Hi all. What to do if the monitor turns on and immediately goes out? Today I had a monitor being repaired with 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 briefly, for about 1-2 seconds, after which the monitor immediately went out.
This monitor uses what is called fluorescent CCFL backlight. To put it simply, this is lighting in the form of llamas, similar to fluorescent lamps, only reduced tenfold. These lamps are lit by a special inverter, which supplies high voltage to their cathodes, which causes these lamps to glow. Most inverters have fault protection, which is triggered if one of the lamps fails, there is a breakdown in the inverter itself, or insufficient power is supplied to it. For an ordinary user, the protection works 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 this very inverter protection includes.
Disassembling this monitor is quite easy. First, unscrew the stand, which is secured with 3 bolts. Everything else is held on by latches. To remove the frame, we drive some object between the body and the frame itself (I use a colloquial card from a bank), and run it around the perimeter of the frame, unlatching the latches.
There are control buttons on the frame, from which you need to carefully disconnect the cable going to the motherboard. After placing something soft on the table, place the monitor face down. On the left we snap off the protective casing and get to the cables for connecting the backlight lamps.
Connectors for connecting monitor backlight lamps
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 must be disconnected by pressing the latches on the sides.
After this, the matrix can be put aside and you can start repairing the boards.
The power supply is held on by 3 bolts, which can be unscrewed to remove the board. After removing 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 rated as follows: 2 820 mf capacitors and one 470 mf capacitor for 25 volts. I replaced them with two 1000uF capacitors and one with the same one as 470 at 25V.
After reassembling the monitor in reverse order, everything worked.
I hope this article will be useful to you. Thank you for your attention.
Electronic repair specialists know that troubleshooting takes up the lion's share of total 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. When preparing this article, we used materials from the MONITOR forum, where repair specialists share their wealth of experience.
Common common problems with LCD monitors
The image is present, but there are the following defects:
The image shows narrow vertical and/or horizontal stripes. The defect is caused by a violation of contact between the contact pads on the flexible cables of the decoder and on the LCD matrix crystal. The defect can be “floating” and can only be eliminated with special equipment and materials;
The image shows wide vertical and/or horizontal stripes. This defect is caused by a violation of contact between the pads on the 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, or the IC of the LVDS transmitter (on the scaler board) or the receiver (on the LCD matrix) fails. Except for the last case, such malfunctions cannot be eliminated.
Instead of an 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 for the LCD matrix is faulty, there is no RESET signal on the LVDS receiver IC, or the IC itself is faulty; in addition, there may be a board defect scaler.
Instead of an image, a black raster is visible. No LVDS signal, receiver defective. The reason could be either the matrix or the scanner board with a 50/50 probability.
Instead of an image, a white raster is visible. There is no supply voltage to the LCD matrix - a break in the fuse in the circuit (spontaneous or due to a short circuit), a break in the power supply circuit of the DC converter, in rare cases the cause is the scaler board (there is no control signal on the key switching the power, or the key is faulty).
The image is present, but there is “noise” throughout the entire raster, a 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, or power supply unit (PSU) is also possible. In some cases, the cause is a leak in the factory flux/varnish under the SMD components in the LVDS IC circuit (both on the matrix and on the scaler board).
LCD panel backlight disappears 1...2 seconds after switching on.
First of all, you need to check the inverter supply voltage (usually 12...15 V), the turn-on signal (3...5 V) and electroluminescent lamps (CCFL) for compliance with the standard by replacing them with known good ones.
If the CCFL malfunctions (one of the lamps is broken, the wire from the “cold” (low-voltage) contact of one lamp is completely burned out), 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 - simply remove the foil.
Here are typical defects of a CCFL inverter:
Break in the high-voltage (HV) winding of one of the transformers (in some cases, break directly at one of the terminals);
Breakdown of key transistors in one of the inverter output channels (may be a consequence of a leak 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 turns in the inductor (choke) of the converter;
Short-circuit turns in the high-voltage windings of an inverter transformer: a short-circuit in the primary winding can be eliminated by impregnating it with epoxy resin (with a smaller amount of hardener than usual). A short circuit in the secondary winding cannot be restored; it must be rewinded.
The backlight disappears after an indefinite period of time (from 10...20 s to several days). Very often the cause is a defect in the lamps - burnout of the “cold” contact (typical of AU Optronics matrices) or the presence of metal foil (see above), or a defect in the inverter itself. You can clarify the reason using external CCFLs.
