ATX Computer Power Supply Repair: I Learnt From My Mistake
I was given this Computer system because it was not powering up when they switch it on. When I powered it myself there was no any sign of life.
To open this Computer system I had to press a button and slide the side cover towards the front. Then I removed the power supply by moving it backward out of the clips. I unscrewed all the screws on the power supply in order to access the inside printed circuit board.
I started with visual inspection and there was not any sign of a damaged component. Then I tested the fuse by using my multimeter set on ohms range. I found out that it was open. I soldered the fuse out and tested again just to verify and it was indeed open. I recalled what I read from the SMPS E book by Mr. Jestine Yong that sometimes a fuse may blow either by itself due to life span or mild surge. Therefore, I just soldered my 100 watts bulb across the points where the fuse was.
I connected the power cable and inserted the plug to the power outlet. I switched on … what a spark!!! My God! As it sparked the bulb was giving out a very bright light. I immediately switched off. I checked what could be causing that terrible spark.
It was my silly mistake! The sharp edge of the cover of the power supply touched one of the EMI coils to the extent of removing the insulation and shorted it.
The spark burnt the coil and broke open its windings. The Printed Circuit Board was also painted black by the smoke produced from the spark.
I believed the spark caused some damage to other components in the circuitry. Patiently, I started scrutinizing the pieces one by one with my meter. The capacitors and coil forming the EMI/RFI circuit, the Varistor, Bridge rectifier, the diodes on the secondary side of the switch mode transformer. In the circuit the Schottky diodes were showing low resistance. I removed them from the circuit and tested them separately. All were testing fine. I think it was the circuit network that was giving a low resistance. I had no means to test the IC.
Thank God nothing was victimized apart from the only two items; a fuse and a coil. I managed to get a fuse of the same rate (250 V 6.3A) from my old PCBs. I bought a new coil and fixed it. With the bulb connected I tested the power supply again and the bulb gave out light at once and then off. Yeah, positive sign.
I connected a jumper between the Green cable and the Neutral (PS – ON and Black) the bulb was giving out bright light ON ,OFF, ON, OFF … During this time the fan was spinning with different speeds, fast, slow, fast, slow … as if it is trying to start and then stops. This is one of the possible outcomes when using a bulb trick on ATX power supply. Since I tested almost all components I expected that the power supply is trying to pick up but the bulb is reducing current necessary to power supply.
I removed the bulb and put a new fuse. Power up the power supply it was silent. I connected a jumper between Green cable and Neutral the fan rotated at a constant speed. I tested the voltage output with my meter set on DC Voltage range, all voltages were available. I assembled the PS and put it back to the CPU with all the connectors in their places. I switched ON the computer … Glory to God it is back to life.
My mistake increased the job and made it more complicated. However, I have learnt from that experience to be always sure that there is no contact between the metallic covers and the PCB or the components of the equipments. I give my humble gratitudes to Mr. Jestine Yong for his informative, educative and down to earth E book on Switch Mode Power Supply Repair. The E – book was my companion during this repair and still will be in the other repairs to come.
Luciano Francisco Thomas Khware (Malawian) studied Electrical and Electronics at Comboni Technical College and at Polytechnic (University of Malawi). Currently, he is a student at Tangaza University in Nairobi Kenya.
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