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

The failure rate of the power board in LCD TVs is quite high and often poses a major challenge during repairs. It's easy to confuse the root cause, especially since the power board supplies energy to both the backlight driver circuit (which can be either a high-voltage board or an LED board) and the mainboard. When a TV fails to boot, shows a black screen, has flickering, or shuts down irregularly, it's hard to determine whether the issue lies with the power board, mainboard, backlight, or driver board, which can lead to unnecessary troubleshooting steps. Drawing from years of repair experience and real-life examples, this article aims to provide practical and straightforward methods for diagnosing and fixing power board issues. The goal is to demystify the process and make it more approachable for technicians and enthusiasts alike. Let’s take the TCL-PWL37C power supply as an example to explore how the power supply works. If you've repaired CRT TVs before, you’ll find that the power supply system in LCD TVs is similar but includes an additional PFC (Power Factor Correction) circuit. ### 1. Standby Circuit When the TV is turned on, the 3rd and 4th pins of the power output socket P3 should provide a +5V voltage to power the motherboard’s CPU. On the hot side, the 4-5 winding of the auxiliary switching transformer T2 also outputs a voltage, which is rectified and filtered to produce +20V, supplying the PFC and PWM circuits. If the output is unstable, check the voltage regulator around IC9 (TL431). Normally, pin 1 should read 2.5V. If not, the TL431 or its surrounding components may be faulty. **Symptom 1:** No +5V output **Diagnosis:** The voltage at the 5-8 pin of IC1 was found to be 0V. After checking, RB13 was desoldered. Replace RB1, RB2, and RB13 with 1W or 2W resistors to prevent recurrence. **Symptom 2:** +5V fluctuates around 3V **Diagnosis:** No-load test showed low voltage, indicating a problem in the standby section. DB5 was found damaged and replaced. **Symptom 3:** +5V only reaches 4V **Diagnosis:** Visual inspection revealed swollen capacitors CB7 and CB8. Replacing them resolved the issue. ### 2. Open/Standby Control Circuit When the mainboard sends a high-level signal to the PS-ON pin of P2, it triggers QS3 through DS9 and RS16. This activates the optocoupler IC6, turning on Q11 and Q12, which then provide +19V and +12V to the PFC and PWM circuits. During standby, the signal is low, and the circuits are inactive. **Fault Phenomenon:** +5V is normal, but no +12V or +24V **Diagnosis:** Measuring the capacitor C5 showed only 300V, indicating the PFC wasn’t working. Checking IC2’s pin 14 revealed 0V, leading to the discovery of a failed Q11 and open R7. Replacing these components restored normal operation. ### 3. PFC Circuit The PFC circuit, controlled by IC2 (L6563), regulates the input voltage to about 380V. If components like QF5 or QF6 fail, it can cause issues such as blown fuses or unstable voltages. **Symptom 1:** Blown fuse **Diagnosis:** QF5 was found shorted. After replacing it, the same issue occurred again. Further checks revealed a faulty C5, which was replaced to resolve the problem. **Symptom 2:** Screen flickers after power-on **Diagnosis:** +12V and +24V were unstable, and C5 measured only 300V. After replacing CF7, the issue was resolved. **Symptom 3:** PFC voltage too high (500V) **Diagnosis:** The sampling resistor RF12 was open. Replacing it fixed the overvoltage issue. ### 4. PWM Oscillation Circuit The PWM circuit, driven by IC3 (L6599), controls the switching transistors QW9 and QW10. A fault in IC3 can lead to no output voltage. **Fault Phenomenon:** +5V is normal, but no +12V or +24V **Diagnosis:** Pin 7 of IC3 had no voltage due to an open RW13. Replacing it restored the output. ### 5. Protection Circuit LCD power supplies include overvoltage and overcurrent protection. If the output drops immediately after startup, it might indicate a fault in the protection circuit. **Fault Phenomenon:** Output drops to 0V after power-on **Diagnosis:** Disconnecting the protection resistors and testing step-by-step revealed a faulty IC4. Replacing it solved the issue. ### Additional Notes: - For screens showing interference after 30 minutes, check filter capacitors and replace if necessary. - Unplanned shutdowns may be caused by faulty ZS2 or other capacitors. - General output voltages vary depending on TV size: 40 inches typically have +5V, +12V, +24V; larger models may include +18V as well. - Power boards can be tested independently by shorting control transistors or using dummy loads like bulbs. - Always inspect for bulged capacitors or burnt components before testing. - Use a 100W bulb in place of a fuse to avoid re-burning. This guide covers key aspects of LCD power board diagnosis and repair. While each model may differ slightly, understanding the core principles will help in tackling various faults efficiently. As always, safety is crucial—avoid direct contact with hot components and ensure proper isolation when testing. For more details, refer to "LCD TV power board repair steps" and the attached schematic diagram.

On-Grid Solar System

On-Grid Solar System,Energy Solar System,Solar Energy System For Air Conditioning,Solar Energy Saving System

JIANGSU BEST ENERGY CO.,LTD , https://www.bestenergy-group.com