Maintenance Fault Analysis of AC Power Supply
Fault 1: The fuse is blown
When this kind of failure occurs, first open the power supply casing and check whether the fuse on the power supply is blown, so as to preliminarily determine whether the inverter circuit has failed. If so, it is caused by the following three situations: a bridge rectifier diode in the input circuit is broken down; the high-voltage filter electrolytic capacitor is broken down; the inverter power switch tube is damaged. The main reason is that the DC filtering and transforming oscillation circuits work at high voltage (+300V) and high current for a long time, especially when the AC voltage changes greatly and the output load is heavy, the fault of fuse blowing is prone to occur. The DC filter circuit is composed of four rectifying diodes, two current limiting resistors of about 100KΩ and two electrolytic capacitors of about 330μF; the conversion oscillation circuit is mainly composed of two high-power switching tubes of the same type installed on the same heat sink.
After the AC fuse is blown, turn off the power and unplug the power plug. First, carefully observe whether the appearance of each high-voltage component on the circuit board has been broken down and burnt or the trace of electrolyte overflowing. If there is no abnormality, use a multimeter to measure the value of the input terminal. If it is less than 200KΩ , indicating that there is a partial short circuit at the rear end, and then measure the resistance between the e and c poles of the two high-power switch tubes respectively. If it is less than 100KΩ, it means that the switch tube is damaged. Measure the forward and reverse resistance of the four rectifier diodes And the resistance of the two current-limiting resistors, use a multimeter to measure the charge and discharge conditions to determine whether it is normal. In addition, when replacing the switching tube, if you cannot find the same type of product and choose a substitute, you should pay attention to the collector-emitter reverse breakdown voltage Vceo, the maximum allowable collector power dissipation pcm, and the collector-base reverse breakdown voltage. The parameters of the crossover voltage Vcbo should be greater than or equal to the parameters of the original transistor. Another thing to pay attention to is: when a certain component is found to be damaged, it must not be turned on directly after replacement, otherwise the replaced component may be damaged because other high-voltage components are still faulty. It is necessary to conduct a comprehensive inspection and measurement of all high-voltage components of the above-mentioned circuit before the fuse blown fault can be completely ruled out.
Fault 2: No DC voltage output or unstable voltage output
Fault analysis and troubleshooting: If the fuse is intact, there is no output of DC voltage at all levels under load conditions. The possible reasons are: open circuit and short circuit in the power supply, failure of overvoltage and overcurrent protection circuit, and failure of the oscillation circuit. The load of the power supply is too heavy, the rectifier diode in the high-frequency rectifier filter circuit is broken down, and the filter capacitor leaks, etc. The treatment method is: use a multimeter to measure the ground resistance of the system board + 5V power supply. If it is greater than 0.8Ω, it means that there is no short circuit on the system board; change the configuration of the microcomputer to the minimum, that is, only the main board, power supply, and buzzer are left in the machine. Measure the DC voltage of each output terminal, if there is still no output, it means that the fault is in the control circuit of the microcomputer power supply. The control circuit is mainly composed of an integrated switching power supply controller (TL-496, GS3424, etc.) and an overvoltage protection circuit. Whether the control circuit works normally is directly related to whether the DC voltage is output or not. The overvoltage protection circuit is mainly composed of low-power triode or thyristor and related components. A multimeter can be used to measure whether the triode is broken down (if it is a thyristor, it needs to be soldered and measured), and whether the related resistance and capacitance are damaged. Finally, use a multimeter to statically measure whether the rectifier diode and low-voltage filter capacitor in the high-frequency filter circuit are damaged.
Fault 3: The power supply has output, but there is no display when it is turned on
Fault analysis and troubleshooting: The possible cause of this fault is that the delay time of the reset signal input by "pOWERGOOD" is not enough, or there is no output from "pOWERGOOD".
After starting up, use a voltmeter to measure the output terminal of "pOWERGOOD" (connected to pin 1 of the host power plug). If there is no +5V output, check the delay components. If there is +5V output, replace the delay capacitor of the delay circuit. Can.
