No Power In 3Com Baseline Switch 2226 Plus

By on July 9, 2012


3 com switch

One of my customers called me to check if my company does repair service for computer network switch. I told him that we only repair no power problem because the mainboard parts are difficult to get from the market. He then told me that the problem was no power. You may click HERE to check the function of a network switch.

Once the cover was removed I could see two boards i.e. the mainboard and the switch mode power supply board. Testing the power supply output connector with power on confirmed that the power supply have problem because the voltmeter reads zero volt on the output lines.  By right it should have 5 and 3.3 volt outputs.

3 com mainboard

Before the repair work start, I did scanned through the whole board to see if there are any burnt component and also to see if there is any bulged e-caps. All components seem to be intact and looks good. The first thing I did was to check on the main fuse and it checked to be good. The next step I used was voltage testing method. I placed the voltmeter across the big filter cap and I’ve got about 300VDC and this indicates the filter cap and bridge rectifier are good.

3 com power supply

Next, I checked on the supply voltage of the SMD power IC UC3842 (Pin 7) and the IC had only 3 volts! A-ha this is the clue. This IC should have about 10 to 16 volt in order to function and 3 volts is just too low. There are few possibilities why the supply voltage was too low:

1)    Shorted or leaky power IC

2)    High ESR ohm in the small filter cap (47uf 50 volt) along the supply line

3)    Start up resistor/s had turned high ohm (increase in resistance value)

4)    Or shorted or leaky component/s around the power IC

Note: I did check on the transformer primary winding, optoisolator IC, secondary output diodes and also secondary filter capacitors but all were tested good. 

The first component that I have checked was the small filter cap (47uf 50v) and it was tested good using my faithful Blue ESR meter. The next move was to replace the SMD power IC since I have the stock. The problem was still the same even with a new power IC. So I left only the last choice which is to check on the SMD start up resistors, SMD resistors and corresponding components (SMD diodes and SMD non polar caps). The SMD start up resistors were ok and only the SMD diodes and SMD non polar caps remain. I did not really focus on the corresponding SMD resistors because shorted caps and diodes have much more failure rate than resistors. All SMD caps were removed and checked with my analogue meter set to x 10 KOhm and they were tested good.

The last step was to check on the SMD diodes. True enough there was a SMD diode (board location D2) that has shorted under analogue meter x 10 k Ohm range.

smd diode

Note: This diode was tested good under x 1 Ohm range. When you check on the SMD resistors, diodes and non polar caps, make sure you test it off board and not onboard. On board will produce misleading result thus it is not recommended.

For your information I do not have data for the below diode. Since this is a SMD diode, I used a normal signal diode 1N4148 as replacement. It turned out to be working well after burning in for few days. The supply voltage to the SMD power IC now is 10.9volt.

Conclusion: What we have learned from this repair case was that it is not necessary must have lots of bad components if you come across no power symptom in switching power supply. A shorted diode is enough to cause the power supply not to function. You need a procedure to properly troubleshoot and repair a fail switch mode power supply and also in other type of electronic circuits. Your testing electronic components skill has to be good because if you miss just one component, the rest of the troubleshooting time will be a waste.

Hope you like this repair article. Your feedback on the post is welcome. Please leave it in the comments.

Jestine Yong



  1. Del Corso A.

    July 11, 2012 at 8:10 am

    Ancora grazie per la dedica del Suo tempo al popolo degli appassionati di elettronica.

  2. noel gervais gerard

    July 11, 2012 at 11:52 am

    Thank  sir for the articles  its very useful for troubleshooting SMPS and know how to replace and testing SMD diode and other component  thank again God bless

  3. Igor Levin

    July 11, 2012 at 4:21 pm

    Hi, Jestine,
    You didn't understand me. I've meant valtage across a diode that you see measuring diode in the
    position 'diode' of multimeter switch. It should read 0.45- 0.67 for diodes, about 0.7 for zeners and
    0.13- 0.16 for schottky diodes. Don't your meter have such a position?

    • Jestine Yong

      July 11, 2012 at 8:46 pm

      Hi Igor,

      Yes i have the meter that have the diode range. The reason I did not use it is because it is not accurate. A leaky diode can still test good using the diode range in digital multimeter! As mentioned in the article, if you miss just one component the rest of the troubleshooting time will be a waste. That's why I prefer to remove the diode out and test it off board with any analogue meter that have the x10 k ohm rage.


  4. Kobina Dankwa-efs

    July 12, 2012 at 1:59 am

    Am i wrong to say you are an angel ? You are always ready to share your experiences. 
    Hi Jestine...thank you and God bless you always.

  5. janardhan

    July 12, 2012 at 8:50 am

    thank you for your information , i brought the laptop repair book  from the link given by your site.
    I hope you will carry this for long time.
    thank you, god bless you

  6. Aleander

    July 12, 2012 at 9:19 am

    A wonderful presentation especially for beginers like me. Short, apt , concise and orderly instructions for locating the cause for the failure(solution for trouble shooting).Hmmm... as you state, finding parts in the market is indeed a great hassle. Yes testing electronic components does present substantial support for this tutorial. Thank you for the step by step directions.
    Alexander Zachariah.

