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Replacing Working Components In Switch Mode Power Supply

By on March 23, 2020







power supply repair

The OLAN 22″LCD TV came in with the complaint of no power symptom. Once the cover was removed I saw four circuit boards:

  1. Inverter Board
  2. Power Supply Board
  3. Main board
  4. USB Port that shared with the AC incoming board

The first thing to check was obviously is the main fuse and it was open circuit. With visual inspection I saw the main filter capacitor already bulged as seen from the photo below:

bulged capacitor

Checking on the power FET on board found it to be shorted. I proceeded to check on other components in the power primary side and found the current sense resistor was open circuit  too. 

shorted power fet

bad components in lcd tv

As a precaution measure, I also replaced three working components ie; Optoisolator IC, TL431 IC and a filter capacitor. The reason I replaced the working components is because from experience, these component although are good at time of testing, it can go bad later. Why not replace it than to worry it will come back later for repair? A repeated call by customer will always be a non-profit repair. Once the components were replaced and powered On, the LCD TV came back to life.

bad components in lcd tv repair

olan lcd tv repair

Conclusion- You need lots of testing components skills to be able to understand the characteristics of electronic components. Some you can check on board, some you need to test it off board and some will tend to breakdown when under full load. If you would like to be good in checking electronic component, I can recommend Jestine’s Ebook. He uses his methods to accurately check on many electronics components with the help of few different meters. 


This article was prepared for you by Suranga Bandara who owns an Electronics repair shop in Anuradapura, Sri Lanka.

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  1. Parasuraman S

    March 23, 2020 at 11:55 am

    Straight to the job and to the point as usual! Well done!

  2. Albert van Bemmelen

    March 23, 2020 at 1:45 pm

    I take it that you did test the TL431 optocouplers, but even if they test good you just replace them out of bad experience. Your argument that any call from a customer is no profit if a repair comes back is completely understandable. Good to know that we better also should replace these optocouplers to be sure!

    • Albert van Bemmelen

      March 24, 2020 at 2:03 am

      I made the tester circuit that can be found online on youtube with the 3 blue,red and green leds that can test both the 3 pin TL431 programmable zener diode simultaneous together with the 4 pin PC817 optocouplers, but I've previously never replaced these components if they tested good in the tester. The TL431 zeners also can be checked onboard incircuit with another made 3 wire test cable. And to make sure some of my other universal portable digital testers also confirm that these parts are good or not. And luckily in most of my defect power boards these parts are almost always still intact.

      • Gassan Ali

        March 24, 2020 at 5:50 pm

        Thanks for putting up educational comments! From where van I buy those meters?

        • Albert van Bemmelen

          March 27, 2020 at 12:11 pm

          Here a new tester I just found online that also tests the pwm uc3843 ICs :

          • Albert van Bemmelen

            March 27, 2020 at 12:17 pm

            And this tester can test many TTL/CMOS IC's, triodes, leds, plus zeners (these less exact though), opamps and some audio amplifier IC's. And the optocouplers!:

            • Albert van Bemmelen

              March 27, 2020 at 12:25 pm

              Here that same tester on Aliexpress since the eBay link to it was sold out

      • Robert Calk Jr.

        March 24, 2020 at 8:47 pm

        It doesn't seem fair to call the programmable precision shunt regulator a zener since it has better characteristics than a zener. I think that many people do that because they don't like to type that long name. We need to get everybody to call it a, "PPSR" instead. That is easier to type!

        • Albert van Bemmelen

          March 26, 2020 at 10:09 pm

          The problem with abbreviations Robert is that we never for certain know what they mean and they only cause more confusion in case we doubt. Why I hate those short naming conventions. Also because they can mean many more things. I neither ever have heard about people I know of naming it a PPSR before and I very much doubt that anyone knows about what it means. You are probably the only one 'I know' of using it? By-the-way: I still don't know what it means. Programmable Precision Serial Resistor? Just writing 'Programmable zener' is not that hard and fully covers it.

          In case someone needs the info on making the testers. Here some links:

  3. Waleed Rishmawi

    March 23, 2020 at 7:28 pm

    a job well done. the only time i replace working parts is when I have a bad short on board. in this case, I would just replace the bad parts, put it under test for an hour or so and give to the customer. so far this method worked just fine with me. have a safe blessed day and thanks for sharing

  4. Daniel Mitchell

    March 24, 2020 at 11:48 am

    As a power supply engineer, unless the power supply got whacked really hard, the opto and Tzl431 were 99% good.

    Depending on the age of the filter electrolytic, it is sometimes worth replacing them.

  5. Gassan Ali

    March 24, 2020 at 5:52 pm

    Are there any books you recommend when learning about power supplies?

    • Waleed Rishmawi

      March 27, 2020 at 12:43 am

      Gassan: yes, there is one by Jestine on this web site. it is an excellent book about SMPS..I recommend it. have a blessed day

  6. Robert Calk Jr.

    March 24, 2020 at 8:39 pm

    Good job, Suranga! If the customer comes back you will also have an angry customer. We have a saying here in America that it is better to be safe than to be sorry.

  7. Yogesh Panchal

    March 30, 2020 at 4:35 pm

    Is there any procedure to check TL431 IC i thing only a way to test this component is using oscilloscope because from multi meter you can only make out for shorting.

  8. Humberto

    April 1, 2020 at 2:35 am

    Hi Suranga, another interesting article. Keep on

  9. ulises aguilar pazzani

    April 17, 2020 at 10:54 am

    good AFTER NOON Mr Suranga, good Job Sir


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