LCD TV power board failure judgment and maintenance - android - Phones Developers

Crystal oscillator

SMF05C SOT363 ESD electrostatic protection tube 5V

The failure rate of the power board of the LCD TV in the whole machine is quite high, and it is also one of the key points and difficulties of repairing the LCD TV, which is easy to be confused. A considerable part of his energy supply to the light board drive circuit (divided into two types according to different sources of light source: high voltage board and LED light board) and the motherboard, once the TV appears not to boot, black screen, ripple interference, irregular shutdown, etc., we It is often unclear whether the fault is on the power board, motherboard, lamp (bar) or lamp driver board, causing many detours for maintenance. Based on my years of maintenance experience, combined with the typical examples encountered in the maintenance process of many users, throwing bricks and jade, using simple and easy to solve methods, to analyze the cause of the failure of the power board and troubleshooting techniques, unsolving the LCD power supply is not " Mysterious veil.
Take the TCL-PWL37C power supply circuit diagram as an example to briefly introduce the working principle of the LCD TV power supply. (Masters who have repaired the CRT color TV power supply should know that the power supply of the LCD TV is similar to that of most CRTs, only one more. PFC circuit only).

1: Standby circuit.

After the power is turned on, the 3 and 4 pins of the power output socket P3 should have a +5V voltage output to supply power to the motherboard CPU circuit. In addition, on the hot side, the 4-5 winding of the sub-switching power supply transformer T2 also outputs a set of voltages, and is rectified and filtered to output +20V, which is supplied to the PFC oscillating circuit of the main power supply and the PWM oscillating circuit. (See Figure 2.) If the output voltage is unstable, check the voltage regulator control circuit centered on IC9 (TL431). In normal operation, the voltage of pin 1 of TL431 is 2.5V. If the voltage of this pin is abnormal, it indicates that the TL431 is damaged or there is a problem with its peripheral components.
Symptom 1: No +5V voltage output.
Analysis and overhaul: Check the standby power supply circuit and find that the voltage of 5-8 pin of IC1 is 0V. After checking the current limiting part of the current limiting resistor RB 13 has been desoldered. It is recommended to replace the three current limiting resistors of RB1, RB2 and RB13 with the same resistance value of 1W or 2W to avoid damage again.
Symptom 2: The +5V voltage fluctuates around 3V.
Analysis and overhaul: No-load test machine, +5V voltage is still low, which indicates that the fault is in the standby power section. Detect the Zener diode DB4 (6.8V) and DB5 (20V) in the output voltage circuit and find that the DB5 is broken down and the fault is eliminated after the replacement.
In addition, the Zener diodes DB5 (20V), DB10 (33V), and DB8 (10V) are easily damaged in this circuit, and the fault phenomenon is mostly manifested by the +5V voltage fluctuating around +4V.
Symptom 3: The +5V output voltage is only +4V.
Analysis and overhaul: No-load test machine, +5V voltage is still low, which indicates that the fault is in the standby power section. Visual inspection found that the +5V filter capacitors CB7 and CB8 have been drummed, and the fault is eliminated after the replacement.

2. When the open/standby control circuit is turned on, the 1 pin (PS-ON) of the socket P2 on the power supply board receives the high-level power-on signal sent from the main board, and is added to the transistor QS3 through the DS9 and RS16 (BT3904, which can be replaced by C1815) The b pole, as shown in Figure 4, when QS3 is turned on, 5V voltage passes through RS 15 and then flows through the pins 1 and 2 of the optocoupler IC6. At this time, the phototransistor inside the pins 3 and 4 of IC6 is turned on, then Q11 Turn on, at this time the standby power supply output about 20V VC voltage, through the Q11 to the PFC circuit to provide about +19V working voltage (VCC2), and then through the Q12 voltage regulator to provide PWM circuit about +12V working voltage (VCC1). (See Figure 3.) During standby, the PS-ON signal is low, QS3 is off due to b is very low, optocoupler IC6 is not conducting, Q11, Q12 are off, PFC and PWM circuit stop working due to power loss .
Fault phenomenon: +5V voltage is normal, but there is no +12V and +24V voltage output.
Analysis and repair: test +5V standby voltage is normal, use the wire to connect +5V voltage to the open/standby control terminal, the voltage across the measuring capacitor C5 is only +300V, which means that the PFC circuit is not working; measure the power supply end of the PFC oscillation block IC2 (L6563) 14 The pin voltage is 0V (normal value is ++19V), which is generated by the 4-5 winding of transformer T2. Check the power supply circuit and find that the transistor Q11 breaks down and the resistor R7 opens. After replacing the damaged component, test the machine and troubleshoot.

