In this article we will look at how you can repair a monitor yourself.
A modern LCD monitor consists of only two boards: a scaler and a power supply
Scaler- This is the monitor control board. His brain. Here the monitor converts the digital signal into colors on the display and also contains various settings. It contains a processor, flash memory where the monitor firmware is written, and EEPROM memory in which the current settings are saved.
Power unit. It supplies power to the monitor circuits. It may also contain an inverter for monitors with LCD backlight. Monitors with LED backlighting do not have an inverter.
The power supply for the monitor looks something like this:
There is also a significant difference. In power supplies for monitors with LCD backlighting, you can see the high-voltage part. He's an inverter. Its presence is indicated by inscriptions such as “High Voltage” and terminals for connecting lamps. Please note that the voltage supplied to the lamps is over 1000 Volts! It’s better not to touch, much less lick, this part when you turn on the monitor.
Swollen capacitors
These are, of course, electrolytic capacitors in the power supply filter.
This is one of the most common failures of LCD monitors. Capacitors are easily and simply soldered. Sometimes boards have a non-standard capacitor rating, for example 680 or 820 uF x 25 volts. If you encounter swollen capacitors of the same value and they are not in your radio store, do not rush to go around all the radio stores in your city in search of exactly the same value. This is exactly the case when “too much is not harmful.” Any electronics engineer will tell you this. Feel free to set 1000 uF x 25 volts and everything will work fine. It's possible even more.
Due to the fact that the power supply emits heat during operation, which has a detrimental effect on the service life of the capacitors, be sure to install capacitors marked “105C” on the case. Also, after soldering the capacitors, it would not hurt to check the fuse of the secondary circuits, which is often a simple SMD resistor with zero resistance, size 0805, located on the back of the board on the trace side.
Zener diode failure
And one more nuance, at the output of the power supply, in front of the power connector going to the scaler, an SMD zener diode is often installed
If the voltage on it exceeds the rated value, it goes into a short circuit and thereby turns off our monitor through the protection circuits. You can replace it with any suitable voltage rating. Can even be used with leads
After everything is done and repaired, we check the voltage at the power connector that goes to the scaler. All voltages are signed there. We make sure that they coincide with the multimeter readings.
Problems in the high-voltage part of the power supply (inverter)
If possible, first of all, always look for diagrams of the device being repaired. Let's look at the high voltage part of one of the monitors
If you see that the monitor's power supply fuse has blown, it means that the resistance between the monitor's power wires (input impedance) has become very low at some point (short circuit). Somewhere around 50 Ohms or less, which in turn, according to Ohm's law, caused an increase in current in the circuit. Due to the high current, our fuse wire burned out.
If the fuse is in a metal-glass case, we can insert absolutely any fuse into the mount and test the resistance between the pins of the plug in 200 Ohm Ohmmeter mode. If our resistance is zero and up to 50 ohms, then we are looking for a broken radio element that rings to zero or to ground.
The steps will be like this:
We insert the fuse, switch the multimeter to 200 ohms and connect it to the power cord plug. We make sure that the resistance is very small. Next, we are in no hurry to remove the fuse.
So let's look at the diagram to see which radio components we can short out. In the photo, those parts that will need to be checked in the event of a short circuit in the high-voltage part are highlighted with colored frames.
All these procedures for measuring resistance are done in order to call the listed parts one by one. That is, we unsolder and again measure the resistance through the plug. As soon as we get high resistance at the input of the plug, having replaced or removed the defective radio element, we can safely plug the plug into the socket and dig further.
No monitor backlight
What is the difference between LCD-backlit monitors and LED-backlit monitors? In LCD monitors, we use CCFL lamps for backlighting. In Russian, this abbreviation sounds like “cold cathode fluorescent lamp”.
These lamps are located at the top and bottom of the display and illuminate the image.
LED monitors use LEDs for backlighting, which are located either on the sides of the display or behind it.
Now all monitor and TV manufacturers have switched to LED backlighting, as it reduces energy consumption by almost half and is much more durable than LCD backlighting.
If there is no backlight, then the problem may be either the CCFL lamps or the LED strip. If they do not light up at all, the image will be so dim that nothing will be visible on the display. Only a careful inspection of the switched-on monitor under lighting can show that there is still an image. Therefore, if there is no image at all, then the first thing to do is inspect the switched-on monitor under a stream of light. If the image is even slightly visible, then take further measures, either change the lamps, or the problem is with the inverter.
Monitor backlight disappears
Our monitor turns on, works for 5-10 seconds and goes out. This indicates that one of the CCFL display backlight lamps has become unusable. Before this, part of the screen may also blink a little. In this case, the inverter will go into protection, which will manifest itself in automatically turning off the monitor backlight.
In order for us to check the lamps and rule out a defective one, we need to buy a high-voltage capacitor from a radio store. 27 picofarads x 3 kilovolts for 17-inch monitors, 47 pf for a 19-inch monitor and 68 pf for a 22-inch monitor.
This capacitor must be soldered to the contacts of the connector to which the backlight lamp is connected. The lamp itself, of course, must be turned off. By connecting a capacitor to each connector in turn, we ensure that our inverter stops going into protection.The monitor will work, although it will be a little dim.
