Troubleshooting Common Failures in Regulated Power Supplies
1. The regulated power supply has output, but it does not perform self check when turned on
This is mainly due to insufficient delay time or no output of the PW-OK signal of the power supply. After turning on, use a voltmeter to measure whether the output terminal (pin 8 of the power plug) of PW-OK has+5V. At this time, check whether the comparator LM393 is damaged. If the delay is not sufficient, the resistor R104 and capacitor C60 in the delay circuit should be checked.
2. The output voltage of the stabilized power supply is inaccurate
If only one voltage deviates from the rated value, while all other voltages are normal, it is due to damage to the integrated voltage regulator circuit or rectifier diode of that voltage. If all deviations from the rated value occur, it is due to a fault in the+5V and+12V automatic voltage stabilization control circuit composed of IC1's 1 and 2 pin error amplifiers, R39 and C32 error amplifiers' negative feedback loops, sampling resistors R33, R34, and R35.
When replacing the diode in the voltage regulator power supply circuit, attention should be paid to the fact that the inverter operates at a high frequency, generally greater than 20kHz, and the load current is also large. Therefore, the+5V stage of the power supply uses Schottky high-frequency rectifier diode SBD, and the other stages also use high-frequency rectifier diode FRD with recovery characteristics. So when replacing, try to find the same type of rectifier diode as much as possible to avoid further damage.
3. The fan does not turn or makes noise
The fan of a computer regulated power supply usually uses a DC fan connected to the+12V DC output terminal. If the power input and output are normal, but the fan does not rotate, it is mostly due to damage to the fan motor. If there is a sound, one of the reasons is that the four fixing screws of the fan are loose due to the long-term operation or intense vibration during transportation of the machine; The second issue is that there is too much dust inside the fan or the oil bearing is lacking oil. As long as it is cleaned in a timely manner or an appropriate amount of lubricating oil is added, the fault can be eliminated.
4. After controlled startup, there is no output from the DC power supply
(1) T2 primary side VT3 and VT4 push tubes are damaged, and the resistance of R54 resistor increases; (2) At least one of the switching tubes VT1 and VT2 in the half bridge power conversion circuit is open circuited; (3) The anti bias magnetic capacitor C8 has a reduced capacity or is open circuited.
5. Poor load capacity of stabilized power supply
The poor load capacity of a regulated power supply is mainly manifested as follows: the power supply can work normally under light load conditions, such as only supplying power to the system board or floppy drive, but when equipped with a large hard drive or expanding other devices, the power supply often does not work properly. This situation is generally due to poor performance of the switching tubes VT1 and VT2 in the power conversion circuit, and insufficient capacity of the filtering capacitors C5 and C6. When replacing the filter capacitor, it should be noted that the capacity and withstand voltage values of the two capacitors must be consistent.
