This article is mainly about the relevant introduction of Samsung 788df display, and focuses on the reasons for the black screen of Samsung 788df display and its common faults.
Samsung 788df display won't turn on/black screen solutionThere are many reasons for the black screen. If the graphics card is damaged or the monitor is disconnected, etc., no signal is transmitted to the monitor, the indicator light of the monitor will keep flashing to indicate that no signal is received. If the resolution is set too high, exceeding the maximum resolution of the display will also cause a black screen, which may destroy the display. However, the current display has a protection function, which automatically protects when the resolution exceeds the set value. In addition, hardware conflicts can also cause a black screen.
A white LCD screen
A. A white screen indicates that the backlight board can work normally. First of all, determine whether the motherboard can work normally. You can press the power switch to check whether the indicator light responds. If the indicator light can change color, it indicates that the motherboard is working normally.
1. Check whether the connection line of the main board signal output to the screen has bad contact (the connection line or screen can be replaced).
2. Check whether the voltage of each operating point of the motherboard is normal, especially the power supply voltage of the screen
3. Use an oscilloscope to check line and field signals and clock signals (from input to output)
B. If the indicator light does not respond or does not light up, it indicates that the motherboard is not working properly
1. Check the voltage of each operating point of the motherboard, pay attention to the voltage of the EEPROM (about 4.8V), the reset voltage (high level or low level, depending on the model), and the MCU voltage. If there is a short circuit in the power supply, carefully find the location of the short circuit. There may be a short circuit in the copper foil of the PCB.
2. Check whether the MCU pins are in good contact with the motherboard:
3. Check whether the motherboard chip and MCU are working. Use an oscilloscope to measure whether the crystal oscillator starts to oscillate.
4. If necessary, replace the MCU or reprogram the MCU;
two. LCD black screen
A. First of all, determine whether it is the motherboard problem or the backlight board problem. You can check whether the indicator light responds. If the indicator light is not on, you should check the motherboard power section "
1. Use a multimeter to measure the working points of the main power supply, whether the fuse is blown, if it is blown, disconnect the power supply, use the resistance file to measure whether there is a short circuit at each main power supply working point, and carefully find the line if there is a short circuit (whether the copper foil of the circuit board is short-circuited ) And various related components (whether damaged or not connected with tin). } |3 S2
2. If there is no short-circuit phenomenon, you can refer to the white screen phenomenon for maintenance to ensure that the voltage and signal input and output of each operating point are in normal working conditions
B. If the working status of the motherboard is normal, check the backlight board
1. Check whether the connection between the main board and the backlight board is in contact with the main beam
2. Use a multimeter to measure the voltage of the backlight. There must be a power supply voltage of 12V, a switching voltage of 3.3V-5V and a backlight adjustment voltage of 0-5V. The switching voltage of the backlight is the most important. If there is no voltage or the voltage is too low, To check whether the output level of the MCU and the working state of the triode are normal, pay attention to whether there is a short circuit, and replace the components if necessary
three. Lack of color on LCD
1. Check whether there is a short-circuit or virtual solder between the main chip and the connector (pay attention to the chip feet, chip exclusion and connector, especially the flat socket)
2. Check whether there is bad contact between the screen and the motherboard connection line such as flat cable
3. If necessary, replace the motherboard, cable, and even the screen to find out the problem
four. The LCD monitor button does not work
1. Measure the voltage to the ground of each button. If the voltage is too low or 0, then check whether there is a short circuit, open circuit, pull-up resistance, false soldering, seat and connecting wire from the button board to the MCU. Poor contact
2. Pay attention to whether the button itself is damaged
Fives. The dual-color indicator of the LCD does not light up or only one color