Of course, rarely anyone does this. The trick is to disable the protection on the PWM chip itself))). To do this, google “remove inverter protection xxxxxxx.” Instead of “xxxxxxx” we put the brand of our PWM chip. Once I turned off the protection on a monitor with a TL494 PWM chip according to the diagram below, soldering a 10 Kilo Ohm resistor. Monique is still working. No complaints).
Good day!
In this post, I would like to consider such a problem of LCD monitors as failed backlight lamps, try to figure out why this happens, and change them accordingly. If you are interested, I ask you to follow the green man.
P.S.
Below the cut there are 27 photos
-Dear people, I apologize in advance for the quality of the subsequent photos, I took pictures on a toaster....
-Also, I would like to note that the monitors do not differ much in their fundamental design, so do not be alarmed if you suddenly do not find screws or anything else in the place shown in the photo, they are somewhere nearby...
So, we have a monitor that works in almost romantic, red-pink tones. The operating time of such a monitor is unpredictable... but as a rule, it does not exceed 2-3 hours, after which your eyes are given time to rest and your brain to think about the issues of existence.
The problem is a failed monitor matrix backlight lamp, but why did this happen?
There are many reasons for this situation:
- manufacturing defect,
- shorting the metal parts of the lamp to the metal frame of the matrix,
- physical damage, etc.
But let's delve a little into the theory.
LCD matrices work through transmission, that is, the monitor must have a light source that illuminates the matrix through. The quality of the monitor depends quite significantly on the light source. For stationary LCD displays and TVs, direct backlighting is usually used, when light sources (lamps or LEDs) are distributed over the entire panel area. ©
But why does he continue to work then? and such a short period of time?
It's simple.
It is worth noting that monitors most often use 2 blocks of 2 lamps ( top and bottom of the monitor), which should distribute the light evenly along the light guide under the matrix.
If one or more lamps fail, the rest continue to work. But the inverter ( which powers them) is a smart thing, and if he “sees” that something is wrong with one or more of his charges, he decides to stop his work so as not to cause harm.
Well, let's get down to business, shall we?
We start by disconnecting all the cables from the inverter unit and the monitor controller,
Remove the back panel with the power supply and controller
Removed? great... What we see, in numbers 1
We have marked the power wires going to the treasured lamps.
2
- a train going to our matrix.
The asterisks indicate places that need to be picked out so that disassembly can continue.
We are removing the panel on the left for now, we don’t need it now
And again we disassemble our “matryoshka” 
Great, we're almost halfway there,
Now let's explain:
5
-our matrix (the same thing with numbers 640x480~1920x1080)
6
-signal decoder connected to the matrix by a row/column data line
7
-light guide with light filters
Next, we again delve into the “wilds of the monitor” and remove the plastic frame around the perimeter... 
Under the black frame there are 2 thin films lying on top of each other, and under them there is a light guide.
8
-light filter
9
-polarizing film
10
-light guide
Now we take out the large acrylic thing ( 10
) and finally we can see the heroes of the occasion...
Those assholes who made us come this far ( 11
)
Gentlemen. I present to your attention broken, faulty backlight lamps!
Speaking of lamps.
Did you know:
that LCD panels use CCFL lamps, which in Russian means cold cathode fluorescent lamp. Its principle is almost the same as hot (in common parlance “fluorescent lamps”). The only difference is that to obtain plasma in the hot one, the initial heating of the cathodes is used, and in the cold one the plasma is obtained due to the high voltage applied to the cathodes. Next, the plasma, which has an ultraviolet radiation spectrum, hits the phosphor, the white coating that you see through the flask, and is converted into visible radiation (white light) ©
As we can see, they really burned out. (the “black marks” around the cathodes hint at this)
We unscrew them, having first pulled out the reflective backing ( or maybe you don’t have to do this on your monitor)
... and swap them ( I want to note that you should be careful, because they are quite fragile. I also advise you to securely fasten the wires and keep a vigil so that there is no breakdown in the future. We isolate everything to the maximum!)
Now we will return our lamps to their place, screw them on, return the reflective thingy and put the light guide in place.
We connect - everything works! ( Before this, it also worked, but not correctly, only 1.5 lamps were on, I didn’t bother to capture this action in disassembled form. I repent)
Well... the hardest part is over, all that remains is to put everything back together.
Let's get started. 
We return the films to their place, cover them with a plastic frame and place our matrix on top, fixing it with a metal frame.
(Here we should not forget about such a thing as dust... before assembling everything, it’s worth blowing air through all components of the monitor, it won’t take long, but it will affect the image quality)
We turn it over and return the last “detail” to its place.
Connect to the “stand” and rejoice!
Everything works, no traces of uneven illumination were noticed,
The flight is normal.
Fin.
_______________________________________________________________________________
What would you like to say in conclusion?
0
It turns out that replacing the lamps yourself is not so difficult, if only you had the desire.
You can also experiment and replace the lamps with LED strip. But you need to remember that the LED strip does not give a completely uniform light + on top of everything, it’s very possible that 1 or more LEDs will burn out/become a little dimmer, and then the backlight will become uneven. Also, don’t forget about the color temperature of the LEDs.
1 .When replacing lamps, you need to know exactly their dimensions; I used this table as a guide.
