During the repair of the switching power supply, first use a multimeter to check for any breakdowns or short circuits in the power components, such as the power rectifier bridge, switching tube, and high-frequency, high-power rectifier tube; and whether the high-power resistor for suppressing surge currents is burned out. Then, inspect the resistance at each output voltage port for any abnormalities. If any of the above components are damaged, they need to be replaced.
Step two completed, the unit still fails to operate normally after powering on. Subsequently, inspect the Power Factor Correction (PFC) module and the Pulse Width Modulation (PWM) component. Review relevant materials to familiarize yourself with the functions of each pin on the PFC and PWM modules, as well as the necessary conditions for their normal operation.
Then, for power supplies with PFC circuits, it is necessary to measure the voltage at both ends of the filter capacitor to see if it is around 380VDC. If the voltage is around 380VDC, it indicates that the PFC module is operating normally. Next, inspect the operating status of the PWM component by measuring its power input terminal VC, reference voltage output terminal VR, and the voltage at the start/control terminal Vstart/Vcontrol. Power the switch-mode power supply using a 220VAC/220VAC isolated transformer, and observe the waveform at the PWM module's CT terminal to ground using an oscilloscope, checking if it is a linear sawtooth or triangular wave. If the TL494 CT terminal is a sawtooth wave, the FA5310 CT terminal is a triangular wave. The waveform at the output terminal V0 should be an orderly narrow pulse signal. Aviation ground power 2, 36V aviation power supply 1, 400Hz variable frequency power supply 7, 115/200V power supply 2, 27V DC power supply 5, 400Hz power supply 12, aviation power supply.
(4) In the practice of switch power supply maintenance, many switch power supplies utilize the UC38×× series 8-pin PWM components. Most power supplies fail to operate due to damaged power-on resistors or degraded chip performance. When the R circuit is open, there is no VC, and the PWM component cannot function, necessitating the replacement of a resistor with the same power resistance value as the original. As the PWM component's startup current increases, the R value can be reduced until the PWM component operates normally. During the repair of a GE DR power supply, the PWM module was a UC3843, and no other anomalies were detected. After connecting a 220K resistor in parallel with R (220K), the PWM component worked, and the output voltage was normal. Occasionally, due to peripheral circuit issues, the VR terminal's 5V voltage drops to 0V, causing the PWM component to fail as well. During the repair of a Kodak 8900 camera power supply, this situation was encountered. By disconnecting the external circuit connected to the VR terminal, VR voltage rose from 0V to 5V, and the PWM component operated normally, with the output voltage being normal.
(5) When there is no approximately 380VDC voltage on the filter capacitor, it indicates that the PFC circuit is not functioning properly. The critical detection pins of the PFC module are the power input pin VC, the startup pin Vstart/control, the CT and RT pins, and the V0 pin. During the repair of a Fujifilm 3000 camera, a test revealed that there was no 380VDC voltage on the filter capacitor of a printed circuit board. The waveforms for VC, Vstart/control, CT, RT, and V0 were all normal. However, when measuring the gate of the field-effect power switch tube, no V0 waveform was found. Since the FA5331 (PFC) is a surface-mounted component, after prolonged use, there was a solder joint issue between the V0 terminal and the board, causing the V0 signal not to reach the gate of the field-effect tube. The V0 terminal was reconnected to the board.
