Today we will look at repairing the Samsung SyncMaster 720n monitor. So, when the monitor is turned on, there is no picture, but the power diode blinks. To diagnose a broken monitor, you need to disassemble it.
The first thing we do is unscrew the monitor leg.
Using a mediator or a slotted screwdriver, remove the front panel.
Disconnect the cable from the settings buttons panel.
We remove the cover.
Turn off the backlight and unscrew the 4 bolts.
We disconnect the cable from the matrix and remove the cover with the power supply and inverter board, as well as the control board.
We unscrew the board of the power supply and inverter.
Visually inspect, the capacitors on the power supply are not swollen, as often happens on such monitors. I measured the resistance of the inverter transformers, it is the same on two transformers, it is unlikely that they both failed. These are the two most common diagnoses for these monitors (Samsung SyncMaster 720n, 740n, 940n).
I also check two voltage stabilizers on the monitor control board (this problem in the form of non-working stabilizers also occurred not for the first time). In order to measure voltage stabilizers, we assemble a monitor without a case and connect it to the network while placing a dielectric under the boards (I use glass for this).
We check the 3.3 volt voltage regulator (AS1117L-33). We measure the input voltage 5v everything is fine.
We measure the output voltage, which should be 3.3 v. When measuring this voltage, the multimeter showed 3.2 v, which is in the normal range.
We check the voltage regulator 1.8v (AS1117L-18).
We measure the input voltage 5v everything is fine.
We measure the output voltage and everything is not so smooth here, the multimeter shows 4v instead of 1.8v, the stabilizer is inoperative and needs to be replaced.
We solder the voltage regulator to 1.8v.
Solder a new stabilizer.
After, we assemble the monitor, also without a case.
Connect to the network and turn on the monitor. And what we see is that the lamps light up, and the power diode stops flickering.
Electronic repair technicians know that troubleshooting takes up the lion's share of all repair time. This material will help to significantly reduce this time and repair the LCD monitor without delving into its circuitry details, but only using external signs of a malfunction. In preparing the article, materials from the MONITOR forum were used, where repairmen share their rich experience.
Common Common LCD Monitor Problems
The image is present, but there are the following defects:
Narrow vertical and/or horizontal stripes appear on the image. The defect is caused by a violation of the contact between the contact pads on the flexible loops of the decoders and on the LCD matrix crystal. The defect can be "floating" and is eliminated only with special equipment and materials;
The image shows wide vertical and/or horizontal stripes. This defect is caused by a violation of the contact between the pads on flexible cables connecting the graphics controller board (scaler) and the contact pads of the decoders located on the LCD matrix. Sometimes one of the decoders fails, or the IC of the LVDS transmitter (on the scaler board) or the receiver (on the LCD matrix). Except for the last case, such malfunctions cannot be eliminated.
Instead of the image, chaotic stripes are visible. The supply voltage of the LCD matrix or its individual components (LVDS receiver, decoders, crystal) is too low or too high, the DC power converter of the LCD matrix is faulty, there is no RESET signal on the IC of the LVDS receiver, or the IC itself is faulty, in addition, a board defect is also possible scaler.
A black raster is visible instead of the image. No LVDS signal, receiver defective. The reason can be both in the matrix and in the scanner board with a 50/50 probability.
Instead of an image, a white raster is visible. There is no LCD matrix supply voltage - a fuse break in the circuit (spontaneous or due to a short circuit), an open in the DC converter power circuit, in rare cases the scaler board is the cause (there is no control signal on the key that switches the power supply, or the key is faulty).
The image is present, but there is "noise" on the entire raster, the predominance of some color (with interference) . As a rule, this is caused by poor contact of the LCD matrix cable, but a defect in the matrix, scaler board, and power supply unit (PSU) is not excluded. In some cases, the cause is leakage through the factory flux / varnish under the SMD components in the LVDS IC circuit (both on the matrix and on the scaler board).
Disappearance of the LCD panel backlight in 1...2 seconds after switching on.
