Viewsonic VG1930WM no power, repaired

I just finished repairing a Viewsonic VG1930WM that had no power, no standby LED, completely dead. I opened up the monitor and checked the output voltages from the power supply and both were present and stable. I verified that the 5V was still present at the main board. Next I checked the output from the 3.3V and 1.8V linear voltage regulator ICs on the main board and the 3.3V regulator's output was considerably low around 2.2V, the output from the 1.8V regulator was also very low but it gets its input from the 3.3V regulator so it makes sense that the 1.8V regulators voltage was low. I replaced the 3.3V regulator after verifying it was faulty and the monitor was still dead when I plugged it in to check voltages again, the 3.3V regulator's output was now within tolerance so I again checked the output from the 1.8V regulator and it was still low about 1.2V, so I then replaced the 1.8V regulator and when I plugged the monitor in to check the output from the 1.8V regulator I saw the standby light turn on, the voltage output from the 1.8V regulator was now within tolerance and the monitor turned on and functioned flawlessly.

Thanks to John Preher
Also don't forget to visit http://www.preher-tech.com/

 

Digital LCD TV Block Diagram

Here’s a block diagram of a Digital LCD TV and if possible you may print it out for future use. For your information I used to understand an equipment block diagram first before i go into the schematic and analyze the circuit functions.This is the first step if you wants to venture into new electronic equipment repair

Repaired BenQ FP93V 19″ LCD Monitor

The complaint of this BenQ FP93V 19″ LCD Monitor with P/N number 9JL1J72-VWM was no power. As usual have to check primary side first and found that four components have problem. It was a 2 ampere main fuse, .22 Ohm current sense resistor, power FET P7NK80ZFP, and PWM IC 1200AP40. Since i do not have the part number for the power FET, K2645 was used instead. After fixed in all of the components and power “On”, the LCD Monitor came back to life again. The monitor is now under burning in test and should not have any problem with substitution number. In fact i have used that part number to substitute into many different types of LCD Monitors. Here are the photos

Permanent Line On LCD Monitor

I got quite a number of emails asked whether one permanent vertical or horizontal line can be repair or not. The line is usually a white line. My answer to them is still “No” if you have confirm that there is no dry contacts between the LCD panel screen and the LCD controller board. If it is a dry contact, you can know it by pressing on the board area that produced the line (vertical or horizontal). The line problem in LCD Monitors are most probably cause by a defective LCD driver IC and there is no way we could replace the IC.


If i come across permanent line on LCD Monitor, i will send back the Monitor to customers (normally customer don’t want to repair because a used or a new lcd panel are very costly) or offer them a good price to buy over so that i could salvage the Main board and power/inverter board and in some models i could have their power pack too for future use

Solved No Power Problem In HP L1706 (PX849A) LCD Monitor

This Monitor came in with no power problem and found the main fuse 3.15 amp open circuited. As usual one has to find out whether other components have short circuit or not before you put a new fuse and retest the set. After scanning all of the primary and some of the major components in the secondary side of power supply i could not locate any shorted components. Placing a 100 watt bulb across the fuse and noticed that the bulb dims down and the big filter capacitor voltage did not drop even though the power jack already been removed.

Since the start up resistors are going to the power IC SG5841SZ (SMD IC) I suspect that this IC could be faulty causing the filter cap voltage remain high. Finding from my spare part compartment could not locate such part number thus i use SG6841SZ SMD IC as replacement to try it out. Before i put in the new IC, i did search from the internet and compare both spec and found that both of the IC’s were not much different from each other.