2
. Why did I decide to write this article?
Faced with monitor repair for the first time, I went into "some search engine", and did not see detailed instructions...
No, I’m not saying that I didn’t find them, they were there, but they seemed incomplete to me, so it was decided to collect this material and post it here. You never know who will find it useful...
3
.Links to similar/used/additional materials:
cheklab.ru/archives/2534 (good article about the design of various types of monitors)
radiokot.ru/lab/hardwork/30 (replacement of backlight bulbs + some background information)
habrahabr.ru/post/182772 (we revive the monitor if there are no new lamps at hand)
radioskot.ru/publ/remont/zamena_ljuminiscentnykh_lamp_podsvetki_v_monitore_na_svetodiodnye/4-1-0-594 (successful replacement of lamps with LED strip)
www.yaplakal.com/forum2/topic471720.html (almost successful replacement of lamps with LED strip)
4
P.S.
If Habra residents are interested in posts about the repair and restoration of equipment, then I will be happy to share the material I have accumulated.
In this material, the author continues the topic started in the article - he describes in detail the diagnostics of power inverters for cold cathode electroluminescent lamps (CCFL lamps). Circuit diagrams of all inverters discussed in the article are given in.
Correct fault diagnosis significantly reduces repair time and costs. The main problem that arises when diagnosing a backlight system is to determine what is faulty: the backlight lamp or the inverter. Practice shows that the malfunction of CCFL lamps manifests itself as follows:
The screen turns red;
When you turn on the laptop, the screen color has a red tint, and then gradually becomes normal;
The panel backlight (the entire image) blinks in time with the changing brightness of the scene;
The panel backlight begins to blink and then turns off.
The malfunction of the lamps with such manifestations is confirmed in approximately half of the cases; in other cases, it is necessary to refer to the methods outlined below.
Structurally, the inverter board and backlight lamps are usually located under the front cover of the laptop screen. The first thing to check is whether the backlight problems are related to a malfunction of the laptop motherboard. If, when connecting external display devices - a monitor, TV, projector, there is an image, then most likely the laptop's backlight system is faulty.
To repair an inverter or lighting system, you must have the minimum necessary measuring equipment at your workplace - a multimeter, an oscilloscope and an autonomous power supply with an adjustable constant voltage from 1.5 to 30 V with current protection (1 A), as well as a working CCFL lamp.
To eliminate the influence of a faulty lamp, an equivalent load is used when repairing the inverter. It is preferable to connect a known good lamp to the inverter being tested. If there is none, then a resistor with a nominal value of 100...130 kOhm and a power of 2...5 W is connected to the output connector of the inverter (as recommended by inverter manufacturers). The resistor is selected based on the required secondary voltage at the feedback output. A ceramic capacitor with a capacity of 20...200 pF and an operating voltage of at least 2 kV can also be used as an equivalent load. Using a capacitor when testing the inverter in operating mode is preferable, however, problems may arise when starting the inverter controller. The inverter can be considered operational if there is a stable sinusoidal voltage at the load equivalent.
Replacing a lamp requires special care and ensuring the cleanliness of the room. Work is carried out with gloves. In some cases, when complete disassembly of the matrix is required, this operation is carried out in “clean” rooms and in special clothing.
Backlight malfunctions are sometimes associated with poor contact at the welding (soldering) site of the inverter wire and the lamp electrode. In this case, it is possible to restore the functionality of the backlight system. To do this, you must have an insulating tube (rubber tip) from a faulty CCFL lamp. It is better to do welding or soldering with hard solder and a gas soldering iron, which creates a high temperature at the soldering site. The tube, previously placed on the wire, is carefully pulled onto the soldering site and the lamp is ready for use.
Malfunctions and repair of SAMSUNG laptop inverter
To access the inverter board and lamp, remove the decorative cover from the LCD panel of the laptop, disconnect the cable connecting it to the motherboard and the lamp connection cable from the inverter.
The screen does not light up
Check the serviceability of the inverter elements by external inspection. In this case, the malfunction of the power elements and, first of all, the transformer, is determined by the darkening of its housing, burnt insulation, darkening and even destruction of the board underneath.
Check the presence of voltages at connector CN1 (Fig. 3c): +12 V on pins 1-2, inverter turn-off voltage on pin 4 and brightness voltage on pin 3.
In normal mode, when loading video card drivers, there should be no voltage on pin 4 of CN1. The inverter turns on automatically when supply voltage is applied. The brightness voltage (pin 3) must be at least 0.5...2 V.
Check the voltage at the emitter of transistor Q4, and if it is missing, check fuses F1, TF1, as well as transistors Q7 and Q5.
Check the serviceability of transistors Q1, Q2. These are digital transistors of the KST1623 type, they are produced in an L4 package, they can be replaced with an analogue of the BSS67R type. If transistor Q1 fails, it is enough to replace only it. If transistor Q2 fails, check the serviceability of transistor Q7 and operational amplifier U1A.
If fuse F1 is good and TF1 (self-resetting fuse) is faulty, then before replacing it, check the serviceability of transistor Q4 and zener diode D2.
