The basic principle of LCD TV and CRT TV is the same in all parts of the circuit, but some maintenance personnel do not understand the logic board of LCD TV, and the maintenance theory and ideas have not been updated. The LCD TV that is in trouble does not dare to repair, and there is no way to make a profit. In fact, the LCD TV logic board, in addition to the LCD screen LVDS program failure, other faults are still good maintenance. Therefore, in response to the doubts of the users in the forum on the logical version of the problem, remove the cumbersome content, and use the easy-to-understand language to publish the LCD TV logic board maintenance ideas to the forum and share it with netizens. R&M Bar,9000Puffs Disposable Vape,9000Puff E-Cigarette Device,R&M Disposable Vape Pen Shenzhen Uscool Technology Co., Ltd , https://www.uscoolvape.com
The logic board is provided by the screen manufacturer and the screen. The logic board is also called: screen driver board, center control board, TCON board. The function of the logic board is to input the LVDS input signal from the digital board. (The input signal contains three categories: RGB data signal, clock signal and control signal.) After processing by the logic board, the LVDS signal inputs the TTL level in parallel. After the RGB data signal is converted into an LVDS signal capable of driving the liquid crystal panel, it is directly sent to the LVDS receiving chip on the side of the liquid crystal panel to drive the liquid crystal screen to display an image.
When we check the logic version, we can test the screen voltage, depending on the 5V or 12V screen model. Then test the voltage on the LVDS output interface to see if the static and dynamic conditions change. If there is no change, you can judge the fault on the logic board. (When measuring, use the pointer multimeter. You can see that the hands have slight jitter, generally 1.2V or so), if you have the conditions, take the same logic board for the most reliable replacement, as long as the format and the screen voltage can be replaced, so that we can find the fault point.
The LVDS line from the main board to the logic board has a certain regularity. The red side is the positive pole of the power supply, the black line is generally the negative pole of the power supply, and the LVDS signal line is twisted together (4 pairs - 6 bits in a single position; 5 pairs - Single 8 bit; 8 pairs - double 6 bits; 10 pairs - double 8 bits), now there are logic boards and screens are connected together, called X + C integrated, generally 32 inches below the screen, 32 The screen above the inch is generally composed of X+C separate type (small screen is mainly considered by cost). Due to the technical and precision features, it is generally not easy to repair. If the principle is thorough, and the welding technology is excellent, the maintenance logic board is not a myth!
Damage to the logic board can cause the screen to not display the image properly, and of course there is no menu display, but the buttons and remote control are functional (remember this), the screen has intermittent color lines in the vertical direction, and no characters (this also Very important).
The intermediate image of the screen with poor contact between the logic board and the screen connection line is mixed with many small color points, which can be inserted and removed to confirm;
Both the logic board and the screen can cause the image to be screened, but the screen of the logic board is different from the screen produced by the screen. The screen of the logic board is a regular flower screen. We can change the signal source or change the 4:3 mode to confirm. .
The following is a quick way to determine the fault of the LCD TV logic board circuit:
(1) Whether the waveform of the input video signal sent by the digital image processing circuit is normal on the detection logic board. If there is a normal waveform input, the previous digital image processing circuit works normally.
(2) Check whether the voltage input from the power supply on the logic board is normal. If the input voltage is normal, the power supply circuit works normally.
(3) Check whether the waveform of the LCD screen drive signal outputted by the screen line interface on the logic board is normal. If there is no normal waveform output of the LCD screen drive signal, there may be a fault in the logic board circuit.
The following are common fault phenomena and maintenance ideas:
Symptom 1: No image.
Analysis and overhaul: Most of these faults are caused by the no-supply voltage of the logic board. Because the screen manufacturing process is different, the screen of 26 inches or more generally has a black screen when the logic board has no power supply voltage. Since the backlight is bright at this time, it sometimes appears as a gray screen; the screen below 26 inches usually appears as a white screen.