Fault 4: Poor power load capacity
Fault Analysis and Elimination: The power supply can work normally when it only supplies power to the motherboard and floppy drive. When the hard disk and CD-ROM drive are connected, the screen turns white and cannot work normally. The possible reasons are: the operating point of the transistor is not selected properly, the high-voltage filter capacitor leaks or is damaged, the Zener diode heats up and leaks, the rectifier diode is damaged, etc.
Change the transistors in the oscillating circuit to increase the gain, or increase the operating point of the transistors. After detecting the problematic parts with a multimeter, replace the thyristor, Zener diode, high-voltage filter capacitor or rectifier diode.
Fault 5: No DC output
The parts that may be faulty are: the fuse is blown, the converter does not work, and the control circuit is faulty. Opened the power box and found that the fuse was removed. According to user feedback, the fuse has been replaced and burned repeatedly. Solder the rectifier diode and the converter power switch tube, and check with a multimeter that everything is normal, and use a high-impedance gear to check that there is no short circuit at the AC input terminal. Check that the rectifier filter capacitor is normal. Judging from the blown fuse, the fault location should be in front of the primary winding of the converter, but no short circuit was found. Had to restore the original state, change the fuse and power on the experiment. Turn on the AC power supply, the fuse is blown, immediately disconnect the AC power supply for inspection, the fuse tube is burnt black. It can be seen that there is a serious short circuit in the AC input circuit, disconnect the AC input of the rectifier bridge. Add fuses at both ends of the AC input of the rectifier bridge, and connect them directly to the AC power supply. Turn on the power supply, the regulated power supply fan rotates normally, and the DC output voltage of each test is normal. It can be seen that the fault location is in the AC filter circuit, but it is useless to detect it with a multimeter. At this time, I thought of an alternative method, and removed two AC filter capacitors from another power supply to replace them. (Because the welding is simple, replace the capacitors first.) Power-on test, the DC stabilized power supply works normally. It can be seen that the fault location is in the two capacitors, tested with a high-voltage insulator, and one of the capacitors has a high-voltage breakdown.
Fault 6: The computer self-checks after starting up, and the boot is normal. When the screen prompts "INSERTSYSTEMDISKINDRIVEAANDPRESSANYKEY", insert a DOS disk, and the floppy drive does not read the disk.
From the analysis of the fault phenomenon, the fault location is in the floppy disk drive, the floppy disk adapter or the system. After the substitution method, it is proved that the floppy disk adapter and floppy disk drive on this machine are good. Finally, remove the motherboard to verify that it is good. After restoring the original state, power on and test, the fault cannot be eliminated. So suspect the power supply part.
Unplug the 5-inch floppy drive power plug in the case. Turn on the power, check the DC output with a multimeter, +5V, +12V are normal. Turn off the power and plug in the power plug of the floppy drive, and then turn it on again, and the fault remains unchanged. Finally, under full load, the DC output +5V is +4.1V, and +2V is +10.4V. The lower output voltage of the power supply affects the normal operation of the floppy drive motor, resulting in failure to read the disk normally. After the cause is found, remove the power supply for maintenance. When the load is light, the power output is normal; when the load is heavy, the power output decreases. It shows that the load capacity of the regulated power supply is reduced. Open the cover of the power supply box, and use an oscilloscope to detect that the waveform amplitude of terminals 8 and 11 of the TL494 component and the signal amplifier tube is not affected by the load. When the waveform of the +5V winding of the converter is detected, the load will affect it, but the change range is very small. Therefore, it is suspected that the forward voltage drop of the +5V rectifier diode becomes larger, resulting in a decrease in output capability. After replacing the +5V rectifier tube, power on the test again, the fault cannot be ruled out, and the maintenance is in trouble at this time. After a calm analysis, the factor affecting the DC output is also the power switch tube. After replacing the power switch tube, start the test. When the load changes, the DC output is normal, and the fault is eliminated. The replaced power tube is tested with JL-1, and the magnification is very small. Later, I learned from the user that this machine has been working continuously for more than 4 years. This is the failure of the aging power tube. From this case, it can be concluded that when the microcomputer fails, the output voltage of the DC power supply should be checked first, which is extremely beneficial for maintenance personnel to narrow the scope of the fault and quickly eliminate the fault.