Check the brightness control voltage on pin 3 of CN1. For diagnostics, pin 3 is supplied with a voltage of about 3 V from an external source. If the screen lights up, then the cause of the problem is the laptop's motherboard. In this case, you can forcibly turn on the screen backlight by applying voltage from a resistor divider (80 kOhm in the upper arm (to +5 V), and 40 kOhm in the lower arm) connected to the +5 V bus. If the screen does not light up, check the serviceability of transistor Q8 .
The backlight turns off 1-2 seconds after the operating system starts loading
First of all, check the serviceability of CCFL lamps. Connect the oscilloscope to pin 1 of connector CN2 (see Fig. 3c) and an equivalent load. If there is a sinusoidal voltage with an amplitude of 500...700 V and a frequency of 60...70 kHz at this ("hot") contact of connector CN1, then the inverter is working and turning off the backlight may be due to a malfunction of the lamp or poor contact between the inverter wire and the electrode lamps. All this requires disassembling the laptop and removing the lamp. Observe the shape and voltage level at an equivalent load for at least 10 minutes, and replace the faulty lamp. If there is no voltage or its waveform is significantly distorted, then the malfunction is due to internal problems in the inverter.
Check the feedback circuit. If, when the inverter is turned on, an oscilloscope registers any signal at the “cold” contact of the lamp (its shape does not matter) with an amplitude of at least 1.5 V, and on the pin. 6 U1 voltage remains unchanged (constant voltage, which is measured with a multimeter), check the serviceability of diode assemblies D4, D5 (they can be replaced with any suitable ones, or with two separate diodes of the BAV99 type in SMD cases). If assemblies D4, D5 and resistor R14 (1 kOhm) are working, then the U1 chip is faulty.
Check the precision stabilizer U2 (TL341). If it is working, then on the pin. 5 U1 should be constant voltage 1.5V. In addition, this inverter protection line is connected with brightness control and overload protection circuit. To determine which of these circuits is faulty, turn them off sequentially (but not simultaneously) for a while. First, the protection circuit D3 R3 R4 is turned off, then the brightness control circuit - transistor Q8. If, when these circuits are disconnected, the lamps work stably, then the fault is in these circuits.
Check the presence of contact in connector CN2. In case of visible burning of the contact, it is restored. If the contact does not cause suspicion, connect an equivalent load. Check the overload protection signal generation circuit D3 C3 C4 D5. The protection may be triggered due to overheating of transformer T1, malfunction (leakage) of transistors Q5, Q6.
Malfunctions and repair of inverter based on MP1101 controller
The screen does not light up
Check for voltage on pins 4 (VCC), 2 (Enable) of connector JP1 (Fig. 4c). In this case, the supply voltage should be 12 V, the Enable inverter switching voltage should be at least 1.5 V. The absence of Enable voltage indicates a malfunction of the laptop motherboard, most likely the video card. The absence of 12 V voltage at the JP1 connector when the cable connecting the inverter to the motherboard is disconnected indicates a malfunction of the motherboard. If there is 12 V voltage at the connector, and at the pin. 6 U1 it is equal to zero, then check the serviceability of the filter capacitors, fuse F1 and controller U1.
Check the inverter switch-on voltage at the pin. 4 U1. If it is missing, check its presence on the contact of the connector disconnected from the inverter board. If there is no voltage, check the laptop circuit. The absence of an inverter switch-on voltage can be associated either with a malfunction of U1 or with a break or “cold” soldering of the resistor REN1 (there are no radio element designations on the inverter board based on the MP1011 controller, so refer to Fig. 4c). To eliminate this problem, simply solder the SMD resistor REN1. Check the serviceability of transformer T1 (see above), CON2 connector and wires.
The backlight turns on for 1-2 seconds and goes off
First of all, check the elements of the feedback circuit D2 (a, b) CSENSE RSENSE. Diodes are checked for open circuit or breakdown. Check the serviceability of the lamp (see above). Connect an equivalent load. Connect the oscilloscope to the Lamp+ circuit (Fig. 4c). If, after the operating system starts loading, a sinusoidal voltage of 500...700 V is present at this pin, then the main inverter board is working and the lamp needs to be replaced.
The reason for the backlight to disappear may be a malfunction of the feedback unit. If when you turn on the screen on the pin. 2, a positive voltage of about 0.5 V appears for a while, but the lamps go out, then the MP1011 controller should be replaced. If the feedback voltage is less than 0.1 V, check all elements in the feedback circuit: D2, RSENSE, CSENSE.
If, when the inverter is turned on, a signal with an amplitude of more than 0.5 V is recorded on the “cold” terminal of the lamp by an oscilloscope, and on the pin. 2 U1 voltage remains unchanged (constant voltage, which can be measured with a multimeter), then check the serviceability of the diode assembly D2, it can be replaced with two diodes of the BAV99 type. If the diodes are working and the RSENSE resistor (140 Ohm) is not broken (cold soldering), then the MP1011 controller is faulty.
Backlight turns off after a few seconds or minutes
In this case, check the T1 transformer, the CSER capacitor (for leakage) and the lamp connection wires for possible insulation damage and contact with metal objects of the housing.