3. PFC circuit consisting of PFC circuit block IC2 (L6563) and switching tubes QF5, QF6 (SK3568, which can be replaced by K2645), as shown in Figure 5. When the 14 pin of IC2 gets power supply of about 19V, IC2 starts to work, and the pulse signal is output from pin 13. The push-pull circuit composed of QF3 and QF4 alternately operates in the on and off states, and the power transistors QF5 and QF6 work in the switch state. .
When QF5 or QF6 is turned on, the rectified mains charges the energy storage inductor L1, and the electric energy is converted into magnetic energy and stored in L1; when the driving signal of the PFC is low, QF5 and QF6 are extremely low due to G. At the end of this time, the magnetic energy stored in L1 is released, and after being filtered by D2, C5 is filtered and superimposed with the VAC voltage, and the voltage VBUS of 370V to 410V is output. (See Figure 6)
The main pin functions of L6563: 1, 2 feet are the regulated sampling control terminal. If the external components of these two pins are damaged, the output PFC voltage is too high or too low; 4 feet are connected with a resistor to detect the flow through the switch. The current is controlled by the internal circuit to control the on-time of the switch. If the voltage of the 4-pin is greater than 1.7V, the IC will be turned off; the 7-pin is the detection terminal of the PFC output voltage, and the external sampling resistor is normally used. The voltage is between 0.26V and 2.5V. If it exceeds this range, the IC will be turned off. The 10th pin will detect the +300V voltage through the resistor divider. When the input power supply voltage is too low, the voltage of the pin is lower than 0.52. When V, the IC is turned off to achieve the input undervoltage protection function.
Symptom 1: Start burning insurance.
Analysis and overhaul: Check and find that the switch tube QF5 breaks down, but it is damaged again after the replacement. Check the Other wearing parts QF6, QF3, QF4, C5 in the part of the circuit and find that the C5 capacity becomes smaller and the fault is replaced after the replacement. exclude.
Symptom 2: After power on, the raster flashes.
Analysis and overhaul: After the power is turned on, the +12V and +24V voltages measured by the power board are unstable, and the voltage across C5 is only 300V, indicating that the PFC circuit is not working.
Check that the 14 pin of IC2 is powered normally. After the power is cut off, the resistance value of each foot of IC2 is measured. It is found that the resistance of the 1 pin to ground is very small. Check the external components and find that the capacitor CF7 is leaking, and the fault is eliminated after the replacement.
Symptom 3: After replacing some components of the PFC circuit, the PFC voltage is as high as 500V.
Analysis and repair: The power board was originally fused, the switch tube QF5, QF6 breakdown, the damaged components were replaced and turned on, measured the voltage across C5 up to 500V, immediately shut down.
The PFC voltage is high, indicating that the voltage regulation control circuit of the PFC oscillation block IC2 is faulty. The pins 1 and 2 of IC2 are the voltage-sampling sampling control terminals. The external components of these two pins are mainly checked. It is found that the external resistor RF12 of the 1-pin is open, and the fault is eliminated after the replacement.

