حل أي مشكلة الطاقة في شاشات الكريستال السائل f1723 إتش بي

Normally
for this BenQ FP71G+ LCD Monitor with the model of Q7T4 the major
complaint was display appears for a while and then shutdown (cut off)
but for this unit the complaint was dim display! That means you don’t
have the chance to see the bright BenQ logo the moment power is “On”.
If you look at the screen closely you could actually see the picture
except that the display is very dark. If you could see the display,
this means 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 most of the LCD Monitors in the market have top and
bottom backlights, chances for both the top and the bottom backlights
failed at the same time is very slim. Due to this description, I have
eliminated backlights as the cause of the problem. So my concentration
will be in the inverter board. Now this assumption came across my mind
even before the LCD Monitor cover was removed! It is good to assume and
expect which circuit that causes the problem as this will motivate you
even more if the assumption is correct. Many suggestions came out from
your mind and you have to decide for the best. If the assumption is
wrong, then you have to ask yourself is there anyway you can improve
it. Don’t put too much pressure on yourself if the assumption is way
off 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 has 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 if you directly install a new transistor in it, I
believe the darkened area could cause intermittent problem or total
destruction again to the new transistor! Why? It is because the
darkened area has become carbonized and current can flow to other
circuits causing the transistor to go 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 can do is to make one hole
in the area. Clean the area with thinner and install the new transistor
and it should not have problem after that.




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 check the surrounding components
that have 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 value of this bad
plastic film polyester type capacitor is .22 uf 160 Volts. If you test
this capacitor with a digital 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 .1uf (104), besides test it with digital
capacitance and analogue meter, I will also use ESR meter to check the
ESR value of the capacitors too. In the above case, the bad cap has 47
ESR Ohm and the good one has 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, you can also use it to test
on the non polarity capacitor too? But the non polarity capacitor value
must me around .1uf or higher. The ESR meter can’t test any value which
is lower than that. If you do not have any ESR meter, I guess this is
the time now to invest one to speed up your repair. With the help of
ESR meter, I have solved lots of intermittent problem caused by
capacitors. Some intermittent capacitor problem can’t be revealed with
the help from applying freezer and heat but surprisingly it can be
discovered faulty by using ESR meter- this really amazed me! If you
want to buy the Blue ESR meter, you can click on 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 along the output line. A good 1000 uf e-cap
should have ESR less than .1 Ohm but the faulty one read 4.9 ESR Ohm! A
new replacement of the Pico fuse, C5707 transistor, FQU11P06 FET,
1000uf 16Volts e-cap and .22uf 250V metallised polyester capacitor
brought the LCD Monitor back to life again.

About C5707 Transistor Replacement In BenQ T705 (Model Q7T4) 17” LCD Monitor



This BenQ T705 LCD Monitor
has a common fault of display shutdown after few you “On” for few
seconds. The cause of the problem was usually a shorted one or two of
the C5707 push pull transistors. Besides the shorted transistor, one
also has to check the tuning capacitor for high ESR value with the help
of ESR meter. If you do not own one, you can use the digital
capacitance meter to check the capacitance value. However, from my
experience, checking only the capacitance value doesn’t guarantee that
the capacitor is good unless you confirm it with ESR meter. The reason
why I emphasis on checking this tuning capacitors was because the
capacitors have a high failure rate that could cause the C5707
transistor to go shorted. You also have to observe the high voltage
transformer pins carefully. I had come across quite a number of shorted
C5707 transistors caused by dry joints in pins of the high voltage
transformer.



In this case, as usual
after I have confirmed that only two C5707 transistors shorted, I began
to test the tuning capacitors and check the pins of the high voltage
transformer. Both the tuning capacitors found to be okay but there were
dry joints in the transformer pins. Fresh solder was applied to the
high voltage transformer pins and I’m ready to “On” the LCD Monitor.
Guess what? The Monitor has the same symptom again which is display
shutdown after few seconds! I don’t believe it as most of the time it
will surely work as this is a common fault!







I started to recheck the
surrounding components that have connection to the C5707 transistors
but found no faults in it. Analyzing the board many times didn’t reveal
any improper soldering job. There must be something that has gone wrong
that had caused the LCD Monitor to shutdown. Deep in my heart I don’t
suspect backlights problem because the C5707 transistors shorted was
caused by dry joints in the high voltage transformer. Beside, the
backlights were quite reliable for this BenQ LCD Monitor model.


Thinking back, I bought a
new badge of C5707 transistors from my electronic suppliers not long
ago. The middle pin (leg) is a bit shorter than the other two as shown
from the photo below. I thought to myself would it be this badge of
transistors that have problem causing the display shutdown after few
seconds? Of course I can’t blindly suspect the transistors unless I
have some proof of it. The finding begins………………


Testing the C5707
transistor with an analogue meter set to X 1 and to X 10 K Ohm doesn’t
seem like it has problem. Comparing the result with an original C5707
transistor did have some difference in the X 10 K Ohm range but the
reading is acceptable from my years of experience in testing
transistor. The only thing that I haven’t compared is the Hfe reading
(forward current gain). I used the famous Peak Atlas components
analyzer to analyze both transistors and I could clearly see the big
difference between the Hfe readings of the two transistors. The
original C5707 transistor has Hfe value of 379 while the new badge of
the C5707 transistor has Hfe value of 161! At last I found the answer!





I did tried replace the 4
C5707 transistors at one go with the new batch of transistors but the
result was the same. Installing the transistors that have the Hfe
reading of 350 to 400 solved the display shutdown problem. This circuit
look like it can only work with the same type of transistor that has
the quite exact Hfe value! That’s why for this type of Monitor circuit,
many technicians hardly can find a suitable replacement. In other
words, even if you could get the substitution transistor that have
higher Volt, Amp and Watt but different Hfe value, you still won’t be
able to bring the LCD Monitor back to life again. From the above test,
this Monitor circuit has voice out loud and clear that it only
need the same part number with the same Hfe value unless you have some
similar transistor replacement specification that is identical to the
original one. I will go back to my electronic supplier to tell them
about his problem and hope to get back the same type of C5707
transistor.



المصدر
المهندس احمد السيد جزاه الله الف خير