Malfunctions of inverters based on the OZ9938 controller
The screen does not light up
Check the serviceability of fuse F1 (Fig. 5c). If it is faulty, then before replacing it, check the serviceability of transformer T1 by external signs (darkening, burnt insulation, burnt board). Then check the breakdown of the transistor assembly of field-effect transistors U1. If the OZ9938 controller is powered by a separate parametric stabilizer (not shown in the diagram), check the serviceability of its elements.
If the inverter circuit is working properly and there is a sinusoidal voltage of 550 V with a frequency of 55 kHz at pin 7 of transformer T1, then check the serviceability of the SG connector.
Check the presence of switching voltage (at least 1 V) on pin 6 of connector CN2. If the voltage is below normal, the pin is unsoldered. 10 controllers from the ENA bus. If the voltage at pin 6 increases to 2 V, check capacitor C18 or replace controller U2. If the voltage on pin 6 remains low, the reason is in the laptop motherboard. You can get out of this situation by applying a voltage of 2 V from an external source.
Check the voltage at the pin. 4 U2, if it is less than 0.1 V, then check the controller, laptop board and capacitor C10. Check the voltage at the pin. 11 U2, which in normal mode should be more than 3 V, with a reduced voltage at this pin, check C14, solder resistor R9. If the specified elements are serviceable, then replace the controller. The backlight turns on for 1-2 seconds and goes off
This defect may be due to a malfunction of the lamp and its connection circuit. If the lamp is working, then check the feedback circuit D1 C22. If, in the absence of a signal to turn on the inverter, the voltage at pin 6 of U2 is more than 1 V, then this microcircuit is faulty and must be replaced. If the voltage at the pin. 6 less than 0.7 V, the lamp is working, and the backlight turns off within a few seconds, check the overload protection circuit D2 R5 R3. If the voltage at the pin. 6 when the inverter is turned on, increases and at one moment exceeds the voltage of 3 V and at the same time the lamps turn off, then the reason is the overload of the inverter output stage. This may be caused by a faulty lamp (start-up problems when the lamp takes a long time to start-up). In addition, overload may be due primarily to the presence of short-circuited turns of the transformer windings.
If the voltage at the pin. 6 does not exceed 3 V, but the lamp turns off, then check for a voltage of no more than 3 V on the pin. 7 U2. If the voltage is below this level, then check capacitor C8 (leakage) or replace controller U2.
The backlight turns off a few minutes after turning it on
Check the overload protection circuits D2 C2 C5. Check the serviceability of transformer T1 (see above). Sometimes the malfunction appears after some time, during which the transformer heats up (above 50°C), then it needs to be replaced. Check the serviceability of the transistor assembly U1 (can be determined by its operating temperature). As a rule, this malfunction disappears while the suspicious elements are “frozen” with Freeze gel. If the time after which the backlight turns off is unstable, then check the serviceability of the lamp and its connector.
Malfunctions of inverters based on the OZ960 controller
The screen does not light up
For inverters such as AMBIT and KUBNKM (see Fig. 6 c) this may be accompanied by a lack of indication on the front panel. In this case, disassemble the laptop and check for +12 V voltage (for KUBNKM inverters the input connector J1 (CN1) is 20-pin, the supply voltage is supplied to the 4 outermost pins, and for AMBIT inverters the connector is 16-pin, and the supply voltage is supplied to the 2 outermost pins contact). If fuse F1 is faulty, check transistor assemblies U1, U3. Check the presence of supply voltage at the pin. 5 OZ960 controllers (U2). This voltage, in contrast to the typical inverter circuit (Fig. 6c), comes from pin 1 of J1 through the stabilizer on transistor Q1 (designation on the board). In AMBIT inverters, controller U2 is powered from pin 4 of J1. There may be no supply voltage at the connector itself due to a faulty laptop power supply or due to a short circuit to ground at the pin. 5 U2. For diagnostics, disconnect the SVDC line from connector J1 and, if voltage appears on the bus, then the inverter is faulty.
Check the presence of the ENA controller turn-on voltage on the pin. 3 U2, it must be at least 2 V. In the KUBNKM inverter, the controller turn-on voltage comes from transistor Q1 (its supply voltage is also removed from it) but through a 10 kOhm resistor. Other modifications of inverters based on the OZ960 controller may also have their own characteristics and differences from the standard circuit, but the troubleshooting technique for them is the same.
If the LEDs on the laptop keyboard panel are lit, there is no screen backlight, and the voltages listed above are present, then check the serviceability of field-effect transistor assemblies U1, U3, as well as zener diodes D1, D2 (4.7 V).
When you turn on the laptop, use an oscilloscope to monitor the presence of rectangular pulses on the pin. 11-12 and 19-20 U2. If there are no pulses and the U1, U3 assemblies are working properly, then check for the presence of a voltage of 2.5 V on the pin. 7 U2. If it is missing or it is underestimated, check C13 and replace the controller. Check for the presence of a sinusoidal signal at the pin. 18 U2 with a frequency of 50.60 kHz. If the frequency differs significantly from the nominal one or there is no signal at all, check elements C5, R4.