Summary: 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 state. When the “standby” button is pressed, the CPU outputs the power-on level, and the PFC circuit works first. After converting the +300V pulsating DC voltage to the normal DC voltage (+380V), the pulse width oscillator of the main switching power supply starts to work. Then, the secondary output of the main switch transformer is +12V, +24V, and the whole machine enters the normal working state.
The PFC circuit is white, that is, the +300V voltage after rectification of the bridge stack is raised to +375V----+400V. 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 difference is that the secondary voltage is lower than that of the CRT. The other places are the same as the ordinary switching power supply. It is measured that the voltage of the large filter capacitor 330U/450V is +375V---+400V, which indicates that the power factor correction circuit works normally; if the voltage across the capacitor is +300V, the PFC circuit is not working, and the PFC oscillation is checked. integrated circuit.
When repairing the LCD power supply, first confirm the status of the fuse, the fuse is intact, and usually 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 charging resistors of several tens of kiloohms or more, indicating that the power supply has no breakdown. If the fuse is damaged, the first one is to check the PFC correction circuit switch, and the second is to check the secondary power IC.

4.PWM oscillation circuit
The PWM oscillator circuit is shown in Figure 7. When the 12 pins of IC3 (L6599) are powered, IC3 starts to work, and the pulse signals are output from pins 11 and 15, and the switches QW9 and QW 10 are turned on and off in turn.
IC3's 7-pin external resistors RM6 and RW 13 detect the PFC voltage. When normal, the pin voltage is between 1.25V and 6V. When the pin voltage is lower than 1.25V, the IC is turned off.
The T1 secondary induced voltage is rectified and filtered to obtain 12V and 24V voltages. Among them, +12V is supplied to the motherboard, and +24V is supplied to the backlight driver board. It is worth mentioning that the rectifier diodes DS1 to DS4 here are Schottky diodes, characterized by a decrease in forward voltage and a large current, and it is not possible to switch to a rectifier diode for a CRT television.
Fault phenomenon: After power on, the +5V voltage is normal, but there is no +12V and +24V voltage output.
Analysis and repair: After starting up, the +5V standby voltage is measured. Connect +5V to the open/standby control terminal (PW) with the wire. The voltage across the capacitor C5 is 380V. This indicates that the PFC circuit works normally. The cause of the fault should be IC3 work. unusual. First check the external pin 7 of IC3 and find that the RW 13 is open, and the fault is removed after the replacement.

5. Protection Circuit The protection circuit of this power supply is shown in Figure 8. If there is a +12V, +24V output at the moment of power-on, then immediately drops to 0V, which usually indicates that the circuit has entered overvoltage or overcurrent protection. In the specific judgment of the cause of the fault, the diodes DS10 and DS11 of the overcurrent protection op amp output terminal, the overvoltage protection input diodes DS7 and DS8, and the short-time energization test machine can be disconnected in turn (the power supply cannot be used for too long, so as to avoid component damage).
If it is protected by overvoltage, if the output voltage is high after disconnecting the diode, check the voltage sampling feedback loop IC7 (TL431) and its surrounding components. If the output voltage is normal, it indicates that the protection circuit itself is faulty, such as overvoltage. Protect the regulator tube from abnormalities, etc.
If it is protected by overcurrent, check the output for short circuit. The overcurrent protection comparator IC4 (LM393) and peripheral components are normal. Here we need to know the working principle of the comparator used in the overcurrent protection circuit: the comparator of one unit has two input pins and one output pin. When the voltage of the positive phase input pin is greater than the voltage of the negative phase input pin, at the output pin A high voltage is output and vice versa.
Fault phenomenon: There is +12V, +24V voltage output at the moment of power-on, and then it drops to 0V.
Analysis and overhaul: After starting up, the standby +5V voltage is measured normally, and it is judged that the fault protection circuit is started. First disconnect all the load of the power board secondary, then disconnect the protection resistor RS20, and then connect the standby +5V voltage to the open/standby control terminal to force the power on. At this time, the +12V and +24V voltage output is normal and stable. There is a problem with the overcurrent protection circuit. Connected to RS20, it is intended to disconnect the overcurrent protection diodes DS10 and DS11 successively. When the diode DS10 is disconnected, the fault no longer occurs. Check that the peripheral components of the comparator IC4 are not abnormal, and the fault is eliminated after replacing IC4.