  7. Igor Levin

    July 12, 2012 at 9:30 am

    Hi, Jestine,
    You have your reason. I am measuring leaking too, but only if reading in opposit direction is suspicious, or I have reason too suspect a diode's or transitor's foul play.

    • Jestine Yong

      July 12, 2012 at 9:07 pm

      Hi Igor,
      Ok noted.


  8. bob lou

    July 12, 2012 at 10:25 am

    Jestine...this diode seems to be orange in color...usually I find that Zener Diodes have this orange color.

    • Jestine Yong

      July 12, 2012 at 9:08 pm

      Hi bob,

      Yes you are right but because the board location is "D" thus higher chances it is a diode and not zener diode.


  9. andra

    July 12, 2012 at 9:28 pm

    oh..i see,tq for sharing,,taht causes mybe overload right?

  10. jose

    July 13, 2012 at 6:10 pm

    Sr. Jestine estoy recibiendo ya varios articulos de reparacion electronica de su parte y la verdad que interesantes ,educativos y de ayuda me esta ofreciendo  y eso sin daber mucho ingles tecnicamente lo puedo enterder es tan facil el concepto ensi de lo que quiere deir que casi no tengo problemas en traducirlo y entenderlo ujala pueda comprar algunosde sus libros electronicos la verdad es lo que me falta .soy de Peru .Gracias

    • Jestine Yong

      July 13, 2012 at 9:06 pm

      Hi Jose,

      Ok thanks for the support!


  11. Basant Kumar Dehury

    July 14, 2012 at 12:48 am

    Thanks very much,
    Its need full for all computer networking group.

  12. gideon

    July 16, 2012 at 8:20 am

    jestine sir thank you very much for giving suggestoins through photograph and making under standing the circuit uc3842 yesterday a philips television(colour television )21 crt semiflat type 21 di 4210/94R has come to me  i saw checked the power supply and noticed that power supply was problem the same power ic was inthe power supply (uc3842)the problem was 110 was not constant by looking in to your circuit i have understood the problem and i havw changed 25v 47mfd condencer and the power supply 110 is  constant now          sir thanking you yours truely gideon

    • Jestine Yong

      July 16, 2012 at 9:39 pm

      Hi Gideon,
      Congratulation. The best test point in the power ic is actually the supply voltage. The power IC has to work first otherwise no point checking other parts.


  13. cristia cris

    July 16, 2012 at 4:00 pm

           thank's  it is the best for sharing your help for every boody
     good time

  14. mustabz

    July 17, 2012 at 7:05 am

    Thanks jestine for your continued sharing it boosts our efforts to getting more experience, However am buffled by this panasonic TV 21 inch that has a wide operating voltage range  from110-230v ac but when plugged into the ac outlet of 220v ac it shuts down after 5 mins and when stepped down to the 110 v ac supply it works perfectly well. could it be the x-ray protect causing this? and do i have to change the flyback?

    • Jestine Yong

      July 17, 2012 at 8:08 am

      Hi Mustabz,

      It could be resistor that have change value in the optoisolator circuit. What is the B+ voltage when you set to 110v and 230vac?


  15. Mirza Faisal baig

    July 17, 2012 at 1:57 pm

    dear jestin yong,
    great troubleshooting, I do repair lots of IT instruments on the basis of basic part testing. but you are excellent troubleshooter.
    Thanks for this website and such valuable repair news letter.
    Mirza Faisal Baig
    IT Engineer at Aurangabad.
    Maharashtra India.

  16. roms

    July 17, 2012 at 8:30 pm

    Hi Jestine,
    Thanks for this wonderful job you've shared. Godbless.

  17. dicksy

    July 18, 2012 at 1:54 am

    hi jestin.
    nice work and  thanx to very important advices

  18. SALIM

    July 18, 2012 at 2:43 am

    Dear Jestine Sir,
    Thank you for sharing all of this things with me. It is not my profession now but hobby. I want to stablish a repair shop at dhaka, in Bangladesh. Please give me good suggestions and suitable help. My email address is given here. Thank you again.

    • Jestine Yong

      July 19, 2012 at 8:39 am

      Hi Salim,

      Make yourself good first in electronics repair then open your in your own house first. Once the customer base is big then only you rent a bigger space and hire worker. Your profits has to come first before anything else.


  19. kimsat25

    July 18, 2012 at 4:41 pm

    Thanks very much,

  20. silvester un

    July 19, 2012 at 1:21 am

    Thank mr jestine yong.can you help me? I need technical refrens electronics.

    • Jestine Yong

      July 19, 2012 at 8:38 am

      Hi Silvester,

      You can buy my ebooks to get started or you can visit the website below for more information:


  21. kamal insan

    July 19, 2012 at 2:11 am

    dear sir
    great troubleshooting
     you are excellent troubleshooter.
    Thanks for this website & repair techniqs..