Reddish tint of the backlight, uneven illumination of the LCD matrix (the side of the “hot” CCFL contact is lighter). In most cases, the lamps have exhausted their service life and must be replaced.
In rare cases, the cause is a low supply voltage at the inverter output.
Now let's move on to considering 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 very distorted. Typical malfunction - the 1.8 V stabilizer type AL1117 fails, the output voltage becomes both less and more than normal. In the latter case, the TSUM16AL-LF scaler IC fails and the contents of Flash memory type 25LV010 are distorted.
A similar defect occurs in Samsung 720N monitors: the image is greatly distorted and the scaler IC gets very hot.
Acer AL1716A
The backlight disappears 2 seconds after switching on. When you turn it off/on again, the situation repeats. Upon inspection, one of the 80GL17T-28-YS type transformers in the inverter circuit had charred windings. As a temporary option, you can remove the defective transformer and leave the backlight only on the upper lamps. It is necessary to adjust the controller protection circuits by selecting current resistors in the circuit of the “cold” terminals of the lamps (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 with an analogue with an operating 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 type PV190WCM with a supply voltage of 5 V; an IC of type AT1380AP is used as a DC/DC converter. The fuse on the panel is broken due to a defect in capacitor C42 (breakdown). After replacing the capacitor, the fuse stopped lighting, but the image did not appear. The 10 mH inductor turned out to be broken. It can be replaced or restored by rewinding - 15 turns of wire with a diameter of 0.1 mm.
Dell E197FP
White raster, no image. The monitor uses an M190EN04 type LCD panel. The 5V fuse on the panel board has blown. The reason is a short-circuited capacitor C24 (in the 3.3 V stabilizer circuit). After replacement, the monitor works. The image and LCD panel backlight periodically disappear. After 20 minutes of warming up, the monitor can work stably. The reason is a defect in the Flash memory microprogram IC type AT49F001NT. To verify this, just warm up the IC a little with a hairdryer - the defect in the monitor is eliminated. It is necessary to replace and flash the IC. Working firmware can be downloaded from the Monitor website forum.
Dell E173FPc
The image blinks and sometimes disappears completely. The cause of the defect is poor soldering (burning of the leads) of the inductor L201, and less often the second inductor L202. Sometimes the image flickers when you tap on the monitor body. There have been more than 15 cases of this defect. Visually, ring-shaped solder cracks are almost invisible.
HP 1702
Rippling, shaking of the image on the screen until it disappears. The malfunction appears unstably (once every 10 minutes or once a day). The cause of the defect is the filter capacitor C1 in the PWB-0706-01 board. A defective capacitor cannot be detected visually or using an oscilloscope, but only by cooling the board under the capacitor with alcohol. There was a whole batch of such monitors.
LG L1953S
The raster is white, there is no image or slight distortion of the image (stripes) is possible. The supply voltage of the LCD panel is 5 V, during control it is lowered to the level of 4.3...4.6 V. The filter capacitor C601 (leakage) and transistor Q602 are faulty.
LG Flatron W2241S-BFT
The monitor does not switch from standby mode to working mode. The reason is a failure of the EEPROM IC firmware of type 24C08. An attempt to restore the FF and 00 codes in advance by writing them to the IC did not help - the device starts up, the firmware version and processor name 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 pressing the Power button several times, but there is no image, and the device switches to standby mode. The reason is a defect in the L102 inductor, and, as a result, a low voltage of 3.3 V supplying the I109 video controller. There were several cases.
LG Flatron L1740PQ
The monitor does not turn on, the power supply unit based on the FAN7601 controller does not work.
When diagnosing it on pin. 7 - signal with a frequency of 1 Hz with a swing of 6...8 V, at pin. 3 - 0.3 V (DC), per pin. 2 - 310 V (DC), on the remaining terminals the voltage is zero. The reason is a leak of a 0.1 µF/50 V SMD capacitor (R osr = 200 Ohm) connected between ground and pin. 8.
LG and SAMSUNG monitors
A number of models have poor build quality. Often the backlight disappears after 1...2 s, which is eliminated by soldering the inverter output circuits (of course, provided that the power supply, control circuit and CCFL are in good working order).
On some models of LCD matrices (for example, M170EG01, M170EN05, QD17EL07, AUхххEN05 (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 - simply remove the foil.
In older models of SAMSUNG monitors, a frequent malfunction is that the operating mode indicator LED blinks briefly and there is no image; when the LCD matrix is turned off, the indicator works normally. The problem lies in the filter capacitors (leakage) of the secondary 3.3 and 5 V 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 inverter printed circuit board 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 hairdryer at 200°C. The break becomes stable and can be easily found using 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, and 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 and the yellow indicator blinks. The reason is that the MICOM processor is faulty - the firmware is faulty, it is read by the programmer with different checksums. It is necessary to reflash MICOM.
