Beijing LG TV after-sales service hotline http://blog.tianya.cn/blogger/blog_main.asp?BlogID=3701288 technical communication. Inspection skills for switch power supply faults in LG LCD color TVs. The switch power supply circuit of LCD color TVs is very similar to the basic principle of CRT color TVs. Therefore, there are also many similarities in the inspection methods for common faults. Below is a brief description of the inspection ideas and methods for the five more common faults in the switch power supply circuit of LCD color TVs.
1. Fuse blown
For fuse blown faults, it is usually necessary to check the filter capacitors and rectifier diodes in the main power rectification and filtering circuit. Of course, faults in the interference suppression circuit can also cause the fuse to blow and turn black. It should be noted that a blown fuse caused by a breakdown of the switch transistor usually results in simultaneous damage to the overcurrent detection resistor and the power control integrated circuit. Negative temperature coefficient thermistors are also prone to burn out with the fuse, so attention should be paid to checking them during repairs.
2. No voltage output, but the fuse has not blown
If there is no voltage output but the fuse has not blown, this indicates that the switch power supply circuit is not working or has entered a protection state after working. During inspection, first measure whether there is a start-up voltage at the start-up pin of the power control integrated circuit.
(1) If there is no start-up voltage or the start-up voltage is too low, then check if there is any leakage in the start-up resistor and the components connected to this pin.
(2) If there is a start-up voltage, then measure whether there is a high and low level signal change at the output end of the power control integrated circuit at the moment of turning on.
- If there is no change, it indicates that there is a fault in the power control integrated circuit itself or its peripheral oscillation circuit components or protection circuit. The fault can be found by first replacing the power control integrated circuit and then checking the peripheral components.
- If there is a change, it is generally due to poor performance or damage of the switch tube itself, which should be checked carefully.
3. Output voltage too low
The reasons for the fault of too low output voltage at the output end of the switch power supply include abnormal voltage regulation control circuit, as well as the following three aspects:
(1) Degraded performance of the switch tube. This situation will lead to the inability of the switch tube to conduct normally, making the internal resistance value of the power supply increase and reducing its load-carrying capacity.
(2) Failure of the output end rectifier diode and filter capacitor. This can be judged by the replacement method to determine if they are damaged.
(3) Short circuit fault in the load of the switch power supply. Especially short circuits or poor performance in DC/DC converters. For this, the method of disconnecting all loads of the switch power supply circuit can be used to distinguish whether it is a problem with the switch power supply circuit or a fault in the load circuit. If the output voltage returns to normal after disconnecting the load circuit, it indicates that the load is too heavy; if it still cannot return to normal, it indicates that there is a fault in the switch power supply circuit.
4. Output voltage too high
When the phenomenon of excessively high output voltage occurs, the fault is mostly in the voltage sampling and voltage regulation control circuit of the switch power supply. All components in the feedback loop composed of sampling resistors, error sampling amplifiers, optocouplers, and power control integrated circuits should be checked. The occurrence rate of degraded sampling resistors, precision voltage regulators, or damaged optocouplers is relatively high.
For switch power supplies with over-voltage protection circuits experiencing excessively high voltage phenomena, the over-voltage protection circuit can be disconnected first, and then the voltage on the main output end of the power supply can be quickly measured at the moment of turning on. If the measured voltage is still higher than the normal value (generally, as long as it is more than 1V higher, it belongs to the fault of excessively high voltage), the aforementioned voltage too high fault should be inspected according to the above.
5. Issues to be aware of when repairing switch power supplies
(1) Discharge large-capacity capacitors at any time. When inspecting the fault of no voltage output in the switch power supply, the discharge of the voltage across the terminals of the large filter capacitor in the main rectification and filtering circuit is extremely slow. When using a multimeter to measure the circuit, these capacitors should be discharged first. The specific method is to parallel a high-power, low-resistance resistor (such as a wire-wound resistor) across the terminals of the capacitor to be discharged, and after a period of time, the parallel resistor can be removed, and then the measurement can be performed. This prevents the charged capacitor from damaging the measuring instrument and even endangering the personal safety of the repair personnel.
(2) Pay attention to the positions of the cold ground line and the hot ground line. When measuring the voltage of the switch power supply circuit, attention should be paid to correctly selecting the positions of the cold ground line and the hot ground line. When measuring the primary circuit of the switch power supply transformer and the circuits before it, the hot ground line, i.e., the ground line before the primary side of the switch power supply transformer, should be selected as the reference ground line. When measuring the secondary circuit of the switch power supply transformer and the circuits after it, the cold ground line, i.e., the ground line after the secondary side of the switch power supply transformer, should be selected as the reference ground line.