Other faults 1: The screen is just about half an hour after the black belt interference analysis and maintenance: This phenomenon is generally caused by the leakage of the filter capacitor or the poor performance signal mixed with AC interference, the key should check the 300V filter and the output of each power supply Filter capacitors, or a relatively straightforward replacement method, can generally be effectively solved. (See Figure 1, Figure 7)
Other fault 2: Untimed shutdown analysis and repair: When the fault occurs, the power indicator does not light. At this time, the 12V output is only about 7V, and then it is reduced to 0. After inspection, it is found that ZS2 is bad, and the fault is eliminated after replacement. Among them, ZS1, CS1, CS2, CS5, CS6 damage is also easy to cause an occasional shutdown. (See Figure 7)


Summary: general output below 40 inches +5V, +12V, +24V three sets of voltage; general output above 40 inches +5V, +12V, +18V, +24 V four sets of voltage. Among them, +5 V is the standby voltage, +12V is for the digital board, +18V is for the sound, and +24 V is for the backlight. In practice maintenance, as long as each group of voltages, the same power board can be replaced.
The power board can be removed from the TV for independent maintenance. The maintenance circuit only needs to short-circuit the switch control circuit transistors C and E (or connect a 1.5K resistor to the +5V output of the sub power supply). When the power is on, all 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 condition. So on the +24 V output you can pick up a 36 V bulb of an electric bicycle for a dummy load (or a motorcycle bulb at the +12V output for a dummy load).
The protection circuit, in the liquid crystal color TV switching power supply, in addition to the common spike absorption protection circuit, is also set in the +24V, +12V and +5V voltage overvoltage, overload protection circuit, the protection circuit uses four operational amplifier LM324, Quad voltage comparator LM339, dual voltage comparator LM393 or dual operational amplifier LM358. Over-current and over-voltage protection circuit can be disconnected during maintenance. If the voltage returns to normal, it means that the protection circuit is caused. In this case, it is necessary to break step by step. Then repair it.
Before starting the machine, first confirm whether there is any explosive or capacitor bulging. If it is necessary, replace it and measure all the related devices. It is recommended to replace all the damaged devices after the test machine, it is best to remove the original fuse, and connect a 220V/100W bulb, which can effectively prevent re-flashing.
The main switching voltage +24V or +12 V has a large output current, and the rectifier diode is required to be high. Generally, a low-power difference high-power Schottky diode is used, which cannot be replaced by an ordinary rectifier diode. In addition, after the load is connected, the voltage rises instead, which is caused by poor power supply filtering.
The power supply has poor load capacity. First, we must check whether the PFC voltage is normal (380 v). If it is normal, the problem is on the thick film of the power supply, usually caused by the poor carrying capacity of the thick film of the power supply. Please pay attention to this.
On the power board, the heat sink with the yellow triangle mark and the circuit under the heat sink are hot. Direct contact with hands is strictly prohibited! Note that any testing equipment can't be directly connected between hot and cold!

The above explanation is only an example of a power supply circuit, and the fault phenomenon caused by the abnormality of the power supply circuit is various and cannot be exemplified one by one. In fact, different power supply circuits are similar, and the circuit principle is basically the same. This requires us to master the most basic circuit knowledge, and to use it flexibly in the maintenance process. The reason why many beginners feel that the power failure is impossible to start is that the basic skills are not solid. In the future maintenance process, if you want to use energy in the drawings, and do not stick to the drawings, you must have the habit of dare to throw away the crutches of the drawings, and lay the basic skills to be able to handle the faults with ease.

Related articles link: "LCD TV power board repair steps"









TCL-PWL37C power line diagram.pdf