The lack of backlight may be due to the lack of (low) voltage at the pin. 14 controllers. If the voltage at this pin is less than 1 V, apply 3 V from an external source. If the screen lights up, then the problem is related to the brightness control voltage supplied from the laptop board. In this case, you can apply voltage from pin 1 of J1 to the brightness control input through a resistive divider, but it must be taken into account that the brightness will not be adjusted
The backlight turns off 1-2 seconds after turning on the laptop
Make sure that the backlight lamp is working properly (see the verification method above). They are connected with an oscilloscope to the “hot” (upper in the diagram in Fig. 6c) output of transformer T1. If, when you turn on the laptop, a sinusoidal voltage with a frequency of 55...60 kHz appears at this pin and immediately disappears, check the serviceability of transformer T1. Then they check the serviceability of the transistor assemblies U1, U2 for leakage: measure the resistance between source and drain with an ohmmeter, and if it shows the final value at the limit of 100 kOhm, then replace the assembly. Check the serviceability of capacitor C4 for leakage (ESR).
Check the presence of feedback voltage at the pin. 8 of the controller, it must exceed 1.25 V. If the voltage is below this value, check the diode assembly CR1, and also solder resistor R8. If there is no result, replace the U2 controller.
Backlight turns off after a few seconds or minutes
In this case, check the surge protection circuit. Disconnect it from the main circuit (just unsolder the CR2 diode assembly). When you turn on the laptop, check for voltage at the pin. 2 controllers (should be no more than 1 V). If this voltage exceeds the specified level, check the threshold value of 2.5 V at the pin. 7. If it is missing or the voltage is too low, replace the controller. If the voltage at the pin. 2 is normal, but when connecting the protection circuit, the voltage becomes higher than 2 V or changes over time, check the serviceability of the transformer, capacitors C7, C11, diode assembly CR2. You can replace the transformer with any type from another inverter (this circuit is insensitive to the type of transformer), the only thing that will need to be adjusted is the feedback voltage coming from the cold end of the lamp (by selecting resistor R8).
In an AMBIT type inverter, which uses an OZ979 chip to power the keyboard LEDs, you can try to restore the screen backlight using a temporary scheme. The lamps are turned off and lines of LEDs are fixed (glued) on the back side of the LCD matrix at the top and bottom of the screen, 3 pieces each. in 5 lines, the first LED is connected to pin 3 of the OZ979, and the last one is connected to the housing. This method is suitable for small screens of 10-12 inches.
You can use an inverter circuit based on OZ960; after the transformer, instead of capacitor C4, a double diode in an SMD package and a quenching resistor with a nominal value of 50 Ohms are installed. The resistance is more accurately selected when installing LEDs to ensure normal illumination and, depending on their operating current, for normal illumination of a 15-inch display, 16 ultra-bright LEDs, for example FYLS-1206W white, are sufficient. LEDs can be glued to fluoroplastic tape and connected with thin conductors. In this case, the input voltage on the first LED should not exceed 80 V at a current of 25-50 mA. The current through the LEDs is set by selecting the value of the limiting resistor.
Some circuits based on OZ960 differ from the standard one, including the name and location of some electronic components.
Sometimes there is a decrease in the brightness of the backlight and its adjustment is not enough. This occurs due to a decrease in the current of the gas-discharge lamp due to an increase in the transition resistance at the contact point on the board of the high-voltage winding of the transformer T1 and the ballast capacitor C4. The problem is eliminated by soldering the capacitor leads.
Literature
1. Vladimir Petrov. Repair and maintenance of power inverters for backlight lamps of LCD panels of laptops. Repair & Service, 2010, No. 3, p. 37-40.
As you know, modern LCD monitors work to “transmit” light - a translucent picture on the matrix is illuminated from behind, the light passing through the matrix and its light filters forms an image. A fairly bright source of white light is used as backlight - the light transmission matrix is more reminiscent of rather dark sunglasses.
Traditionally, cold cathode fluorescent lamps, or CCFL – Cold Cathode Fluorescent Lamps, have been used for this. These lamps are glass tubes with a diameter of 2-3 mm, the inner surface of which is coated with phosphor. The tubes are filled with mercury vapor. When an electric discharge passes through the gas, radiation is generated, causing the phosphor to glow. To operate such a lamp, a high alternating voltage is required - about 1500 V with a frequency of about 40-50 kHz.
Among the most common malfunctions of liquid crystal panels is the failure of the backlight or inverter - a device that converts direct voltage (usually 12-18 V, depending on the monitor's power supply) into alternating voltage to operate the lamp. This manifests itself in a sharp decrease in the brightness of the screen, usually from one of the edges, or in the complete shutdown of the backlight - in this case, the image on the screen is barely visible.
In branded services, such faults are “cured” by replacing the entire panel, especially in the case of laptop panels. It’s quite expensive, but in the case of a monitor, it’s easier to buy a new one. Fortunately, there are not only branded services in the world, but also a considerable number of “craftsmen” who have mastered the operation of replacing backlight lamps or inverters.
Replacing the backlight lamp is a simple operation, which is provided “structurally” in some monitors. If anyone hasn’t read Igor Pichugin on RadioKot, I’ll give a short “squeeze” from it.
The lamps are mounted along one side of the display in a “pencil case”. Typically, removing the pencil case requires disassembling the LCD panel, that is, removing the metal casing and the panel itself. A thin (about 0.5-1 mm) control board is mounted behind the panel, connected to the matrix itself with several cables. To remove the liquid crystal screen, you must carefully peel off (do not cut under any circumstances! Damaged data lines on flexible cables cannot be restored) the protective film.
