Tips for LED TV Repair

Wed Sep 05 00:00:00 CST 2018

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Tips for LED TV Repair


1. After the LCD power is turned on, the auxiliary power supply works first, and the +5v voltage is output to the CPU on the digital board. At this time, the whole machine is in the standby mode. When the “standby” key is pressed, the CPU outputs the boot level, and the PFC circuit works first. After converting the +300V pulsating DC voltage into a normal DC voltage (+380V), the pulse width oscillator of the main switching power supply starts to work, and then the secondary output  +12V, +24V voltage of the main switching transformer, The machine is in normal working condition.


2. What is a PFC circuit? PFC circuit is to increase the +300V voltage to +375V---- +400V after the bridge pile rectification. This is the first point of difference between the power supply of the LCD TV and the power supply of the CRT TV. The second point of difference is that the secondary voltage is lower than the CRT. The rest is same as the normal switching power supply. The voltage across the 330U/450V of the large filter capacitor is +375V-----+400V. It indicates that the power factor correction circuit works normally. If the voltage across the capacitor is +300V, the PFC circuit is not working. Mainly check PFC oscillation integrated circuit.


3. When repairing the LCD power supply, first confirm the status of the fuse, the fuse is intact, and the switch tube in the PFC correction circuit does not fail. Then measure whether there is a short circuit between the large electrolytic capacitor and the ground, and There are several dozen kiloohms of charging resistors, indicating that the power supply has no breakdown. If the fuse is damaged, the first one is check the PFC correction circuit switch, the second is to check the secondary power IC.


4. The general output are +5V, +12V, +24V below 40inch; the general output are +5V, +12V, +18V, +24V four sets of voltage above 40inch . Among them, +5V is the standby voltage, +12V is for the digital board, and +24V is for the backlight. In practice maintenance, the board can be replaced as long as each group of voltages and power are same.


5. The power board can be removed from the TV for independent maintenance, maintenance only need to short the switch control circuit transistors C, E (or a 1.5K resistor connected to the +5V output of the sub-power supply). The whole machine is in the power-on state, and all the voltages are output. In the switching power supply of some LCD TVs, only the +12V or +24V output has a certain power load, and the main switching power supply is in normal working state, so On the +24V output, you can connect a 36V bulb of an electric bicycle to make a dummy load (or connect a motorcycle bulb to the +12V output to make a dummy load).


6. protection circuit, in the switching power supply liquid crystal TV, in addition to the common spike absorption protection circuit, it is also set in +24V, +12V and +5V voltage over-Asian, overload protection circuit, its protection circuit uses four operational amplifier LM324 The quad voltage comparator LM339, the dual voltage comparator LM393 or the dual operational amplifier LM358 pass through the overcurrent protection circuit. It can be disconnected during maintenance. If the voltage returns to normal, it means that the protection circuit is caused. At this time, it is necessary to disconnect and Check which circuit is working, then repair it.


7.When booting , first confirm whether there are Components explosion or Capacitance bulge. If have this kind of situation pls change the related components and measure all the related components again, it is recommended to replace all damaged components 、remove the original fuse and connect a 220V/100W bulb when test , which can effectively prevent the components explode again.


8. The main switch voltage +24V or +12V output current is large, the rectifier diode is required to be high. High-power Schottky diodes with low dropout voltage are generally used., which cannot be replaced by ordinary rectifier diode. After the load is connected, the voltage is reversed Rising, mostly due to the poor power supply filtering.


9. First, we must check whether the PFC voltage is normal (380V) if the power supply with poor load capacity. it is normal, the problem is on the thick film of the power supply. Usually, the thick film load capacity of the power supply is poor. Please pay attention to this.


10. On the power board, the heat sink with the triangular mark and the circuit under the heat sink all are hot. Strict direct contact with hands, pay attention to any testing equipment, can not be directly connected between hot and cold



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