During the actual repair, first check whether the power supply voltage of the logic board is normal. If the voltage is normal, the power supply circuit of the signal board works normally. At this time, check whether the fuse on the logic board is blown. If the fuse is blown, you can replace the fuse resistor by confirming that there is no short circuit in the rear stage. If there is no voltage or the voltage is low, then the upper screen cable plug of the logic board can be unplugged, and then the voltage of the upper screen sent by the signal board is normal. If it is normal, it indicates that there is a short circuit fault on the logic board, otherwise the problem is in the signal board. on.
Symptom 2: There is an image, but the color or contrast of the image is abnormal.
Analyze and overhaul: The data signal at the measurement logic board is normal. If there are conditions, the oscilloscope can be used for measurement. Since the actual measured waveform is not very clear, in order to judge, the closed route can be pulled out, that is, when there is no signal input, the waveform is measured again, so that the presence or absence of the data signal is judged by comparison. If there is no oscilloscope, you can use the method of measuring DC voltage to make a rough judgment.
After a few minutes of power on, touch the surface temperature of the integrated block on the logic board by hand. If it is normal, it should be slightly hot. If the temperature rises too fast, it generally indicates that the circuit is faulty.
Under normal circumstances, the failure phenomenon caused by the damage of the logic board is as follows: the image has no layer, lack of detail or negative image; or the moving image is tailed, the definition is poor; or the image is embossed, the line is hard; or the local area on the image appears Color spots, color blocks.
Symptom 3: Vertical bars appear on the screen
Analysis and overhaul: This fault is divided into two categories: one is that the full screen is a color vertical bar; the other is a vertical bar in the upper part of the image. Because this type of fault is very similar to the fault phenomenon of LCD screen damage, it is generally used to replace the same type of logic board to accurately determine the fault location. If the fault persists after replacing the logic board, it indicates that there is a problem with the LCD screen; otherwise, it indicates that the original logic board is faulty.
Maintenance example:
1: Model: TLM4236P
Movement: LCD-MST6
Fault phenomenon: no sound, no black screen analysis and maintenance: boot backlight check light, test screen power supply 12V normal. Remote control switch machine is normal, which means that the motherboard control part works normally. Therefore, the focus is on the check of the logic board.
The logic board is mainly composed of a format converter circuit and a DC/DC converter circuit. Because the screen cannot be lit, the DC/DC converter circuit is regarded as a key inspection. For the convenience of maintenance, the control of the circuit is briefly analyzed. process.
VCC-PANEL enters the 20th, 21st, and 22nd pins of UP1, and outputs 2.5V voltage from the 18th pin (SWB) through the internal conversion of the chip. 1.8V is obtained by UP5 voltage regulation to supply power to the format conversion chip. The 11th pin (DRN) of UP1 The booster circuit composed of DP7, C227, and UP1 pins 11 outputs a VGL voltage of about -5.6V to supply negative voltage power to the row and column drive.
After the 2.51 normal operation is detected in the UP1 chip, the boost circuit consisting of LP7, LP2, DP2, DP6, UP1, 4th (SW), 5 pin (SW) internal circuit starts to work, and outputs about 16V voltage, UP1 The 27-pin (GD) provides an enable signal for QP1 and QP2. The 16V voltage passes through QP1, and QP2 obtains the VDA voltage to supply power to the row and column driver circuits.
After the above circuits are working normally, the VAAP starts normal operation through the booster circuit composed of DP5, CP18, and UP1 pin 10 (DRP). The VGHP voltage is obtained by RP21 current limit, and VGHP outputs VGH voltage of about 22V through QP8. The row and column drivers provide power.
It can be seen from the above analysis that there is a strict timing relationship when the circuit starts normally. Therefore, the voltages of each channel are checked according to the timing relationship, and it is found that the VGHP voltage is only 10.5V, while the normal voltage is 19.5V. The VGH voltage is 0V. Normally it should be 18V. Obviously the problem is caused by the VGHP voltage not being properly boosted.
After detecting the voltage of the 10th pin of UP1 is 0V, the normal voltage should be able to detect the DC voltage of 2.25V, and the AC voltage is about 5V when the AC detection is detected, but the measured AC and DC voltages are not detected. There is no abnormality in the ground resistance. It is suspected that the internal pin 10 of UP1 is damaged, and the fault is removed after replacement.
