1. When repairing the LED display screen power supply, we first need to use a multimeter to detect whether there is a breakdown short circuit in each power device, such as the power rectifier bridge, switch tube, high-frequency high-power rectifier tube, and whether the high-power resistor that suppresses surge current is burned out. Then, we need to detect whether the resistance of each output voltage port is abnormal. If the above devices are damaged, we need to replace them with new ones.

2. After completing the above tests, if the power supply is turned on and it still cannot work properly, we need to test the power factor module (PFC) and pulse width modulation component (PWM), review relevant information, and familiarize ourselves with the functions of each pin of the PFC and PWM modules and the necessary conditions for their normal operation.

3. For the power supply with PFC circuit, it is necessary to measure whether the voltage at both ends of the filter capacitor is about 380VDC. If there is a voltage of about 380VDC, it indicates that the PFC module is working normally. Then, it is necessary to detect the working state of the PWM module, measure its power input terminal VC, reference voltage output terminal VR, start and control Vstart/Vcontrol terminal voltage, and use 220VAC/220VAC isolation transformer to supply power to the led display screen, Use an oscilloscope to observe whether the waveform of PWM module CT end to ground is Sawtooth wave wave or triangle wave with good linearity. For example, TL494 CT end is Sawtooth wave wave, and FA5310 CT end is triangle wave. Is the waveform of output V0 an ordered narrow pulse signal.

4. In the maintenance practice of LED display screen power supply, many LED display screen power supplies use UC38×& Times; Most of the 8-pin PWM components in the series do not work due to damage to the starting resistance of the power supply or a decrease in chip performance. When there is no VC after the R circuit is broken, the PWM component cannot work and needs to be replaced with a resistor with the same power resistance value as the original one. When the starting current of the PWM component increases, the R value can be reduced until the PWM component can operate normally. When repairing a GE DR power supply, the PWM module was UC3843, and no other abnormalities were detected. After connecting a 220K resistor to R (220K), the PWM component worked and the output voltage was normal. Sometimes, due to peripheral circuit faults, the 5V voltage at the VR end is 0V, and the PWM component does not work. When repairing the power supply of the Kodak 8900 camera, this situation is encountered. The external circuit connected to the VR end is disconnected, and the VR changes from 0V to 5V. The PWM component operates normally and the output voltage is normal.

5. When there is no voltage of around 380VDC on the filtering capacitor, it indicates that the PFC circuit is not working properly. The key detection pins of the PFC module are the power input pin VC, the start pin Vstart/control, CT and RT pins, and V0 pins. When repairing a Fuji 3000 camera, test that there is no 380VDC voltage on the filter capacitor on one board. VC, Vstart/control, CT and RT waveforms as well as V0 waveforms are normal. There is no V0 waveform at the G pole of the measuring field effect power switch tube. Since FA5331 (PFC) is a patch element, after a long time of use of the machine, there is a faulty soldering between the V0 end and the board, and the V0 signal is not sent to the G pole of the Field-effect transistor. Weld the V0 end to the solder joint on the board, and use a multimeter to measure the 380VDC voltage of the filtering capacitor. When the Vstart/control terminal is at low power level and the PFC cannot operate, it is necessary to detect the relevant circuits connected to the periphery at its endpoint.