To demonstrate “classic” backlight technology, I use an LG Flatron L1970H LCD monitor.
Let's start disassembling the monitor by removing the stand. You need to remove the plastic casing from the back that covers the backlight mount and the cables from the connectors on the stand.
After removing the stand, remove the LCD module from the case. The front frame is secured with latches and can be easily separated from the rear of the case.
The LCD module is covered with a metal casing. Through the holes in it, inverter transformers with menacing inscriptions are visible.
Unscrew the screws securing the casing.
Now you can see in detail the monitor’s control electronics board and the inverter, which is made as a separate unit.
The electronics board is connected by a separate harness to the LCD matrix decoder, covered with a thin self-adhesive film.
The decoder is connected to the matrix using thin flexible cables. If you have to remove the panel, carefully peel off the protective film - the flexible data lines cannot be restored, in which case the matrix will have to be thrown away.
The inverter mounted behind the monitor can often be replaced with a similar one. It is enough to know the supply voltage and the number of lamps. In addition, the inverter in monitors is usually large and repairable.
The lamps will be connected to the inverter using standard connectors.
In this monitor, the lamp cases can be removed without disassembling the panel. You just need to unscrew the screw...
...and pull out the pencil case.
The lamps are mounted in canisters of two. A sign of an “old” lamp is black rings around the cathodes. In burnt-out lamps they are much wider and darker.
It was no coincidence that I needed lamps. They brought a battered Fujitsu-Siemens Amilo M7800 laptop to “look at” with a diagnosis of “very dark image on the screen.” The company service asked for some unrealistic money for repairs - apparently they were going to change the matrix. I had just read an article on “cat” and was going to try to change the lamp.
To access the LCD panel, the first step is to remove its frame. Usually it is secured with latches, but some laptop models may also have screws hidden under rubber plugs.
At the bottom of the laptop's LCD screen, between the hinges, is the inverter in a protective casing.
It makes sense to check whether the lamp is really faulty, or whether the inverter has failed. To do this, just connect a known-good lamp to the inverter.
Laptop inverters are quite miniature and in case of malfunction they are usually replaced entirely. Replacement with a similar one from another model is acceptable, since they are available both at radio flea markets and at Dealextreme.
When replacing an inverter, it is advisable to determine how the on/off and backlight brightness are controlled. Typically, for this purpose, the cable going to the inverter contains DIM signals (brightness control, usually varying from 1 V - the lowest brightness to 3 V - the highest) and ENABLE (0 V - backlight off, 3 V - backlight on). Usually, their correct connection is not necessary for the new inverter to work, but it allows you to retain some energy saving features.
To replace the lamp we will need to remove the LCD panel. You need to unscrew the screws that secure it to the hinges and lid of the laptop.
There are metal guides on the sides of the panel that must be removed for further disassembly.
In the panel used in this laptop, you can get to the lamp without disassembling the entire panel. You just need to remove one side of the metal frame and open the plastic case.
It would seem that the next step is obvious - we go to the radio market, buy the necessary lamp and put it in the laptop. The reality turned out to be somewhat more complicated. On Mitino there were no lamps of suitable length; they were either very short (15 mm shorter) or very long (15 mm longer). In the Istok-2 kiosk (this is a kiosk with radio tubes and all sorts of lighting equipment, located at the end of the basement floor farthest from the entrance, it lights up like a Christmas tree) they advised to use a line of super-bright LEDs.
The width of such a ruler is about 3 mm. The diodes on it are installed in groups of 3 pieces, each about 15 mm long. Accordingly, you can cut the ruler to the required length with acceptable accuracy.
Now, with the development of technologies for manufacturing high-power white LEDs, LED backlighting has begun to be installed in some LCD monitors and televisions. In fact, anyone can join the “leading edge” of technology by installing such lighting to replace the burnt-out “lamp” one. Yielding to the persuasion of the “originists,” I bought a ruler 300 mm long for 250 rubles (approximately the cost of the lamp).
The LED line fits wonderfully inside a standard pencil case.
To check the LED backlight, simply connect the ruler inserted into the matrix to a suitable power source. When turned off, the screen should glow milky white.
Reassemble in reverse order (c).
Instead of an inverter thrown out as unnecessary, you can assemble a circuit like this:
We select resistor values depending on the parameters of the DIM and ENABLE signals and the supply voltage.
In conclusion, I want to say a few words about why LED backlighting is disgusting.
Firstly, the emission spectrum of LEDs does not quite match the spectrum of lamps. Therefore, on monitors designed for working with graphics, such a replacement can only do harm.
Secondly, sometimes there are “smart” inverters controlled by digital signals (usually via the I2C bus, but there are also more exotic ones). If there is no inverter, the LCD panel may not turn on.
Thirdly, the main disadvantage of LED backlighting assembled “on the knee” is some unevenness of the glow near the lamp.
It is noticeable in the photo that the illumination of the lower part of the screen is not very uniform, and the lower right corner is generally dark; unfortunately, the ruler turned out to be a little short.
