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Servicing ZEBRONICS ATX Power Supply
My persistent requests were responded and I was allowed to start a service centre within the Hospital for in-house servicing of defective electronic devices. So, I am now-a-days able to spend my free time to do my favourite service! I have already cleared four ATX SMPS, all of which were having the usual problems like capacitor burst and/or switching transistors short etc. and I did not bother to write an article, as the faults were quite common and there was nothing new in it! But the present case is a very interesting one, reasons for which are in climax!
This ATX Power Supply was brought to me, as the service provider declared it to be beyond economical repairs and recommended a replacement. As usual, the first job was to do a thorough cleaning up. Noticed that a couple of capacitors in the secondary were bulgy. One resistor which had distinct marks of Red, Black, Black and Gold (R8) and another Brown, Black, Black and Gold (R6) (20 Ohms and 10 Ohms), were both open. Fuse was intact. Noticed a lot of dry solder. So, patched up dry solder and cleaned up the board thoroughly. Replaced the two resistors as well as all the electrolytic capacitors on the board as some of them had ESR out of range. Just see the board after this and also the inside after replacement of capacitors:
Since I have set up the workshop myself, I had added one AC power outlet extension, which had one ELCB fit on it, along with three 3 pin female sockets with switch and one wired through a series bulb holder with a switch. Just see the power outlet and my service table:
Connected the Power Supply to the Series Bulb socket and switched on, the fan purred, but the bulb remained lit and the two resistors which I had replaced started burning! Switched off and after referring to Jestine Yong’s world famous SMPS Repair book, and another ATX Power Supply circuit, I tried without shorting the green to earth wire at the output. The bulb did not remain lit! So, I knew that the primary was definitely ok. Something in the secondary is wrong! My suspicion was that the value of the resistors could be wrong. So, I looked through the number of SMPS waiting in queue and found a similar one. Found that the R6 and R8 resistors in that were not open.
Removed them and tested it with a Multimeter. Just guess what I found! The one with Red, Black, Black and Gold was reading 277 Ohms, and the one with Brown, Black, Black and Gold was reading 410 Ohms! I could not believe it myself! How can it be! Then I picked up another resistor removed from a salvaged SMPS, but different colour code. The code was Red, Black, Black and Gold. But see the values these were showing:
So, I tried using different value resistors for R6 and R8, with the same result. Finally, I decided to put these two resistors that I removed from another SMPS. The Power Supply worked perfectly, with all its output voltages of 12, 5 and 3 V perfect! Now, let us have a look at the board with these two resistors in place:
I switched it on for a couple of hours, without any problem. The SMPS fan ‘purred’ smoothly! Here are the pictures after assembled it back and components replaced:
Now, why the values of the resistors are totally different from the codes on the resistors? Since this is a China make, are they doing it deliberately to fool the service technicians, who will be forced to recommend a replacement? Well, I have no idea! Another possibility of changing colours due to heat in the resistor is also ruled out as yellow cannot become brown, and second code cannot become ‘black’! Anyhow another job completed satisfactorily, though the mystery remains unsolved!
This article was prepared for you by Parasuraman Subramanian from India. He is 66 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company.
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You may check on his previous repair article below: