. Both being the tuning capacitors found in order, but it was dry joints in the transformer pins. Fresh Lot was to apply the high voltage transformer pins, and I am ready to "On" the LCD monitor. Guess what? The monitor has the same symptoms again the display shutdown after few seconds! I do not think so, because most of the time it is safe because this is a joint workFault!
I started to the surrounding components that connect to the C5707 transistors have, but found no fault in check again. The analysis of the Board many times, there were no improper soldering job. There must be something that had gone wrong, that the LCD monitor has led to the shutdown. Deep in my heart I guess no backlight problem, because the C5707 transistors shorted was caused by dry joints in the high-voltage transformer. In addition, the backlight has been very reliable for thisBenQ LCD Monitor model.
When I think back, I bought myself a new badge of the C5707 transistors from my electronic suppliers not long ago. The middle pin (leg) is slightly shorter than the other two from the pictures shown below. I thought to myself it would have this badge of transistors, which is causing the problem, the display shutdown after few seconds? Of course I can not blindly assume the transistors, if I have a proof of it. The statement begins ..................
Test the C5707 transistorcontact with an analog meter to X 1 and X 10 K ohm does not seem like it has the problem. Comparing the results with an original C5707 transistor has some differences in the X-10 K ohm range have, but the reading is from my years of experience in testing transistor acceptable. The only thing I have not compared, the Hfe reading. I have the famous Atlas Peak-component analyzer for both transistors, and I could see clearly, analyze the big difference between the readings of the two harp transistors.The original C5707 transistor has Hfe value of 379, whereas for the new badge of the C5707 transistor Hfe a value of 161! Finally I found the answer!
I have tried to replace the 4 C5707 transistors in one fell swoop with the new character of the transistors, but the result was the same. Solved Install the transistors, the Hfe reading of 350 to 400 have the display shutdown problem. This circuit looks can only work with the same type of transistor that has exactly Hfe value! Therefore, forto find this kind of monitor circuit, many technicians hardly a suitable replacement. In other words, even if you could substitute the transistor, which will receive a higher volts, watts and amps, but with different Hfe value, to awaken you not yet in a position, the LCD monitor back to life. From the above test, this monitor circuit voice loud and clear that it is only the same part number with the same Hfe value if a similar transistor replacement specification haveidentical with the original.
For my readers that in this electronic repair consistent, even if some of you might not be repaired on the LCD monitor, I hope you will absorb all this information and apply it in your area. Who knows, one day you could be facing the same problem that I am, and this article could easily help you quickly solve your problem. Do you have a good and a blessed day!
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