First of all, you need to check for compliance with the norm the inverter supply voltage (usually 12 ... 15 V), the turn-on signal (3 ... 5 V) and electroluminescent lamps (CCFL) by replacing them with known good ones.
In the event of a CCFL malfunction (one of the lamps is broken, the wire is completely burnt out from the "cold" (low-voltage) contact of one lamp), they are replaced or the contact is restored. In some models of LCD matrices, the low-voltage wire is glued to the lamp body with metal foil, the edges of which cut through the insulation - just remove the foil.
Here are the typical defects of the CCFL inverter:
Breakage of the high-voltage (HV) winding of one of the transformers (in some cases, a breakage directly at one of the terminals);
Breakdown of key transistors in one of the output channels of the inverter (may be due to leakage in the resonant capacitor);
Breakdown of the key transistor of the DC converter in the 1st stage (in the case of a 2-stage inverter topology) due to short-circuited (short-circuit) turns in the inductor (choke) of the converter;
Short circuit turns in the HV windings of the inverter transformer: A short circuit in the primary winding can be eliminated by impregnating it with epoxy resin (with less than usual amount of hardener). A short circuit in the secondary winding cannot be restored; it must be rewound.
Disappearance of the backlight after an indefinite time (from 10 ... 20 s to several days). Very often the cause is a defect in the lamps - the burnout of the "cold" contact (typical for AU Optronics matrices) or the presence of metal foil (see above), or this is a defect in the inverter itself. You can clarify the cause using external CCFLs.
Reddish tint of the backlight, uneven illumination of the LCD matrix (on the side of the "hot" contact CCFL is lighter). In most cases, the lamps have reached their end of life and must be replaced.
In rare cases, the cause is an underestimated supply voltage at the output of the inverter.
And now let's move on to the consideration of typical defects of specific monitor models.
Typical defects of specific models and brands of LCD monitors
Acer AL1716As, Samsung 720N
There is no image or it is severely distorted. A typical malfunction is a 1.8 V stabilizer of the AL1117 type fails, the output voltage becomes either less or more than normal. In the latter case, the TSUM16AL-LF scaler IC fails and the contents of the 25LV010 type Flash memory are distorted.
A similar defect occurs in Samsung 720N monitors, the image is strongly distorted and the scaler IC is very hot.
Acer AL1716A
The backlight disappears 2 seconds after switching on. When you turn it off / on again, the situation repeats. When viewed from one of the transformers of type 80GL17T-28-YS in the inverter circuit, the windings are charred. As a temporary option, you can remove the defective transformer and leave the backlight only on the top lamps. It is necessary to adjust the protection circuits of the controller by selecting current resistors in the circuit of "cold" lamp leads (about 2300 ohms).
Acer AL1521
When turned on, there is no image or disappears after a few minutes, only the LEDs on the front panel light up. The defect is the Schottky diode D201 (short circuit under load), it is replaced by an analogue with a working current of 3 A. Sometimes it is enough to solder this diode and the inductor in its circuit.
ASUS VW191S
No image, white raster. The monitor uses an LCD panel of the PV190WCM type with a supply voltage of 5 V, an IC of the AT1380AP type is used as a DC / DC converter. The fuse is broken on the panel due to a defective capacitor C42 (breakdown). After replacing the capacitor, the fuse stopped burning, but the image did not appear. It turned out to be a 10 mH choke in the open. It can be replaced or restored by rewinding - 15 turns of wire with a diameter of 0.1 mm.
Dell E197FP
The raster is white, there is no image. The monitor uses an LCD panel type M190EN04. Blown fuse in 5V circuit on panel board. The reason is the short-circuited capacitor C24 (in the 3.3 V stabilizer circuit). After replacement, the monitor works. The image periodically disappears, the backlight of the LCD panel. After 20 minutes warm-up, the monitor can work stably. The reason is a defect in the Flash-memory IC of the firmware type AT49F001NT. To be convinced of this, it is enough to warm up the IC a little with a hairdryer - the monitor defect is eliminated. It is necessary to replace and flash the IC. Working firmware can be downloaded from the Monitor website forum.
