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How to repair led electronic display LED display fault judgment method

How to repair led electronic display LED display fault judgment method

2019-08-14

How to repair led electronic display?

When the display is the most important modern display device, a few simple maintenance analyses are performed below.

The tools you need are: multimeters, soldering irons, blades, tweezers.

Judging the problem must be handled first and second, and the problem will be dealt with obviously and severely.

The short circuit should be high priority. 1. Resistance detection method. Adjust the multimeter to the resistance file, check the resistance value of a certain point on the normal circuit board, and then test whether the same point of the same circuit board is different from the normal resistance value. If it is different, it is determined. The scope of the problem.

Let's take a closer look at the related knowledge of LED electronic display repair!

1:The role of the chip

 

The role of 74HC245

Signal power is amplified. The first pin DIR is used for input/output port conversion. When DIR=“1” high level, the signal is output by “A” terminal input “B” terminal. When DIR=“0” low level, the signal is input from “B” terminal. "End output. Pins 2-9 "A" signal input and output, A1 = B1,,,,,, A8 = B8, A1 and B1 are a group, if DIR = "1" G = "0", then A1 input B1 output, Other similar. If DIR = "0" G = "0", B1 inputs A1 output, and the others are similar. Pin 10 GND, power ground. Pins 11-18 "B" signal input and output, the function is the same as the "A" end, not described. The 19th pin G, the enable terminal, if the pin is "1", the signal of the A/B terminal will not be turned on. When only the "0" is enabled, the A/B terminal is enabled, and the pin acts as a switch. The 20th foot VCC, the positive power supply.

The role of 74HC04

6-bit inverter. Pin 7 GND, power ground. The 14th foot VCC, the positive power supply. The signal is input from the A terminal to the Y terminal inverting output, A1 and Y1 are a group, and so on. Example: A1=“1”, then Y1=“0”, A1=“0”, then Y1=“1”, the other groups have the same function.

 

The role of 74HC138

 

Eight-bit binary translation decimal decoder. Pin 8 GND, power ground. Pin 15 VCC, power supply positive pin 1-3, A, B, C, binary input pin. The 4th to 6th chip select signal control, only when the 4th and 5th feet are "0" and the 6th pin is "1", it will be strobed, and the output is controlled by the A, B, C signals. Any other combination will not be strobed, and the Y0~Y7 outputs are all "1". It is cascaded by controlling the strobe pin to extend it to sixteen bits. Example: G2A=0, G2B=0, G1=1, A=1, B=0, C=0, then Y0 is “0” Y1-Y7 is “1”.

The role of 74HC595 

 

LED driver chip, 8-bit shift latch. Pin 8 GND, power ground. 16th pin VCC, positive power supply Pin 14 DATA, serial data input port, display data is entered by this, must be matched with the clock signal to move in. Pin 13 EN, enable port, when the pin is "1", the QA~QH ports are all "1". When it is "0", the output of QA-QH is controlled by the input data. The 12th pin STB, the latch port, when the input data is in the incoming register, only the input of a latch signal can send the shifted data to the QA-QH port output. Pin 11 CLK, clock port, each clock signal will shift a bit of data into the register. The 10th pin SCLR, reset port, as long as there is a reset signal, the data moved in the register will be cleared, the display does not need this pin, generally connected to VCC. The 9th pin DOUT, the serial data output, passes the data to the next one. Pins 15, 1-7, the parallel output port is also the drive output port, driving the LED.

 

The role of 4953 

Line drive tube, power tube. The inside is two CMOS tubes, 1, 3 pin VCC, 2, 4 pin control pin, 2 pin control 7, 8 pin output, 4 pin control 5, 6 pin output, only when 2, 4 feet are "0" ", 7, 8, 5, 6 will output, otherwise the output is high impedance.