So the result was the LCD Monitor power came back a life after the replacement of the SG6841SZ IC and a fuse. This HP L1706 Monitor use some of the part numbers such as :

BIT3713 (INVERTER IC), 40P03GJ (FET), 2761I (Power FET), 817B (Optoisolator IC), 1506 (Voltage regulator IC) and AZ431 (Adjustable Precision Shunt Regulator IC). Please jot down in your diary book that the SG5841SZ can be replace with SG6841SZ

No Power In LCD Monitor

Whenever you switch “On” LCD Monitor and the Power LED indicator doesn’t lit don’t always assume it was the power supply problem. You must understand that the source of voltage to the power LED indicator was from the Mainboard particularly from the MCU. If the MCU have problem or the internal DATA had been corrupted then it won’t be able to send an “On” signal to the LED. This problem happened many times in the ACER and the BenQ LCD Monitor and replacement of the Main board solved the problem.

In order to isolate whether is the Mainboard or the power supply problem, you just need to measure the output voltages from the power supply. If you got all the voltages to the main board and the LED does not light then suspect a faulty Main board. If there is no voltages at all then suspect the power supply fault. Be remind that i had came across before the Power On/Off micro switch button failure and with a new replacement brought back the display-of course the LED lit! You can easily test the On/Off button using an ohmmeter or even with the help of continuity check

Two In One High Voltage Transformer

If you repair LCD Monitor you must have come across before the high voltage transformer that is located at the inverter board. Normally to drive the top and the bottom backlights/lamps, LCD Monitor designer usually use either one transformer to drive one or two lamp (top and bottom back light) or two transformers to drive the top two lamps and two transformers to drive the two bottom lamps. These designs are very common and could easily be found in most LCD Monitors.

However, since there are many choices for LCD Monitor designers, some prefer to use one transformer to drive 4 backlights! The transformer that you are seeing in the picture actually have two transformers! One transformer for the two top backlights and one for the the two bottom backlights. So please don’t be mistaken thinking of only one transformer! The most important thing is you must understand the theory of how the LCD Monitor inverter circuit work. Why? Because in the future no matter what kind of inverter designs coming out in the market or what kind of high voltage transformer used, you will still be able to test and repair the unit

One Vertical Or Horizontal Line Across LCD Monitor Screen

If you come across any LCD Monitor that have a permanent one vertical or one horizontal line across the LCD screen then most probably you need to replace the LCD Panel as both the problems can’t be repair due to the fault could be in the LCD driver IC, LCD Controller board or even the broken TCP package or problem with the TAB bonding.

One vertical or one horizontal line problem in LCD Monitor is totally different from when you are repairing the CRT Monitor. If there is one vertical line problem, you will be looking at the fault at the horizontal section or even with an open horizontal yoke coil (which is very rare) and if there is one horizontal line across CRT Monitor screen, then you will be concentrating on the vertical section. It could be a drj joints, bad vertical IC, missing supply IC, bad corresponding components or even missing the main waveform from the horizontal/vertical oscillator IC.

The problem in CRT Monitor is quite direct and you can solve the problem fast but this is different in the LCD Monitor as it contains more complex circuit as most of the signals running are digital signals. LCD Monitors have many databus lines carrying lots of digital signals. The conclusion is the only choice to solve the permanent one vertical or one horizontal problem is to directly replace the LCD panel

About LCD Driver IC In LCD Monitor

I have question from Friend asked whether we can repair/replace the LCD driver IC in the LCD Monitor? My answer to them was-not possible. Why? Because the LCD Driver IC was located in between the TCP package and solder gun will definitely melt the package. First by looking at how this IC constructed in the package, you would have already know that it can’t be repaired due to lots of tiny circuit lines. Those circuit lines could only be clearly seen by using at least a 10 magnifying glass. There are lots of them and there is no room for us to do the soldering job.

These LCD driver IC is not like our usual type of linear IC where we can easily read the part number , find the replacement and replacing it. It is just totally different from other ICs. Due to this problem, the best way to solve the problem that relate to LCD driver IC is to replace the LCD panel

How To Test Horizontal Driver Transformer

If you have the flyback tester then it would be easy for you to test the horizontal driver transformer. The testing method is the same as when you test on the smps transformer. The horizontal driver transformer primary winding is not as high as the smps transformer thus you cannot expect it to show many LED bar from the test. A good one sometime only shows one bar while to some it may go up to few LED bars.