2: Model: TLM40V68P
Movement: LCD LCD-MST6M68FQ
Fault phenomenon: white screen analysis and maintenance: the boot check found that the whole machine starts normally, but the screen lights up for a long time, and after the light is on, it presents a white screen. The sound and other functions of the whole machine are normal, so the fault is determined on the logic board. .
Firstly, the power supply of the logic board is detected. It is found that there is no voltage at the VGHP detection point. When normal, the detection point should have a voltage of 19.5V. Then measure the voltage of other detection points. Remove the logic board and measure the resistance of the VGHP voltage output to ground. It is 0 ohms. Obviously, this problem is caused by the absence of VGHP voltage. When the filter capacitor CP19 is removed, the VGHP output terminal ground resistance is restored to normal, and the logic board is returned to the original machine. The startup fault remains the same, and the retest still has no VGHP voltage, forward measurement UP1 10th foot has normal DC 2.25V and AC 5V output, VAA detection point also has normal 13V voltage, thus determining that no VGHP voltage is caused by DP5 open circuit, after direct replacement and then start detection The VGHP voltage has a normal 19.5V voltage output and the unit returns to normal.
The fault is formed because the VGHP voltage is the high potential provided for the gate level, that is, the voltage of the gate level is turned on. When the liquid crystal LCD panel loses the voltage, the internal TFT of the liquid crystal LCD panel may not work normally and such a fault occurs.
3: Flower screen (with universal significance)
In general, the user menu is normal, but the image appears to be blooming. The problem is usually on the computer board and the scaler processing chip circuit. In this case, the repair welding can be judged. By pressing the chip, it is observed whether there is a significant change in the screen phenomenon. If there is a significant change, it is caused by the soldering of the chip. The special tool, such as a heat gun, heats the IC and will contact. The bad foot can be re-welded.
If there is a malfunction of the screen and the untimely flower screen, it is common for the LVDS patch cord to be in poor contact. Then clean the cable.
If the stripe and color of the flower change after the TV screen is repeatedly powered on, then the driver board is generally faulty, and the driver board can be replaced.
If the flower stripe and color do not change after repeated power-on, then the screen itself is broken. At this time, the driver board must be replaced.
A screen that can not be ignored: the screen parameter error screen parameter is the data signal sent by the motherboard signal to the screen logic board. Specifically, the motherboard of the manufacturer can be equipped with various types of LCD screens, but it needs to be corresponding in the bus. The adjustment, just like the crt color TV is changed after the tube is changed, the dark balance pillow is adjusted. The signal data is different for different screens.
Images and characters appear on the screen (this is a significant difference from the logic version of the fault), there may be screen parameter errors, then you can enter the bus screen parameters.
Take Changhong LCD TV as an example, generally enter the bus mode: in the TV mode, reduce the volume to 0, press and hold the mute button 3S on the remote control and then press the local menu button to enter the factory mode menu. Adjustment method and data: The number after Index represents the index number of the current adjustment. Under this page, you can switch to different index numbers by pressing the “P+/P-†button. During operation, press the number key directly or press the “P+/P-†key to select the adjusted item. Press the “V+/V-†key to adjust the parameters of the item.
If the LCD TV software version is too low, the screen data will be easy to change. Take Changhong as an example. You can also use USB to upgrade the machine. First decompress the software and put it in the root directory of the USB flash drive. Insert the USB flash drive. USB1 of this machine, after the whole machine is turned on, in the TV state, press the “Menu†button of the remote control and press 6—1—6—9 continuously, the machine will automatically detect the U disk and prompt whether it needs to be upgraded.
If you can't judge the reason why the TV screen is used, it is recommended to ask a professional screen repairer for repair, because there may be differences between different brands and models of LCD TVs, and some LCD TVs may have defects in the screen itself. As far as my level of screen repair is concerned, because there is no special pressure screen machine and other professional equipment, plus the limitations of its own technology, it is impossible to compare with the master screen repairer. As a result, the original screen that can be repaired is hard to repair and scrap. Therefore, it is better to be cautious about this situation.