 

 The role of TB62726 

 

LED driver chip, 16-bit shift latch. Pin 1 GND, power ground. Pin 24 VCC, positive power supply, pin 2 DATA, serial data input pin 3 CLK, clock input. Pin 4 STB, latch input. Pin 23 output current adjustment terminal, connect resistor to adjust pin 22 DOUT, serial data output pin 21 EN, enable input other functions similar to 74HC595, except TB62726 is 16-bit shift latch with output current adjustment Function, but there is no high level on the parallel output port, only high resistance state and low state. The 74HC595 parallel output has high and low outputs. The TB62726 has the same pin function as the 5026 and is similar in structure.

 

 

LED display common signal

CLK clock signal:

 A shift pulse supplied to the shift register, each pulse causing data to be shifted in or out of one bit. The data on the data port must be coordinated with the clock signal to transmit the data normally. The frequency of the data signal must be 1/2 times the frequency of the clock signal. In any case, when the clock signal is abnormal, the whole board will be displayed in a mess.

 

STB latch signal: 

The data in the shift register is sent to the latch and its data content is displayed by the drive circuit lighting LED. However, since the driver circuit is controlled by the EN enable signal, it must be lit to enable the on state. The latched signal also needs to be coordinated with the clock signal to display the complete image. In any case, when the latch signal is abnormal, the whole board will be displayed in a mess.

 

 EN enable signal:

 

 The full screen brightness control signal is also used for display blanking. The brightness can be controlled by adjusting its duty cycle. When the enable signal is abnormal, the whole screen will appear unlit, dark or trailing. Data signal: Provides the data needed to display an image. It must be coordinated with the clock signal to transfer the data to any of the display points. Generally, the red, green and blue data signals are separated in the display. If a data signal is short-circuited to the positive or negative pole, the corresponding color will appear full or no light. When the data signal is suspended, the corresponding color is displayed. The situation is uncertain.

 

ABCD line signal: 

Only existed in the dynamic scan display, ABCD is actually a binary number, A is the lowest bit. If the ABCD signal is controlled by binary, the maximum range is 16 lines (1111), as long as the AB signal is 1/4 scan, because AB The representation range of the signal is 4 lines (11). When an abnormality occurs in the line control signal, misalignment, highlighting, or image overlap will occur.

 

Third, common troubleshooting methods (tools: multimeters, electric iron, blades, screwdrivers, tweezers, etc.)

Fourth, LED electronic display repair

 

1. Judging the problem must be handled first and second, and the problem will be dealt with obviously and severely.

The short circuit should be the highest priority.

1.Resistance detection 

Adjust the multimeter to the resistance file, check the resistance value of a certain point on the normal circuit board, and then test whether the same point of the same circuit board is different from the normal resistance value. If it is different, it is determined. The scope of the problem.

2. Voltage detection method 

Adjust the multimeter to the voltage file to detect the ground voltage at a certain point of the circuit suspected of having a problem, and compare whether it is similar to the normal value. Otherwise, the range of the problem is determined.

Short circuit detection

 Adjust the multimeter to the short-circuit detection block (some diode voltage drop or resistance file, generally have alarm function), detect whether there is a short circuit phenomenon, and find that the short circuit should be solved first, so that it does not burn other devices. This method must be operated in the event of a power failure to avoid damage to the meter. 

3. Pressure drop detection 

Turn the multimeter to the diode drop detection file, because all ICs are composed of a large number of basic unit parts, only miniaturized, so when there is current on one of its pins, it will exist on the pin. Voltage drop. Generally, the voltage drop on the same pin of the IC of the same model is similar. According to the voltage drop value on the pin, it is necessary to operate under the condition that the circuit is powered off. This method has certain limitations. For example, if the device under test is high-resistance, it will not be detected. 

Cell board common problem handling

Cell board failure 

A. The whole board is not bright

 1. Check if the power supply and signal cable are connected. 

2. Check whether the test card is identified by the interface. If the red light of the test card flashes, it is not recognized. Check whether the light board is connected to the test card, or the signal board is shorted by the ground and the interface is unrecognizable. (smart test card)

3. Detect whether the 74HC245 has a short circuit or not, and the corresponding enable (EN) signal input and output pin on the 245 is soldered or short-circuited to other lines. Note: Mainly check the power and enable (EN) signals.