The screen has a white background. When checking, no power is supplied to the LCD matrix. When the power is turned on, the voltage at the collector of transistor Q406 drops from 12V to zero. When checked with an ohmmeter, a leak was detected between the collector and emitter of Q406 (type PMBS3904).
The screen has a white background. There is no 5 V voltage on the LCD matrix from the output of the switch on transistor Q405. At the base of Q406, the voltage is 0.2 V - it is locked. The reason is that capacitor C425 (100 nF) is faulty.
Roverscan JS588
The backlight disappears as it warms up (10...60 minutes). The screen may begin to flicker before turning off. The cause of the malfunction is defective SMD capacitors with a capacity of 1 µF (leakage when the temperature rises) connected to the pin. 6 and 11 of the BA9741 inverter controller. The second reason is an increase in the value of SMD resistors R951, R956.
Samsung 740N LS17HAAKS
No image. The monitor responds to the power button. Typically, MICOM processor memory is read and written using standard means. The SE16AWL scaler IC is faulty. In total there were more than 10 cases of such a defect.
Samsung 920NW and similar widescreen models
The panel backlight 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, or more precisely, in the backlight lamps. If you look at the panel from the back, then, as a rule, in its upper right corner, at the end, you can see a white insulating pad at the ends of the CCFL. Without disassembling the panel itself, you can carefully cut and remove this insulation, get to the lamp terminals and see the very reason - more often the black wire breaks (burns). It is better to solder both lamp terminals. Afterwards, it is advisable to fill the terminals 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, or, as a last resort, disassemble it and rewind it. The transformer is quite easily disassembled after boiling for 5 minutes 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 autovarnish (acrylic varnish in an aerosol).
Internet sources
1. MONITOR website forum http://monitor.net.ru/forum/download.php?id=103206
2. MONITOR website forum http://monitor.net.ru/forum/download.php?id=133246
Here are the TOP 10 most common malfunctions of LCD monitors that I have personally experienced. The rating of malfunctions was compiled according to the personal opinion of the author, based on his experience working in a service center. You can take this as a universal repair manual for almost any LCD monitor from Samsung, LG, BENQ, HP, Acer and others. Here we go.
I divided the malfunctions of LCD monitors into 10 points, but this does not mean that there are only 10 of them - there are many more of them, including combined and floating ones. Many of the breakdowns of LCD monitors can be repaired with your own hands and at home.
at all, although the power light may blink. In this case, the monitor lights up for a second and goes out, turns on and turns off immediately. In this case, tugging at the cable, dancing with a tambourine and other pranks do not help. The method of tapping the monitor with a nervous hand usually doesn’t help either, so don’t even try. The reason for this 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 supply have become fashionable. This is good because the user can simply change the power source in case of breakdown. If there is no external power source, you will have to disassemble the monitor and look for a fault on the board. in most cases it is not difficult, but you need to remember about safety precautions.
Before fixing the poor guy, let him stand for 10 minutes, unplugged. During this time, the high-voltage capacitor will have time to discharge. ATTENTION! It is DANGEROUS TO LIFE if the PWM transistor also burns out! In this case, the high-voltage capacitor will not discharge in an acceptable time.
Therefore, EVERYONE check the voltage on it before repairing! If dangerous voltage remains, then you need to discharge the capacitor manually through an insulated one of about 10 kOhm for 10 seconds. If you suddenly decide to short-circuit the leads, then protect your eyes from sparks!
Next, we proceed to inspect the monitor’s power supply board and replace 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 inspect the soldering under a microscope for microcracks.
From my own experience I will say that if the monitor is more than 2 years old, then 90% 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. To the extent that the active flux is not washed away - which leads to failure of the monitor after a year or two. Yes, yes, just when the warranty ends.
when you turn on the monitor. This miracle directly indicates to us that the power supply is faulty.
Of course, the first thing you need to do is check the power and signal cables - they must be securely fastened in the connectors. A flickering image on the monitor tells us that the monitor's backlight voltage source is constantly jumping out of operating mode.
after the time has elapsed or does not turn on immediately. In this case, again, there are 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 typically operates at frequencies between 30 and 150 kHz. If its operating mode is disrupted, vibrations can occur in the audible frequency range.
but the image is visible under bright light. This immediately tells us that LCD monitors are faulty in terms of backlighting. In terms of frequency of occurrence, it could be placed in third place, but it is already taken.
