Solutions to common faults of regulated power supplies
1. The regulated power supply has output, but it does not self-check when it is turned on
This is mainly caused by insufficient delay time or no output of the PW-OK signal of the power supply. After starting up, use a voltmeter to measure whether there is +5V at the output terminal of PW-OK (pin 8 of the power plug). At this time, check whether the comparator LM393 is damaged. If the delay is not enough, check the resistor R104 and capacitor C60 in the delay circuit.
2. The output voltage of the regulated power supply is inaccurate
If only one gear voltage deviates from the rated value, while the other gear voltages are normal, it means that the integrated voltage regulator circuit or rectifier diode of this gear voltage is damaged. If all deviate from the rated value, it is the error amplifier of pin 1 and pin 2 of IC1, the negative feedback loop of the error amplifier of R39 and C32, and the sampling resistor R33, R34 and R35, which constitute the +5V and +12V automatic voltage stabilization control circuit is faulty.
Pay attention when replacing the diode in the regulated power supply circuit, because the inverter has a high operating frequency, generally greater than 20kHz, and the load current is also relatively large, so the +5V range of the power supply uses a Schottky high-frequency rectifier diode SBD , The rest of the gears also use high-frequency rectifier diodes FRD with recovery characteristics. Therefore, try to find the same type of rectifier diode when replacing it, so as not to damage it again.
3. The fan does not turn or makes noise
The fan of the computer regulated power supply usually adopts a DC fan connected to the +12V DC output terminal. If the power input and output are all normal, but the fan does not turn, the fan motor may be damaged. If there is a noise, one of the reasons is that the four fixing screws of the fan are loose due to the long-term operation of the machine or the intense vibration during transportation; the other is that there is too much dust inside the fan or the oily bearing is short of oil. The best quality lubricant, the fault can be ruled out.
4. DC power supply has no output after controlled startup
(1) The push tubes VT3 and VT4 on the primary side of T2 are damaged, and the resistance value of R54 becomes larger; (2) At least one of the switch tubes VT1 and VT2 of the half-bridge power conversion circuit is open; (3) The capacity of the anti-bias capacitor C8 becomes smaller or open circuit.
5. The load capacity of the regulated power supply is poor
The poor load capacity of the regulated power supply is mainly manifested in: the power supply can work normally under light load conditions, such as when it only supplies power to the system board and floppy drive, but when it is equipped with a large hard disk and other equipment is expanded, the power supply often does not work normally. This situation is generally due to the poor performance of the switching tubes VT1 and VT2 of the power conversion circuit, and the capacity of the filter capacitors C5 and C6 is insufficient. When replacing the filter capacitor, it should be noted that the capacity and withstand voltage of the two capacitors must be consistent.