Dell E173FPc
Flashes, and sometimes the image completely disappears. The reason for the defect is poor soldering (burning of the leads) of the L201 inductor, and less often the second L202 inductor. Sometimes the image flickers when tapping on the monitor case. There were more than 15 cases of such a defect. Visually, ring cracks in soldering are almost invisible.
HP 1702
Ripples, jitter of the image on the screen up to its disappearance. The malfunction manifests itself unstable (once every 10 minutes or once a day). The cause of the defect is the filter capacitor C1 in the PWB-0706-01 board. Visually and with the help of an oscilloscope, a defective capacitor is not determined, but only by cooling the board with alcohol under the capacitor. There was a whole batch of such monitors.
LG L1953S
The raster is white, there is no image or slight image distortions (bands) are possible. The supply voltage of the LCD panel is 5 V, during control it is underestimated to the level of 4.3 ... 4.6 V. The filter capacitor C601 (leakage) and the transistor Q602 are faulty.
LG Flatron W2241S-BFT
The monitor does not switch from standby to working mode. The reason is the failure of the firmware of the IC EEPROM type 24C08. An attempt to restore the codes FF and 00 by pre-writing the IC in the IC did not help - the device starts up, the firmware version and the name of the processor are displayed on the screen and remains in this mode. Working firmware can be downloaded from the Monitor website forum.
LG 556LE
The monitor turns on after a few presses of the Power button, but there is no image, and the device switches to standby mode. The reason is a defect in the L102 choke, and, as a result, an underestimated voltage of 3.3 V, which supplies the I109 video controller. There were several cases.
LG Flatron L1740PQ
The monitor does not turn on, the PSU power supply based on the FAN7601 controller does not work.
When it is diagnosed on the output. 7 - a signal with a frequency of 1 Hz, a range of 6 ... 8 V, on pin. 3 - 0.3 V (DC), on pin. 2 - 310 V (DC), the voltage on the other pins is zero. The reason is the leakage of a 0.1 μF / 50 V SMD capacitor (R osr \u003d 200 Ohm) connected between ground and pin. 8.
Monitors LG and SAMSUNG
A number of models have poor build quality. Often, the backlight disappears after 1 ... 2 s, it is eliminated by soldering the output circuits of the inverter (of course, provided that the PSU, control circuit and CCFL are in good condition).
On some models of LCD matrices (for example, M170EG01, M170EN05, QD17EL07, AUxxxEN05 (the latter is used in PHILIPS monitors), the low-voltage wire is glued to the lamp body with metal foil, the edges of which cut through the insulation - just remove the foil.
In older models of SAMSUNG monitors, a frequent malfunction is the operation mode indication LED flashes briefly and there is no image; when the LCD matrix is turned off, the indicator works normally. The problem lies in the filtering capacitors (leakage) of the 3.3 and 5 V secondary power supply, the elements of which are located on the scaler board.
NEC 1701, Mitsubishi NX76
The backlight disappears. These models use an inverter of the PTB-1427 type, the printed circuit board of the inverter is of very low quality - the contact in the vias periodically disappears, as a result, the defect appears periodically. To localize the defect, it is necessary to treat the board with flux and heat it with a hair dryer at 200°C. The break becomes stable and is easy to find with an ohmmeter.
Proview 700P (SP716P)
When the power is turned on, the LED indicator glows green, the monitor does not respond to the Power button, there is no image. The reason is a defective quartz resonator of the processor with a frequency of 14.318 MHz.
Philips 170S7
The monitor spontaneously switches to "sleep" mode, the yellow indicator blinks. The reason - the MICOM processor is faulty - firmware failure, it is read by the programmer with different checksums. It is necessary to reflash MICOM.
The screen has a white background. When checking, the LCD matrix does not receive power. When the power is turned on, the collector voltage of the 12V transistor Q406 drops to zero. When checking with an ohmmeter, leakage was detected between the collector and emitter of Q406 (type PMBS3904).
