Backlight Panel Repair Method

1. When overhauling the backlight board, it is necessary to master the characteristics of the backlight tube, the lighting principle, the composition and working principle of the backlight board. Although the shape and component model composition of the backlight board are different, the basic principle is the same. As long as the theoretical basis is provided, the fault can be analyzed. There are only 3-4 types of faults of the backlight board. The principle of the power amplifier circuit is simple, and the full-bridge/half-bridge drive is mostly used, and MOS tubes or power modules are generally used.

2. The maintenance of the backlight board must first ensure that the power supply part is working normally. That is, the standby power supply is 3.3V or 5V, 12V, 24V, and the PFC circuit is working normally, only then can the maintenance of the backlight system be carried out.

3. During maintenance, you can remove the cable from the power supply board to the main board, and connect the standby voltage 5V with the three pins of STB (standby control), BRI (dimming) and BL-ON (backlight control) respectively. When it is normal, the backlight board should be If it can light up normally, this step can determine whether it is a motherboard problem or a power board problem.

4. If the backlight tube still does not light up, it means that there is no pulse signal added to the backlight inverter. When we overhaul the backlight circuit, we can regard the backlight system as a line scan circuit to overhaul

5. The backlight control chip has a feature. When the chip is just powered on for about 2 seconds, it is not controlled and protected by any feedback pin, and it has output pulses. After starting, it will get the normal feedback signal of its detection pin. will enter the normal working state. If there is no feedback signal or abnormal feedback signal after startup, the chip will enter the protection state and stop outputting the excitation signal

6. Using this feature, first check whether the pulse output pin of the backlight control IC has voltage output when powered on. If possible, you can use an oscilloscope to measure the signal waveform of the pulse output pin, or you can use the AC voltage of a multimeter. Check whether there is pulse output.

According to the voltage value here, we judge whether the fault point is in the circuit with the backlight control chip as the core or in the subsequent circuit. (It should be noted that after the chip is protected, the output will only be available for about 2 seconds after the power is turned off and then restarted)

7. If there is a signal output, it can be judged that the fault is in the post-drive part.

Otherwise, the fault is in the backlight chip control part.

We can measure whether there is an AC excitation signal output at the output of the excitation transformer at the rear stage (because the operating frequency of the backlight circuit exceeds the frequency response of our ordinary multimeter at about 56KHZ, so there is no accurate voltage value, and the digital meter is generally ~30V or so , the mechanical watch is around ~10 volts. It should be emphasized that no matter what the voltage value is, the voltage value obtained by the two windings must be the same, otherwise there is a problem.

8. When the output pulse of the excitation transformer is normal, we continue to measure step by step to the next stage until the short circuit point of the pulse is found.

The above maintenance process is a quick and effective maintenance method to repair the backlight circuit that does not light up after turning on.