Test also the primary and secondary winding with ohmmeter just to make sure the winding is not broken internally. Secondary winding have very less turn thus your result is the LED bar may not light up at all! Please take note of this and do not think that it has problem. I have repaired thousands of CRT Monitors and so far i have only seen few bad horizontal driver transformer and the problem was open circuit in the primary winding.

Well, hope with this repair tips, you can easily test horizontal driver transformer less than a minute. Take care bro

Philips 170s LCD Monitor Shutdown After “On” For A Second

This Philip 170s LCD Monitor was repaired by someone before and the problem was the display appeared for less than a second and then shut down. This usually prompt to a failure in inverter board or bad backlights. After opened the cover and test for the high voltage transformer, i found one of the transformer secondary winding has an open circuit. I tested it with ohmmeter. Even if you set your analogue meter to the highest range (X10 k ohm) there is still no reading!

Due to the no availability of the high voltage transformer spare parts i used the modification method to solve the problem. It took me about 15 minutes to completely finish the job. Now the Philips 107s LCD Monitor works beautifully

I came across many times in Monitor repair that the five band resistor had gone opened circuit. For your information, five band resistor value is more

This BenQ T705 LCD Monitor came in with no power symptom-power LED didn’t light up. The common fault for this monitor was short circuit in C5707 push pull transistor, FQU11P06 P-channel FET, high ESR in tuning capacitor .22uf 160v, dry joints in high voltage transformer and an open pico fuse. Normally after the replacement of the above parts, the LCD Monitor would start working again but this time “No” even though all the voltages now measured normal! I began to test the SMD Volt voltage regulator and found zero volt output. A-ha! at least i know the problem is in the Mainboard.


My concentration is 100% in the Mainboard and start checking why there was no voltage from the voltage regulator. Upon close inspection, i found two pins shorted together in the Scalar IC. I took a blade and separate the two pins (leg) from shorting and i got to use a magnifier glass to help me to do the job. To my surprised the LCD Monitor starting working when the front power switch was pressed! I don’t know why the two pins can short together perhaps it could be some touch up by repair technicians from other company. The reason i say that was because i could see a scratch mark underneath the front LCD cover

Could A 15″ LCD Monitor Drive A 19″ Backlight

There was a 19″ HP LCD Monitor came in for repair and the complaint was display shutdown after “On” for few seconds. From my experience, there are many cases of display shutdown due to defective a backlight. In order for me to confirm whether the backlights are good or not or if it the soucrce of the shutdown problem, i used a 15″ LCD Monitor to test it out since both of the LCD Monitor have the same type of backlight connector.



I removed the good working 15″ LCD Monitor backlight connector and use the 19″ LCD Monitor backlight connector to plug it in into the 15″ LCD inverter board. Guess what happen? The display works good and there is no flicker in the 19″ LCD display. I tested on both the top (2 backlights) and the bottom (2 backlights) and the 15″ LCD Monitor didn’t shutdown. This proof the 19″ LCD Monitor backlights have no problem and the display shutdown could be in the inverter board itself. It could be a defective inverter IC or surrounding components.

Although the 19″ LCD Monitor have display after plugged into the 15″ LCD Monitor inverter, the display was not that bright. This mainly due to the output ac voltage from the 15″ LCD inverter was not high enough to drive the 19″ backlight. Hope this repair tips can helps you in your LCD Monitor repair

Repairing BenQ 17″ LCD Monitor

I came across a common fault problem in BenQ 17″ LCD Monitor. The problem is have power but no display. Once you has opened the LCD cover, check the pico fuse for open circuit (very common), the push pull transistors C5707 for short circuit and the Buck royer circuit p-channel fet FQU11P06 also for short circuit too. For your information, if you can’t get the FQU11p06 you can always use FU9024N as a replacement. Resolder all the high voltage transformer pins and replace the components above and I’m sure you will bring back the life of this BenQ 17″ LCD Monitor.