1. Check the circuit of the indicator light. Whether the level of the triode controlled by the MCU output to the indicator light is normal, usually a high level 3.3V and a low level 0V. When the switch is switched on and off, the two levels will be reversed. If it is not normal, check whether there is a short circuit between the circuit and the MCU, or the solder joint
2. Check whether the power supply voltage (5V) of the triode is normal, whether the output of the triode is normal, and the voltage at both ends of the indicator light can be measured
3. Check whether there is poor contact between the motherboard socket and the key board, and whether the circuit board is short-circuited to the ground
4. It is necessary to replace the indicator
six. LCD color cast:
1. Check the main board signal R\G\B from the input to the main chip part of the circuit (whether there is a short-circuit with a virtual solder, and whether the capacitor resistance has an incorrect value)
2. Enter the factory mode, adjust the white balance, whether the normal color can be recalled
3. Replace the MCU or reprogram the MCU if necessary
Seven. LCD screen flower screen
1. Measure whether the main board clock output is normal
2. Check the main board signal R\G\B from the input to the main chip part of the circuit (whether there is a short-circuit with a virtual solder, and whether the capacitor resistance has an incorrect value)
3. Check whether there is a short circuit between the main board signal output and the connection socket output to the screen (IC pin exclusion and dual row pins, pay special attention to flat sockets)
4. Replace the screen if necessary
Eight. No signal on LCD:
A. No input signal appears after power-on (NO VGA INPUT)
1. Check the VGA cable connection
2. Check the main board from the line field input (note whether there is a short circuit between the line field of the VGA female socket and the ground) to the inverter output and then to the main chip part of the line (whether there is a virtual solder short circuit, whether the capacitor resistance has an incorrect value)!
3. Check the voltage of each operating point of the motherboard (it may be due to damage to the main chip)
B. Exceeding display (VGA NOT SUPPORT) or (FREQENCY OUT OF RANGE) appears after power-on
1. Check whether the computer input signal is out of range
2. Check the voltage of each working point of the motherboard (it may be due to damage to the main chip)
nine. LCD screen flickers (word jitter)
1. Whether the "phase" can be adjusted with automatic adjustment or manual adjustment
2. Check the voltage of each working point of the motherboard (it may be due to damage to the main chip)
3. Check whether the capacitor resistance of the phase-locked loop has an incorrect value
4. Check the main board from the line field input to the inverter output and then to the main chip part of the circuit (whether there is a false solder short-circuit capacitor resistance and whether there is an incorrect value)
ten. LCD ghosting
1. Check the input signal, whether it is caused by the connection and distribution or the VGA cable is out of specification
2. Check whether the motherboard VGA socket is soldered or connected
3. Check the main board from the signal input to the chip part of the line for false soldering, short circuit, wrong value of capacitance and resistance
4. Check the voltage of each operating point of the motherboard (it may be due to damage to the main chip)
Samsung 788df display circuit analysis 1. Rectifier filter circuit
When the display has 220V mains voltage input, the AC voltage of about 220V is filtered by R601, R602, L601, C602, C603 and C604 to filter the high frequency interference in the AC, and then it is divided into two outputs: one is sent to the controlled degaussing circuit; Two channels are bridge rectified by D601 rectifier bridge to obtain pulsating DC voltage. This voltage is not only limited by the negative temperature coefficient thermistor TH602, generates a DC voltage of about 300V across the filter capacitor C605, but also is stepped down by the resistors R606 and R605, and then filtered by the filter capacitor C606 to establish a starting voltage across C606. .
2. Start-up and oscillation circuit
The 300V voltage is sent to the 1 pin of the power supply thick film integrated circuit IC601 (8S0765RRC) through the primary winding (8-4 winding) of the switching transformer T601, which supplies power to the switch tube connected to the 1 pin of the 8S0765RRC. At the same time, the start-up voltage at both ends of C606 is sent to pin 3 of 8S0765RRC, which supplies power to the control circuit in 8S0765RRC. After pin 3 of 8S0765RRC has power supply, the circuit starts to work, and the control circuit generates an excitation voltage to make the internal switch tube work in the on-off state.