There are two options - either the power supply and inverter board burned out, or the backlight lamps are faulty. The last reason is not common in modern monitors. If the LEDs in the backlight fail, then only in groups.
In this case, the image may darken in places at the edges of the monitor. It is better to start repairs by diagnosing the power supply and inverter. The inverter is that part of the board that is responsible for generating a high-voltage voltage of about 1000 Volts to power the lamps, so under no circumstances should you 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 off due to poor contact near the tip of the backlight. This can be cured by carefully disassembling the matrix in order to get to the end of the lamp and solder the high-voltage wiring.
A simpler way out of this unpleasant situation can be found if your friend-brother-matchmaker has the same monitor lying around, but with faulty electronics. It won’t be difficult to make two monitors of similar series and the same diagonal.
Sometimes even the power supply from a monitor with a larger diagonal can be adapted for a monitor with a smaller diagonal, but such experiments are risky and I do not recommend starting a fire at home. But in someone else’s villa, that’s a different matter...
Their presence means that the day before you or your relatives had a fight with the monitor over something outrageous.
Unfortunately, household LCD monitors are not equipped with 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 left, the stain will still begin to grow over time under the influence of temperature and voltage applied to the liquid crystals. Unfortunately, it is not possible to restore dead monitor pixels.
That is, a white or gray screen appears on your face. First, you should check the cables and try connecting the monitor to another video source. Also check whether the monitor menu is displayed on the screen.
If everything remains the same, look carefully at the power supply board. The power supply of an LCD monitor usually generates voltages of 24, 12, 5, 3.3 and 2.5 Volts. You need to use a voltmeter to check if everything is okay with them.
If everything is in order, then carefully look at the video signal processing board - it is usually smaller than the power supply board. It contains a microcontroller and auxiliary elements. You need to check if they are getting food. Touch the contact of the common wire with one (usually along the outline of the board), and with the other, walk along the pins of the microcircuits. Usually the food is somewhere in the corner.
If everything is in order regarding the power supply, 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 perhaps you are faced with a case of broken firmware or microcontroller failure. This usually happens from surges in the 220 V network or simply from aging of the elements. Usually in such cases you have to study special forums, but it’s easier to use them for spare parts, especially if you know a familiar karateka who fights against unwanted LCD monitors.
This case can be easily treated - you need to remove the frame or back cover of the monitor and pull out the board. Most often there you will see a crack in the board or in the solder.
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 must be glued to strengthen the structure.
This occurs due to aging of the backlight bulbs. According to my data, LED backlight does not suffer from this. It is also possible that the parameters of the inverter may deteriorate, again due to the aging of the components.
This often happens due to a bad VGA cable without an EMI suppressor -. If replacing the cable does not help, then it is possible that power interference has penetrated into the imaging circuits.
Usually they are eliminated from the circuit design by using filter capacitors for power supply on the signal board. Try replacing them and write to me about the result.
This concludes my wonderful rating of the TOP 10 most common malfunctions of LCD monitors. The bulk of data on breakdowns was collected based on 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 to and in the comments.
Sincerely, Master Pike.
The most common questions when disassembling LCD monitors and TVs are how to remove the frame? How to release the latches? How to remove the plastic case? etc.
One of the craftsmen made a good animation explaining how to remove the latches from engagement with the body, so I’ll leave it here - it will come in handy.
To view animation— click on the image.
Today we will look at repairing the Samsung SyncMaster 720n monitor. So, when you turn on the monitor, there is no picture, but the power diode blinks. To diagnose a monitor failure, 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 setup button panel.
We remove the casing.
Turn off the backlights 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.
Unscrew the power supply and inverter board.
We inspect visually, 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 the two voltage stabilizers on the monitor’s control board (this problem in the form of non-working stabilizers has also occurred not for the first time). In order to measure voltage stabilizers, we assemble the 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 stabilizer (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 within the normal range.
Checking the 1.8v voltage stabilizer (AS1117L-18).
We measure the input voltage 5v, everything is fine.
We measure the output voltage and not everything is so smooth here, the multimeter shows 4v instead of 1.8v, the stabilizer is not working and needs to be replaced.
Unsolder the voltage stabilizer to 1.8v.
Solder a new stabilizer.
Afterwards, we assemble the monitor, also without a housing.
Connect to the network and turn on the monitor. And what we see is that the lamps lit up and the power diode stopped flickering.