The screen has a white background. There is no voltage of 5 V on the LCD matrix from the output of the key on the Q405 transistor. At the base of Q406, the voltage is 0.2 V - it is locked. The reason is the capacitor C425 (100 nF) is faulty.
Roverscan JS588
The backlight disappears with warming up (10 ... 60 minutes). The screen may flicker before turning off. The cause of the malfunction is defective 1 uF SMD capacitors (leakage when the temperature rises) connected to the pin. 6 and 11 of the BA9741 inverter controller. The second reason is the increase in the value of SMD resistors R951, R956.
Samsung 740N LS17HAAKS
No image. The monitor responds to the power button. As a rule, MICOM processor memory is read and written by standard means. Faulty IC scaler SE16AWL. In total there were more than 10 cases of such a defect.
Samsung 920NW and similar widescreen models
The backlight of the panel disappears, and when replacing the inverter, the defect may not appear. In fact, the defect is not in the inverter, but in the LCD panel itself, more precisely, in the backlight lamps. If you look at the back of the panel, then, as a rule, in its upper right corner, at the end, you can see a white insulating overlay on the ends of the CCFL. Without disassembling the panel itself, you can carefully cut it, remove this insulation, get to the terminals of the lamps and see the very reason - the black wire breaks (burns) more often. It is better to solder both terminals of the lamps. After that, it is desirable to fill the conclusions with sealant.
Samsung SM 940N
The monitor uses an IP-board type BN44-00123E. A frequent defect of this board is the failure of the TMS91429CT type transformer in the backlight inverter circuit. It is better to replace it, in extreme cases - to disassemble and rewind. The transformer is quite simply disassembled after 5 minutes of boiling or using solvent 646. The secondary windings consist of sections of 3 x 460 + 4 x 97 turns of copper wire with a diameter of 0.04 ... 0.05 mm. After winding, the windings are covered with auto varnish (acrylic varnish in an aerosol).
Internet sources
1. Forum site MONITOR http://monitor.net.ru/forum/download.php?id=103206
2. Forum site MONITOR http://monitor.net.ru/forum/download.php?id=133246
Here are the TOP 10 most common malfunctions of LCD monitors that I felt the hard way. The rating of malfunctions was compiled according to the personal opinion of the author, based on experience in a service center. You can think of this as a universal repair guide for almost any LCD monitor from Samsung, LG, BENQ, HP, Acer and others. Here we go.
I divided LCD monitor malfunctions into 10 points, but this does not mean that there are only 10 of them - there are many more, including combined and floating ones. Many of the breakdowns of LCD monitors can be repaired with your own hands and at home.
1st place - the monitor does not turn on
generally, although the power indicator may flash. At the same time, the monitor lights up for a second and goes out, turns on and immediately turns off. At the same time, cable jerking, dancing with a tambourine and other pranks do not help. Tapping the monitor with a nervous hand usually doesn't work either, so don't even try. The reason for such a malfunction of LCD monitors is most often the failure of the power supply board, if it is built into the monitor.
Recently, monitors with an external power source have become fashionable. This is good, because the user can simply change the power supply in case of a breakdown. If there is no external power source, then you will have to disassemble the monitor and look for a malfunction on the board. in most cases it is not difficult, but you need to remember about safety.
Before you fix the poor fellow, let him stand for 10 minutes, unplugged. During this time, the high-voltage capacitor will have time to discharge. ATTENTION! DANGER TO LIFE if the PWM transistor is also burned out! In this case, the high-voltage capacitor will not discharge in an acceptable time.
Therefore, ALL before repair, check the voltage on it! If a dangerous voltage remains, then you need to manually discharge the capacitor through an insulated one of about 10 kOhm for 10 seconds. If you suddenly decide to close the conclusions, then take care of your eyes from sparks!
Next, we proceed to inspect the monitor power supply board and change all burnt parts - these are usually swollen capacitors, blown fuses, transistors and other elements. It is also MANDATORY to solder the board or at least examine the soldering under a microscope for microcracks.