In any case, replacing the CCFL lamp with LEDs is an affordable and inexpensive way to restore LCD monitors. The existing shortcomings cannot be called critical, but in the case of lamps of non-standard sizes, like mine, this is quite justified.
The entry was published on the blog of Shura Lyuberetsky. You can leave your comments there using your LiveJournal username (OpenID login).
Television manufacturers regularly introduce users to new technologies that improve image quality. Approaches to combining TV screens and LED elements have long been mastered by major companies. Recently, the source of bright and soft glow is also moving to the displays of mobile devices. Users of traditional LED-based lighting can also appreciate the advantages of this solution, but, of course, the backlight of LED screens on TVs looks most attractive. Moreover, it is complemented by other high-tech inclusions used by the developers of this technology.
Backlight device
When creating modules to implement backlighting, LED arrays are used, which can consist of white LED elements or multi-colored ones, such as RGB. The design of the board for equipping the matrix is specially designed for the purpose of integrating a specific media model into the device. As a rule, on the left side of the board there are contact connectors, one of which provides power to the LED backlight, and the others are designed to control its operating settings. A special driver is also used, the function of which is interfaced with the controller.
In its finished form, it is a row of miniature lamps that are connected in groups of 3 pieces. Of course, manufacturers do not recommend interfering with the design of such tapes, but if desired, you can physically shorten or, conversely, make the device longer. Also, the standard backlight of the LED screen provides the ability to adjust the brightness, supports soft start and is equipped with voltage protection.
Classification of lighting by installation type
There are two ways to integrate LED backlighting - direct and edge. The first configuration assumes that the array will be located behind the LCD panel. The second option allows you to create very thin screen panels and is called Edge-LED. In this case, the tapes are placed around the perimeter of the inside of the display. In this case, the uniform distribution of LEDs is carried out using a separate panel, which is located behind the liquid crystal display - usually this type of LED screen backlight is used when developing mobile devices. Adherents of direct illumination point to the high-quality result of the glow, which is achieved thanks to a larger number of LEDs, as well as local dimming to reduce color stains.
Application of LED backlight
The average consumer can find this technology in TV models from Sony, LG and Samsung, as well as in products from Kodak and Nokia. Of course, LEDs have become more widespread, but it is in the models of these manufacturers that qualitative shifts towards improving the consumer qualities of this solution are observed. One of the main tasks that faced the designers was to maintain the performance of the screen with optimal characteristics in conditions of direct exposure to sunlight. Also recently it has improved in terms of increasing contrast. If we talk about advances in screen design, there are noticeable reductions in panel thickness, as well as compatibility with large diagonals. But there are still unsolved problems. LEDs are not able to fully reveal their capabilities in the process of displaying information. However, this did not prevent LED technology from displacing CCFL lamps and successfully competing with the new generation of plasma screens.
Stereoscopic effects
LED-based modules have many capabilities to provide various effects. At this stage of technology development, manufacturers are actively using two stereoscopic solutions. The first provides for angular deflection of radiation fluxes with support for the diffraction effect. The user can perceive this effect while viewing with or without glasses, that is, in holography mode. The second effect involves a shift in the light flux, which is emitted by the backlight of the LED screen in the direction of a given trajectory in the liquid crystal layers. This technology can be used in combination with 2D and 3D formats after appropriate conversion or recoding. However, regarding the possibilities of combination with three-dimensional images for LED backlights, not everything is smooth.
3D Compatible
This is not to say that LED-backlit screens have serious problems interacting with the 3D format, but for optimal perception of such a “picture” by the viewer, special glasses are required. One of the most promising areas of this development is stereo glasses. For example, several years ago nVidia engineers released shutter 3D glasses with liquid crystal glass. To deflect light flows, the LED backlight of the LCD screen involves the use of polarization filters. In this case, glasses are made without a special frame, in the form of a ribbon. The built-in lens consists of a wide array of translucent ones that perceive information from the control device.
Benefits of backlighting
Compared to other backlighting options, LEDs significantly improve the consumer quality of television screens. First of all, the immediate characteristics of the image are improved - this is expressed in increased contrast and color rendition. The highest quality processing of the color spectrum is provided by the RGB matrix. In addition, the backlight of the LED screen has reduced power consumption. Moreover, in some cases, a reduction in electricity consumption of up to 40% is achieved. It is also worth noting the possibility of producing ultra-thin screens that are lightweight.
Flaws
Users of TVs with LED backlighting have criticized them for the harmful effects of blue-violet radiation on the eyes. Also, bluish tint is observed in the “picture” itself, which distorts the natural color rendition. True, in the latest versions of high-resolution TVs, the LED backlight of the screen has practically no such defects. But there are problems with brightness control, which involves pulse width modulation. During such adjustments, you may notice screen flickering.
Conclusion
Today, the segment of TV models with LED technology is in its infancy. The consumer is still assessing the capabilities and advantages that an innovative solution can provide. It should be noted that the operational disadvantages that LED backlight has do not confuse users as much as the high cost. Many experts consider this factor to be the main barrier to the widespread popularization of the technology. However, the prospects for LEDs still remain promising, since their costs will decrease as demand increases. At the same time, other lighting qualities are also being improved, which further increases the attractiveness of this proposal.