During the on-time of the switch tube, because the rectifier tube connected to the secondary winding of T601 is in the off state, the electric energy is stored in the T601 in the form of magnetic energy; during the off-time of the switch tube, the magnetic energy is converted into electric energy and coupled by the secondary winding of T601, and then through The rectifier filter circuit generates a DC voltage to provide a supply voltage to the load. At the same time, T601 generates 2 positive and 1 negative pulse voltages from the feed winding. This voltage is rectified by D606, current limiting by R607, and rectified by D607. The DC voltage generated at both ends of C606 is then passed through switches SW1 and SW2 to provide the 8S0765RRC with a working voltage after the start is completed.
3. Voltage stabilization control circuit
When the mains power rises or the load becomes lighter, the output voltage of the switching power supply rises, the pulse voltage at pin 3 of the switching transformer T601 is rectified by D605, and after filtering by C616 and C615, the generated DC voltage rises. Through ZD602, control Q601 conduction With enhanced performance, the Q601 collector voltage drops, causing the 4 pin potential of the 8S0765RRC to drop. The on-time of the switch tube in the 8S0765RRC is shortened, and the output voltage drops to achieve the purpose of stabilizing the output voltage.
When the output voltage drops, the voltage stabilization control process is reversed.
4. Line frequency trigger circuit
During the normal operation of the color display, the OFF1 signal output by the microprocessor IC201 is at high level and Q602 is turned on, so the CLAMP pulse synchronized with the horizontal frequency output from pin 27 of IC201 is added to the primary winding of the transformer T602 via C621. The secondary winding of T602, C613, C611 are coupled to pin 5 of 88S0765RRC. After the 8S0765RRC has a synchronous trigger signal input, it will work on the horizontal frequency, which avoids the difference in the operating frequency of the main power supply circuit and the horizontal scanning circuit, which affects the normal operation of the display.
5. On/off control circuit
After the power switches SW1 and SW2 are turned on, R611 is short-circuited, so that pin 3 of the 8S0765RRC has a normal self-feeding voltage input, and the switching power supply is in a normal working state. When SW1 and SW2 are disconnected, pin 3 of 8S0765RRC cannot get the starting voltage, and the switching power supply cannot work. Therefore, the purpose of shutting down can be achieved.
6. Protection circuit
(1) Spike absorption circuit In order to prevent the switch tube in the 8S0765RRC from breaking down the 8S0765RRC by the peak of the induced pulse voltage at the drain of the switch tube during the cut-off period, the main power supply circuit of this machine is set by D603, R603, C608 and D604, R604, C609. It consists of two sets of spike absorption loops.
(2) Overvoltage protection circuit. When the 50V voltage output by the switching power supply is too high for some reason, the Zener tube ZD201 breaks down, and the current is limited by R297 and then added to the 19th pin of the microprocessor IC201, and the 19th pin of the IC201 becomes At high level, IC201 controls STV9118 via IIC bus to stop outputting line and field driving pulses and +B driving pulses, and the line and field scanning circuit and +B power supply stop working.
7. Degaussing circuit
The degaussing circuit of the display adopts a degaussing circuit composed of a microprocessor and a relay. The specific working process is: at the moment of entering the power-on state, the degaussing control terminal 10 pin of the microprocessor IC201 outputs a high level, so that the current amplifying tube Q621 is turned on. After Q621 is turned on, the 14V voltage passes through R621, the coil of relay RL601, and the ce structure of Q621 to form a conduction loop, so a magnetic field is generated in the drive coil of RL601, so that the AC contact of RL601 is attracted. After the AC contact is attracted, the degaussing circuit is connected to the circuit. At this time, using the thermal sensitivity of the degaussing resistor TR601, an alternating magnetic field from strong to weak is generated in the degaussing coil to complete the degaussing process of the kinescope and its accessories. After about 2s, the pin 10 of IC201 becomes low level. , The AC contact of the relay RL601 is released. In this way, not only the failure rate of TH601 is reduced, but also the efficiency of the degaussing circuit is improved.
This is the end of the relevant introduction about the Samsung 788df display. If you have any deficiencies, please correct me.
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