  22. Okore Kingsley

    July 20, 2012 at 4:38 am

    Thanks for the detailed repair steps on the switch power suply, the pictures made it so nice and easy to understand. I also benefitted from the advise you gave to Salim, I am also planning to open my own electronic repair shop. I want to realy understand the different stages of regular power suply and how some of the regularly used components functions

  23. Komane Molefe

    July 21, 2012 at 10:16 am

    Every time I read your publications, I really get motivated. I can see that you are not only after money, but also much interested in assisting people. That is really good of you and I enjoy your staff. Keep it up!!

    • Jestine Yong

      July 21, 2012 at 11:55 pm

      Hi Komane,

      You are welcome. Everyone needs money to survive and even need money to pay for the maintenance of my websites/blog. I work hard to sell my ebooks and at the same time work hard to provide good information for my readers. The more you give the more you will receive.


  24. hasan

    July 23, 2012 at 10:00 am


  25. shahid

    July 27, 2012 at 10:14 am

    excellent testing procedure

  26. Nicetune

    July 28, 2012 at 12:23 pm

    Jestine, God bless u for this information, surely i improved since started with u with these magazines. its really wonderful.

  27. Abdoulie Bah

    July 31, 2012 at 11:08 am

    Thanks you very much for that hard working tested you done great job,  you are the electronics doctor Mr Yong, we are proud of you.

  28. cashzola

    August 2, 2012 at 4:05 am

    hello Justine,...i want to order some of your repairs ebooks, do i get that paid, can contact me on the email address above and below here,

    • Jestine Yong

      August 3, 2012 at 6:58 am

      Hi Cashzola,
      I have sent to you an email. Please check your inbox.


  29. jomar

    August 3, 2012 at 3:40 am

    sir,i ask ,how to test the tv if there is vertical line appear on the screen?

    • Jestine Yong

      August 3, 2012 at 7:00 am

      Hi Jomar,

      CRT or LCD?


  30. XauS

    September 7, 2012 at 11:12 am

    Thanks for your article!
    At my work I have a similar problem, I found 2 components to have a "leak". Is there some place I can order a new "D1" & "R38" component?

    • Jestine Yong

      September 7, 2012 at 9:08 pm

      Hi Xaus,

      Try this link:


  31. ketan

    September 27, 2012 at 5:43 am

    Hi Jestine,
                            I am having a problem with my monitor its  compaq MV7540. When I power on my monitor is completely dead, led is not on even. I have also tried to replace IC 5SO765C but that want work. Please help me out with this deadlock.
    I will be very thankfull to you.

    • Jestine Yong

      September 27, 2012 at 7:48 pm

      Hi Ketan,

      You may need to check on all the components un the primary side. Check also for any shorted secondary output diodes too.


  32. mohamed khan

    December 1, 2012 at 5:53 pm

    hi Jestine, i'm interested in electronics, i bought ur e-book on SMPS .Amazing...i think i'm getting somewhere ,i also get afew good technics from ur site .keep the good work up...THANK friend..

    • Jestine Yong

      December 2, 2012 at 8:33 am

      Hi Mohamed,

      Thanks for the support!


  33. Robert

    October 15, 2013 at 5:46 am

    Thanks for the article, Mr. Yong. That is the first time I've ever heard anyone talk about a diode needing to " burn in" for a few days. What do you mean by that??

    • Jestine Yong

      October 15, 2013 at 9:48 am

      HI Robert,

      This means I have to let the whole unit run and make sure the substitute diode will not break down. It has to be on burn in test for few days to check for the reliability of the diode.


      • Robert

        October 15, 2013 at 4:12 pm

        Oh, ok, you mean the ammeter test that you mention in the SMPS book. Ok, thanks.

  34. raj dann

    November 17, 2013 at 2:03 am

    hey i got a smps p/s but the voltage keeps cycling.i check the components in the secondary side of the smps trans. all good aslo change the semi conductorws in the oscillator stage of the smps...only found 2 bad resistors a 10m and a 220k replace them and stil the same.....any thoughts?

    • raj dann

      November 17, 2013 at 2:07 am

      its a autec 8011 igt power unit use in casino gaming machines

    • Jestine Yong

      November 17, 2013 at 4:01 pm

      HI Raj,

      Have you checked all the e-caps with ESR meter?


  35. Yazid

    October 27, 2014 at 9:09 am

    Thanks for the article, Mr.Yong. its very informative troubleshooting I like your style of teaching how to troubleshooting electronics PCB thanks.

    • Jestine Yong

      October 27, 2014 at 11:18 am

      Hi Yazid,

      You are welcome!


  36. Amir Mukhtar

    February 12, 2015 at 3:10 am

    Sir, The above Article is too Informative and Learnable .

    Thanks you for your sharing valuable articles with great guidance.

    yours Student
    Amir Mukhtar AShrafi


    May 16, 2017 at 5:51 pm


    • Jestine Yong

      May 18, 2017 at 9:07 pm

      Hi Atiku,
      Just use your voltmeter and measure the output voltages from the power supply.



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