From my own experience I will say - if the monitor is more than 2 years old - then 90% that there will be microcracks in the soldering, especially for LG, BenQ, Acer and Samsung monitors. The cheaper the monitor, the worse it is made at the factory. Up to the point that they do not wash out the active flux - which leads to the failure of the monitor after a year or two. Yes, just as the warranty expires.
2nd place - the image flashes or goes out
when the monitor is turned on. This miracle directly indicates to us a malfunction of the power supply.
Of course, the first step is to check the power and signal cables - they must be securely fastened in the connectors. A flashing image on the monitor tells us that the monitor's backlight voltage source is constantly jumping off the operating mode.
3rd place - spontaneously turns off
after the time has elapsed or does not turn on immediately. In this case, again, three common malfunctions of LCD monitors in order of frequency of occurrence - swollen electrolytes, microcracks in the board, faulty microcircuit.
With this malfunction, a high-frequency squeak from the backlight transformer can also be heard. It usually operates at frequencies between 30 and 150 kHz. If its mode of operation is violated, oscillations can occur in the audible frequency range.
4th place - no backlight,
but the image is viewed under bright light. This immediately tells us about the malfunction of LCD monitors in terms of backlighting. In terms of frequency of appearance, one could put it in third place, but it is already taken there.
There are two options - either the power supply and inverter board burned out, or the backlight lamps are faulty. The latter reason is not often found in modern monitors. If the LEDs are in the backlight and fail, then only in groups.
In this case, there may be a darkening of the image in places at the edges of the monitor. It is better to start repairs with diagnostics of the power supply and inverter. The inverter is the part of the board that is responsible for generating a high-voltage voltage of about 1000 volts to power the lamps, so in no case do not try to repair the monitor under voltage. You can read about it on my blog.
Most monitors are similar in design, so there shouldn't be any problems. At one time, monitors simply fell down with a broken contact near the tip of the backlight. This is treated by the most careful disassembly of the matrix in order to get to the end of the lamp and solder the high-voltage wiring.
An easier way out of this unpleasant situation can be found if your friend-brother-matchmaker has the same monitor lying around, but with faulty electronics. Blinding from two monitors of similar series and the same diagonal will not be difficult.
Sometimes even a power supply from a larger diagonal monitor can be adapted for a smaller diagonal monitor, but such experiments are risky and I do not advise starting a fire at home. Here in someone else's villa - this is another matter ...
6th place - spots or horizontal stripes
Their presence means that the day before you or your relatives had a fight with the monitor because of something outrageous.
Unfortunately, household LCD monitors do not provide shockproof coatings and anyone can offend the weak. Yes, any decent poke with a sharp or blunt object will make you regret it.
Even if there is a small trace or even one broken pixel, the spot will still grow over time under the influence of temperature and voltage applied to liquid crystals. Unfortunately, it will not work to restore the broken pixels of the monitor.
7th place - no image, but backlight is present
That is, a white or gray screen on the face. First you should check the cables and try connecting the monitor to a different video source. Also check if the monitor menu appears on the screen.
If everything remains the same, look carefully at the power supply board. In the power supply of the LCD monitor, voltages of 24, 12, 5, 3.3 and 2.5 Volts are usually formed. You need to check with a voltmeter if everything is in order with them.
If everything is in order, then we carefully look at the video signal processing board - it is usually smaller than the power supply board. It has a microcontroller and auxiliary elements. You need to check if they are getting food. With one touch the contact of the common wire (usually along the circuit of the board), and with the other go over the pins of the microcircuits. Usually food is somewhere in the corner.
If everything is in order in terms of power, but there is no oscilloscope, then we check all the monitor cables. on their contacts. If you find something, clean it with isopropyl alcohol. In extreme cases, you can clean it with a needle or scalpel. Also check the board with the monitor control buttons.
If all else fails, then you may have encountered a case of a flashed firmware or a microcontroller failure. This usually happens from surges in the 220 V network or simply from the aging of the elements. Usually in such cases you have to study special forums, but it's easier to use it for spare parts, especially if you have a familiar karateka in mind who fights against objectionable LCD monitors.
8th place - does not respond to control buttons
This case is easily treated - you need to remove the frame or the back cover of the monitor and pull out the board. Most often there you will see a crack in the board or soldering.
Sometimes there are faulty or. A crack in the board violates the integrity of the conductors, so they need to be cleaned and soldered, and the board glued to strengthen the structure.
9th place - reduced monitor brightness
This is due to the aging of the backlights. According to my data, LED backlighting does not suffer from this. It is also possible that the performance of the inverter may deteriorate, again due to the aging of the constituent components.
10th place - noise, moiré and image jitter
Often this happens due to a bad VGA cable without an EMI suppressor -. If changing the cable does not help, then power interference may have entered the imaging circuits.
Usually, they are eliminated by circuitry using filter capacitances for power supply on the signal board. Try to replace them and write me about the result.
This concludes my wonderful rating of the TOP 10 most common LCD monitor malfunctions. Most of the data on breakdowns is collected based on the repairs of such popular monitors as Samsung, LG, BENQ, Acer, ViewSonic and Hewlett-Packard.
This rating, it seems to me, is also valid for and . What is your situation on the LCD monitor repair front? Write on and in the comments.
Yours sincerely, Pike Master.
P.S.: How to disassemble the monitor and TV (how to snap off the frame)
The most common questions when disassembling LCD monitors and TVs are how to remove the frame? How to release latches? How to remove the plastic housing? etc.
One of the craftsmen made a nice animation explaining how to disengage the latches from the body, so I'll leave it here - it will come in handy.
To view animation- click on the image.
Hi all. What to do if the monitor turns on and immediately goes out? Today I had a monitor for repair with such a problem that even a novice radio amateur can solve. But first things first.
So, a Samsung monitor, namely Samsung SyncMaster 940nw. The malfunction was that a picture appeared on the screen for a short time, for about 1-2 seconds, after which the monitor immediately went out.
This monitor uses what is called fluorescent CCFL backlight. In simple terms, this is a backlight in the form of llamas, similar to fluorescent lamps, only reduced tenfold. These lamps are ignited by a special inverter that supplies high voltage to their cathodes, which causes the glow of these lamps. Most inverters have a fault protection that is activated if any of the lamps fail, the inverter itself breaks down, or there is insufficient power supplied to it. For an ordinary user, the protection operation looks like this: the monitor turns on, at this moment the inverter analyzes whether everything is normal with the backlight, and if it finds a breakdown, it immediately turns off the backlight. This is what we see in our example. It remains only to find out what includes this very protection of the inverter.
Disassembly of this monitor is quite easy. First, unscrew the stand, which is mounted on 3 bolts. Everything else is held on by clips. To remove the frame, we drive some object between the body and the frame itself (I use a common card from the bank), and draw it along the perimeter of the frame, snapping off the latches.
There are control buttons on the frame, from which you need to carefully disconnect the cable that goes to the motherboard. Putting something soft on the table, put the monitor face down. On the left, we snap off the protective cover, and we get to the loops for connecting backlight lamps.
Monitor backlight connectors
As you can see from the image, this monitor uses four monitor backlights. Disconnect the lamps from the board. Now you can carefully lift the boards from the matrix. Next, we see a cable connected to the matrix, which, by pressing the latches on the sides, must be disconnected.
After that, the matrix can be put aside, and repair the boards.
The power supply is held on by 3 bolts, by unscrewing which, you can remove the board. Having removed the board, you can immediately see the cause of the monitor malfunction. It lies in three swollen electrolytic capacitors.
This is a very common breakdown of these monitors, the repair of which takes no more than 15 minutes. The swollen capacitors were nominal: 2 capacitors 820 uf and one 470 at 25 volts. I replaced them with two capacitors of 1000 microfarads each and one with the same one as it was 470 at 25v.
Reassembling the monitor in reverse order, everything worked.
I hope this article will be useful for you. Thank you for your attention.