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Found A Diode Shorted In Computer SMPS

By on December 12, 2019
power supply repair atx












how to repair atx power supply

This Beetel Computer SMPS Model BT-450R was handed over to me by my technician friend saying that it was dead and he did not have time to attend to it, and the customer wanted it to be delivered urgently. Ever since I had the great opportunity to buy and study Jestine Yong’s book on SMPS Repair, in which he encourages us to take up service of SMPS, because of its challenges, I never miss any opportunity to do so. So, I brought it home and opened it and dismantled the PCB and cooling fan. Then I cleaned the inside thoroughly, which is my usual practice.

After cleaning it thoroughly, when I took it up for troubleshooting, I observed a lot of dry solder on the board.

smps repair



atx power supply repair

After finishing retouch of all solder joints on the board from one end to the other, I checked ESR of all capacitors and found these to be ok. I applied power through a Series Bulb, after shorting the green pin to ground and found it to be still dead. Checked DC Voltage at the tank capacitors and found it to be ok. Disconnected and discharged the caps. Did a ring test on the primaries of the three SMPS Transformers. The large one and the next small one were showing correct reading. But the third one showed a short. In order to access the components on the board, I removed both heat zincs along with the mounted components.

testing power supply atx

Checked all the components on the Heat Zincs for any faults using my Peak Atlas. Did not find anything wrong. I even removed the opto-coupler and checked. It was ok. From the secondary of the SMPS from the suspected section, I disconnected one end of both the diodes. Then I did ring test of that SMPS transformer and found it to be reading well. So, checked the diodes and found one FR104 short in it. Then checked all semiconductors and resistors on the board for any defect and did not find anything else. Replaced the diode with FR107.

Re-fixed the Heat Zinc mounted components and cleaned the board thoroughly. Again shorted the green wire to ground on the secondary side and applied power. This time, the fan whirred well indicating that the output was coming.

fixing atx power supply

So connected my SMPS tester and checked:

how to fix and repair atx power supply

Here is the picture of the defective Diode:

how to fix and repair atx power supply easily

Another work got added to my satisfaction bag, which happily readjusted itself to accommodate the new entrant!

This article was prepared for you by Parasuraman Subramanian from India. He is 69 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.

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You may check on his previous repair article below:




  1. Albert van Bemmelen

    December 12, 2019 at 10:52 pm

    Luckily it was not the transformer that short circuited! Otherwise repair would have been very hard without a new identical replacement transformer. Your repair was as always professional and to the point!

    • Parasuraman S

      December 14, 2019 at 7:51 am

      Yes, you are right! Getting identical Transformer is very difficult. We may have to look through the junk. But chances are remote.

  2. Devon Wilson

    December 13, 2019 at 4:29 am

    Good work. Time consuming I suppose. However, result is quite rewarding, if not financially, satisfactorily.

    • Parasuraman S

      December 14, 2019 at 7:54 am

      It takes less than 15 to 20 minutes to re-touch solder on an ATX SMPS board. Yes, result is rewarding. I always get paid for the work done and that is enough!

  3. Gary Gemmell

    December 13, 2019 at 5:29 am

    Your bag must be overflowing by now Para!

    • Parasuraman S

      December 14, 2019 at 7:56 am

      Ha! Ha! The bag is ever adjusting to suit all the inflow!

    • Paris Azis

      December 14, 2019 at 6:49 pm

      Hi Gary,

      He purposely left a wide open hole at the bottom of that bag...

      • Parasuraman S

        December 14, 2019 at 11:09 pm

        Ha! Ha! I checked and did not find a hole! (LOL)

  4. Waleed Rishmawi

    December 13, 2019 at 5:07 pm

    a job well done my friend. although in my home town, these are considered too cheap to repair. I could buy one for less than ten dollars. they come in a very good quality ones and the fan is very smooth when spinning. be blessed my friend.

    • Parasuraman S

      December 14, 2019 at 7:59 am

      Yes, many resort to getting a replacement, causing increase in E-Waste and environmental pollution. But, we electronic hobbyists not only get pleasure in repairing defective electronic devices, but also arrest the pollution. So, that's a double benefit!

  5. beh

    December 14, 2019 at 12:39 pm


  6. Robert Calk Jr.

    December 16, 2019 at 2:14 pm

    Good job, Parasuraman!

  7. humberto

    December 18, 2019 at 12:56 am

    Great repair Mr. Parasuraman. You are a Master

  8. Andy

    December 18, 2019 at 4:10 pm

    Thank you for another excellent repair article.
    In my own experience, I have discovered that the power supply tester that you use is not reliable as it does not put a load on the power supply.
    I have had a few cases where the tester showed all lights green, but the computer would not boot.
    After replacing the power supply the computer booted.

    • Parasuraman S

      December 18, 2019 at 7:20 pm

      That's because the value of Capacitors would have dropped down. This light can be used only after enduring that what needs to be done in the board is completed. If all the components are checked and found alright, this can be used. Even a 50W 12V bulb is enough if you desire to check on load.

  9. Yoges

    December 21, 2019 at 3:43 pm

    Good Job Sir


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