Technical description and composition of the LG 55UH6257 TV, panel type and applicable modules. The composition of the modules.
LG LED
Model: 55UH6257 ZA
Chassis / Version: UD65R
Panel: LC550EGE (FJ) (M3)
T-CON: 6870C-0647A DESC: V16 43/49 / 55_UHD_TM120_v0.1
LED driver (backlight): integrated into PSU; 186V 180mA
Power Supply (PSU): EAX66944001 (1.3) LGP55LIU-16CH2
MainBoard: EAX66882503 (1.0)
IC MainBoard: Audio: NTP7515D
Tuner: TDJM-G301D
Technical Specifications 55UH6257
LG 55UH6257
General repair guidelines for TV LCD LED
Possible malfunctions
- The LG 55UH6257 TV does not turn on. There are no signs of operability. The indicators on the front panel do not light or flash, the TV does not respond to the remote control and control buttons.
Often in such cases, the EAX66944001 power module is faulty. Then we recommend that you measure its output voltages and, if they are absent, you should check the health of the power switches and rectifier diodes of the converters for a probable short circuit.
In case of breakdowns of semiconductors in the secondary circuits of any converter, as a rule, it can operate in emergency short-circuit mode without output voltages, and during short circuit in the elements of the primary circuit, the main fuse and, less often, the current sensor at the source of the key are most often cut off.
Mos-Fet power switches, used in switching power supplies, sometimes fail due to a malfunction of any other elements that can put it out of operation in key mode or create an excess of the maximum permissible parameters of the switch. These can be elements supplying the PWM controller, frequency-setting or damping circuits, or negative feedback elements in the stabilization circuit. PWM controllers, in the absence of visible damage or frank short-circuit between the terminals, are checked by replacing with new, or obviously working.
- There is no image, there is sound, it responds to the remote control. Or the image may appear for a second when turned on and immediately disappears.
A malfunction is usually most likely in these cases at the backlight nodes of the display matrix. The reason for this may be a break in the LED circuit, or a problem in stabilizing their power supply.
It is impossible to check the line of series-connected LEDs without a special current source, and for this purpose it is not safe to use any voltage sources above 12V for them. Then it remains only to open the panel and check each LED separately. Usually, Chinese 9V-powered multimeters light up a single 3-volt LED, if you connect the probes in the forward direction, the red probe to the anode, and the black to the cathode. In dual 6-volt LEDs, the PN junction of its emergency zener diode can serve as a health indicator. In the event of a LED malfunction, its zener diode will either be cut off or broken into short-circuit.
If the LG 55UH6257 has no picture, but there is sound, the LCD backlight LC550EGE (FJ) (M3) often simply does not work. Moreover, in LED TVs, the LEDs inside the panel usually fail, less often the LED driver.
In such cases, you can see a barely noticeable image with a successful hit of external light on the screen.
Replacing the LEDs requires disassembling the LC550EGE LCD panel - a process that requires experience and qualifications from the technician. We do not recommend doing such work on your own to the owners of TVs and craftsmen who do not have experience in repairing LCD panels
If there is no image, and the backlight is on, the screen becomes slightly lighter when turned on, and in an open TV, light is visible through the holes in the panel case.
In this case, the panel may also be faulty - damage to strips, loops, for example, as a result of moisture ingress after unsuccessful washing of the screen, the T-CON (Timing Controller) board may be faulty. But the main board MainBoard EAX66882503 often turns out to be faulty, a software failure is also possible.
Then, first of all, you need to check the panel supply voltage, the fuses on the T-CON board, as well as the supply and reference voltages of the column and line drivers - VGH, VGL, Vcom.
If these measurements are within normal limits, it is necessary to trace the passage of LVDS signals from the MainBoard EAX66882503 with an oscilloscope, as well as synchronization signals to the loops with drivers.
Flooded or unsuccessfully washed screens of LED TVs can be restored, in about half of the cases, usually if significant damage has not formed in the loops and panel strips.
- The indicator on the front panel blinks, the TV does not turn on in operating mode, it does not respond to the remote control.
Repair or diagnostics of the EAX66882503 motherboard should begin by checking the stabilizers and power converters needed to power the chips and matrix. If necessary, update or replace the software (software). In cases of attempts to repair the MB board (SSB), you must check the health of its elements - Audio: NTP7515D which may require replacement with new ones. If a BGA processor is installed, there is a possibility of a violation of the soldering of the contacts of its conclusions with the board (it is diagnosed by heating).
The TDJM-G301D tuner malfunction is established after checking the software and all supply voltages at its terminals. The data exchange of the tuner with the processor via the I2C bus can be controlled by an oscilloscope.
Backlight driver current limiting. MAP3516.
General recommendations
To reduce the backlight current in forward LED drivers with a two-channel MAP3516 step-down controller, you can proportionally increase the resistance of the current sensors at the sources of the power switches of the converters.
The resistor values must be increased equally in both channels.
Currently, many TV manufacturers set a dubious thermal regime for backlight LEDs, which negatively affects the durability of the device. In half of the cases, after 2-3 years of operation, the LEDs fail due to overheating, usually this can be seen from the destroyed phosphor on the diode bodies. As a rule, the LEDs still manage to work out the warranty period.
Even if the maximum permissible current is within the normal range, the cooling of the LEDs is not always effective enough, which can be seen from the traces of overheating - dark spots on the textolite strips on the back. And in modern LG TVs, diodes with an external phosphor coating are used, which after a year or two crumbles and the crystal directly shines in purple. Nobody thinks about how such a source of ultraviolet radiation can harm the health of users.