 B. When the oblique scan is performed, the regular interlaced display does not light up and the screen overlaps.

1. Check whether the A, B, C, D signal input port to 245 is broken or short-circuited or short-circuited.

 2. Detect whether there is an open circuit or a virtual solder or a short circuit between the output terminals A and B, C and D corresponding to 245.

 3. Detect whether there is a short circuit between the signals of A, B, C and D or a short circuit between the signal and the ground. Note: The ABCD line signal is mainly detected. 

C. One or a few lines are not lit when fully illuminated

 

1. Detect whether the line between 138 and 4953 is open circuit or virtual welding or short circuit.

 D. In the line scan, two lines or several lines (usually a multiple of 2, regular) are lit at the same time

1. Detect whether there is a short circuit between the signals of A, B, C and D. 2. Check if the output of the 4953 is shorted to other outputs. 

E. There is a single point or multiple points (irregular) when it is fully lit.

1. Find whether the control pin corresponding to the module is short-circuited with the line. 2. Replace the module or single lamp. 

F. One column or several columns are not lit when fully illuminated 

1. Find the pin that controls the column on the module and measure whether it is connected to the output of the driver IC (74HC595/TB62726).

 G.Have a single point or single column highlight, or the entire line is highlighted and uncontrolled

1. Check if the column is shorted to the power ground. 2. Check if the line is shorted to the positive terminal of the power supply. 3. Replace the driver IC.

 H.The display is confusing, but the signal output to the next board is normal.

 1. Detect 245 corresponds to whether the STB latch output is connected to the latch terminal of the driver IC or the signal is shorted to other lines. I. Display confusion, output is not normal 1. Check whether the clock CLK latches the STB signal for short circuit. 2. Check if the clock CLK of 245 has input and output. 3. Check if the clock signal is shorted to other lines. Note: The main detection clock and latch signal.

J.Display lack of color 

1. Detect whether there is input or output on the data end of the color of 245. 2. Check if the data signal of the color is shorted to other lines. 3. Check whether the cascaded data port between the driver ICs of this color has an open circuit or short circuit or a virtual solder. Note: It is easier to find the problem by using the voltage detection method. Check whether the voltage of the data port is different from normal and determine the fault area.

 K.Have a problem with the output

1. Detect whether the output interface to the signal output IC is connected or shorted. 2. Check if the clock latch signal of the output port is normal. 3. Check if the cascaded output data port between the last driver IC is connected to the data port of the output interface or is shorted. 4. Whether the output signals are short-circuited to each other or short-circuited to the ground. 5. Check if the output cable is good. 

Full screen failure

A. The whole screen is not lit (black screen)

1、Check if the power supply is powered.

2、Check if the communication line is connected, and if there is any error. (synchronous screen) 

3、The sync screen detects whether the green light of the sending card and the receiving card is blinking. 

4、Whether the computer monitor is protected, or the display area is black or pure blue. (synchronous screen)

B. The entire unit board is not lit (black screen) 

1、The horizontal direction of several consecutive boards is not bright, check whether the cable connection between the normal unit board and the abnormal unit board is connected; or whether the chip 245 is normal.

2、If several consecutive boards are not lit in the vertical direction, check if the power supply of this column is normal. 

C. The unit board does not light up.

1、Check if the foot is connected to the output of the 4953.

2、 Check if 138 is normal. 

3、Check if the 4953 is hot or burnt. 

4、Check if the 4953 has a high level.

5、 Check if the 138 and 4953 control pins are connected.

D. The unit board is not bright

1、Check if 595 is normal. 

2、Check whether the corresponding pin of the upper and lower modules is connected.

3、Check if the 595 output pin is connected to the module pin.

E. Cell board lack of color

1、Check if the 245 R.G data has output. 

2、Check if the normal 595 output pin is connected to the abnormal 595 input pin.

 


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