You may go to this fairchild semiconductor website to download the spec of the FQU11P06 P-channel FET

LCD Monitor Common Fault

LCD Monitor common fault were display dim, no power, white screen,no display, rainbow colors, one vertical and horizontal line across display. Among all the problems, display dim is by far the most i had repaired in LCD Monitor. Inverter circuits is just like the flyback transformer/ high voltage circuits that involved high voltage. These are the area that usually breakdown first. If you have been repair the CRT Monitor you would know that HOT, flyback transformer and dry joints were the common problems.

A defective high voltage transformer, ballast capacitor, switching transistor/fet, dry joints, faulty lamps could cause LCD Monitor display to become dim. In the future if you encountered the display dim symptom, i believe you will know where to begin to tackle this LCD Monitor common fault problem

Samsung 713n LCD Troubleshooting

The common fault for this samsung 713n LCD Monitor was no display and OSD running with the word “Not Optimum Mode Recommended mode: 1280×1024 60Hz”. Many other users thought that it was setting problem but actually it was the fault of the microcontroller. You either replace it with a new IC that have programs in it or reprogram it with a programmer that support this 44 pins IC chip.

The shocking news is that not only this samsung 713n have this kind of problem, it also happen to SAMSUNG 153V and Samsung 510n! If all these SAMSUNG LCD Monitor came in with no display symptom, optimum mode, intermittent no display, and other weird problems then suspect this faulty microcontroller

UC3842 Test Circuit

UC3842 IC can be easily found in most of the electronic equipment switching power supply. Normally whenever the power fet shorted, chances are very high that the uc3842 pulse width modulation (PWM) IC may shorted too. Although sometime you can use your meter to measure certain pins of this ic to confirm whether it works or not , i found it to be not accurate. I personally had tested some of the defective UC3842 IC where it tested good with meter but failed when on load (in circuit).

AP4411 IC In HP Pavillion f1703 LCD Monitor

What type of ic is this AP4411 used in the inverter section of HP f1703 LCD Monitor? At current moment it is hard to find such datasheet or specification for this 4411 IC. It is actually a P-channel enhancement mode mosfet which built into the SOC 8 package ic where pin 4 is the gate, pin 1-3 are source and pin 5-8 are drain. You can test it with a meter if you want but because of the IC in small size, i have to use the peak electronic atlas analyzer to measure it.

This AP4411 IC is part of the buck royer inverter circuit in the HP f1703 LCD Monitor. If this IC has become defective, it will cause power to blink or even dim display

SG6841D PWM IC Internal Diagram

If you are repairing LCD Monitors, i’m sure you had came across this SG6841D PWM IC used in the power section. This IC function is the same as the famous UC3842 where it use to switch the power fet except that the output (gate) is from the fet’s (see the diagram). For UC3842 IC, it is from the transistors. Usually if you discovered that the power fet had developed a short circuit, the SG6841 PWM IC need to be changed.

Substitution For C5707 Transistor Use In Dell LCD Monitor

If you repair Dell LCD Monitor (E173Fpb and etc) you would know that most of the power blinking problem was due to a shorted C5707 transistor. This transistor is located nearby the high voltage transformer and when it have dry joints, the push pull transistor C5707 would go shorted. If you want to replace this transistor, you have to replace the other one also (because it comes in a pair to switch the high voltage transformer). The question is if you can obtain the original part number then it would not be a problem for you but what if you can’t get one? As for me i could source this part locally. If you can’t get the original part then the best is to get a substitution part number.

This morning while spoke to one of my repair friend in Miri Sarawak, he told me that he used C3303 as a substitute for C5707 transistor! I personally haven’t try it out but at least for those who do not have the original one can test and see. The spec for C3303 is a little bit lower than the original one but my repair friend told me it works! That’s the good thing about knowing lots of repair friends out there or in the market