Chinese manufacturers through Aliexpress supply diodes and LED strips in sets of any assortment, but the same happy owner of the TV pays for them and for repairs. After replacing one or several faulty LEDs, it is useful to look closely at the rest, if the phosphor is cracked, it is advisable to replace all such diodes. If you measure the voltage drop across overheated LEDs,
it will be slightly higher than that of neighboring less worn out or new ones, which indirectly indicates the presence of parasitic active resistance (ESR). Further operation of such LEDs is even more questionable. If you reduce the current in the diodes, the power dissipation and the real operating temperature decrease, then there is a chance that the old ones will still work
Ways to Limit Current in LED Backlight Drivers
There is a lot of information on the Internet about ways to limit the current in backlight LEDs for different TVs and LED drivers. Much is written plausibly, but sometimes people far from electronics write in order to publish any popular content on hot topics to raise the rating of sites.
Within the framework of one article, it is impossible to tell about each case separately, because even in the same models, different panels and different boards with their own driver options can be installed. But there are basic principles that are clear to masters even with minimal knowledge and skills.
There are three main ways to reduce the backlight current.
1. Increasing the resistance of the LED current sensor - low-resistance measuring resistors in the cathode circuit (LED-).
2. By increasing the resistors at the ISET input (current setting) of the LED driver microcircuit.
3. By changing the values of the resistors in the divider at the control input ADIM (Dimming - the brightness of the glow).
The fundamental difference between the ISET input and ADIM is that ISET is an inverting input, like FB, and ADIM is direct.
Let's consider these options in more detail.
Step-Up Led Drivers:
The first method is the simplest and most popular, it is used in simplified drivers that usually do not have an ISET input, and current regulation and stabilization is carried out according to the general principle of PWM modulation through Negative Feedback (OOS), for example OB3350CP, OB3353CP, SN51DP, BIT3267. Such microcircuits are often made in 8 pin planar packages.
In fact, this is a typical circuitry of a flyback step-up converter with load current stabilization. In this case, the voltage from the current sensor is supplied to the inverting input FB of the PWM microcircuit (FlyBack - feedback). BIT3267 has this pin designated INN.
The contacts of the LED-connector from the LED strips can be connected to the current sensor directly ), or through MosFet keys that perform the On / Off function, then the current sensor is included in the source of the key.
Low-resistance resistors are usually used as a current sensor, one or several, connected in parallel. More often their denominations are in the range of 1 - 4.7 ohms.
It may be enough to change the ratings, or simply remove one or two resistors from the common assembly, then the resistance of the sensor will increase, the voltage on it and at the FB input will proportionally increase, and the PWM in OOS will work towards decreasing the current.
The dependence is inversely proportional, if you double the total resistance of the sensor, the current will be halved.
To calculate the total resistance when resistors are connected in parallel, you can use a calculator to make the necessary proportion to set the desired current. It can be difficult to count even two denominations orally, because conductivity is added - values inversely proportional to resistances. 1 / Rtot = 1 / R1 + 1 / R2 then Rtot = (R1 R2) / (R1 + R2).
Owners and users of the LG 55UH6257 TV should remember that self-repair without special knowledge, skills and qualifications can be fraught with negative consequences that can lead to complete non-repairability of the device!
DOWNLOAD SCHEMATIC DIAGRAM / SERVICE MANUAL ------------------> CHASSIS : UD65R FOR MODELS: 55UH615*, 55UH615*-ZB, 55UH62**, 55UH62**-ZA, 55UH620V, 55UH6257, 55UH625V, 55UH6157, 55UH615V.
The appearance of the MainBoard EAX66882503 is shown in the figure below:
EAX66882503
The appearance of the power supply
Main features of the device LG 55UH6257:
Installed matrix (LED-panel) LC550EGE (FJ) (M3).
The matrix control uses the Timing Controller (T-CON) 6870C-0647A.
To power the backlight LEDs, a converter is used, combined with a power supply.
The formation of the necessary supply voltages for all nodes of the LG 55UH6257 TV is carried out by the EAX66944001 power module, or its analogues.
MainBoard - the main board (motherboard) is an EAX66882503 module, using Audio chips: NTP7515D and others.
The TDJM-G301D tuner provides TV reception and channel tuning.
LG 55UH6257 TV with LCD display LC550EGE-FJM3, diagonal 55 "(140 cm) 16: 9 format. This model uses a panel with edge LED backlighting - Edge LED.
The LC550EGE-FJM3 sensor and MainBoard EAX66882503 software are capable of realizing the highest UHD image quality at 3840x2160 resolution in 4K UHD, HDR. Image processing is carried out using modern High Dynamic Range (HDR) technology with an extended range of brightness gradations.
TV model 55UH6257 has built-in SMART software and webOS operating system installed. The connection to the network can be carried out via Wi-Fi wireless connection. Supported media file formats: MP3, WMA, MPEG4, HEVC (H.265), MKV, JPEG.
Speaker power 20 W (2x10 W) is provided by two speakers. The NICAM stereo audio processing system and the Sound Surround function are used.
Communication with other devices via an external interface is supported by standard input and output connectors: antenna input (RF), AV, component, HDMI x3, USB, RS-232, Ethernet (RJ-45), Wi-Fi, WiDi, Miracast. A headphone jack is provided. The digital tuner is capable of receiving TV channels in the DVB-T, DVB-T2, DVB-C, DVB-S2 standards.
The TV uses an EAX66944001 power supply.
Dimensions: With stand 1240x778x226 mm, Without stand 1240x722x57 mm.
Attention! The LC550EGE-FJM3 panel of the LG 55UH6257 LED TV is an expensive and unreliable element.
Do not allow any impact on the glass and pressure on the screen surface to avoid damage to the LED panel!
It is also unsafe for liquid to spill onto the screen. Sometimes one drop is enough if it drips down the glass onto the cables to make the TV unrepairable.
