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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. Noel

    July 9, 2012 at 8:56 pm

    Hi Jestine,
    Thanks! for the article you shared. another knowledge i've learn.

  2. Jonny Pridgen

    July 9, 2012 at 9:30 pm

    I would like to know how to post a question. The question I want to post is about a sony kdl40hx800 lcd tv shutting down and starts blinking 7 times. I found that lowering the backlight setting from the default of 7 to 4 eliminated this from happening for a while, but now has started shutting down again.

    • Jestine Yong

      July 9, 2012 at 11:40 pm

      Hi Johnny,

      Display shutdown could be the inverter or one of the backlight have problem. Make sure the inverter ic is getting a good supply voltage. You may need a backlight tester to check on the backlight or have good backlight for comparison purpose.


  3. Les Hodgson

    July 9, 2012 at 9:44 pm

    Hi Jestine, I See Your Fault finding articles are getting better all the time.  I see One Chap asking you put the electrical circuit in as well As The Write up . I believe a picture is worth a thousand words. He Has no Idea How time  consuming your work is. I believe we are very lucky to have some one like you devoting his time to these articles. Please Keep up the the excellent articles . Les H.

    • Jestine Yong

      July 9, 2012 at 9:49 pm

      Hi Les,

      Thanks for the understanding. Now in the office doing the support and repair work if no classes "On".


  4. malamig

    July 9, 2012 at 10:53 pm

    Dear Jestine,
    Thank you for this wonderful article, It's good to learn new things each day.

  5. ashok

    July 9, 2012 at 11:08 pm

    thankyou for your valuble advice,..
    God bless you..

  6. senthil

    July 9, 2012 at 11:22 pm

    We have lots of servo motor drives driven by SMPS and PWM control circuit and IGBTs.
    1) Do you have any E-books for repairing these drives.
    2) If i send pictures of the PCB ,will you guide me to repair ?
    3) Which cheaper oscillocope would you recommend to test the power modules,mosfets,opto couplers like (m601,m 456), smd ics like m 51955,74 hc 14,sp 232,74 hc 04,LV 04 A,24 LC 16,,,,,,?
    4) What probes & connectors should i buy with the oscilloscope ?

    • Jestine Yong

      July 11, 2012 at 5:15 am

      HI Senthil,

      Sorry I do not have those books. As for the repair I will personally guide my students only or those who have bought my SMPS repair ebook ( As for the scope get an analog or a digital scope with at least 100mhz for electronics troubleshooting. The probe will come together when you buy the scope.


  7. beh

    July 10, 2012 at 12:11 am

    thanks for good articles please tell me how long this repaier got your time

    • Jestine Yong

      July 10, 2012 at 12:35 am

      Hi beh,

      Do you mean how long it has taken me to fix the switch? If yes then it has taken me about an hour plus but the time spent was well worth it as all of us could learn from this repair tip.


  8. Charles

    July 10, 2012 at 12:19 am

    Hi Jenstine
    It good to know  we can learn thing is not to think too complicate first , simple can save time and works  thanks for your share of experience.

    • Jestine Yong

      July 10, 2012 at 12:33 am

      Hi Charles,

      Yes you are right-have to learn thing not in a complicated way.


  9. Boling

    July 10, 2012 at 12:47 am

    Thanks for the good repair tips. It's like repairing monitor primary section power problem, I think I should have to add another items too for my repair work, as long as it is a power problem.

    • Jestine Yong

      July 10, 2012 at 2:13 am

      Hi Boling,

      Yes you should add more to increase your profit.


  10. Freddie

    July 10, 2012 at 12:49 am

    Hello Jestine,
    Thanks for another great repair tip. It is very useful information for me as an electronic technician also. More power to you Jestine.

  11. pramod m. pakhare

    July 10, 2012 at 1:08 am

    Hello jastine sir,
    thanka a lot for valuable tips

  12. beh

    July 10, 2012 at 1:36 am

    ok i understood and thanks again

  13. Peter Osborne

    July 10, 2012 at 1:40 am

    Hi Jestine,
    Thank you for this article, you have hit the nail on the head regarding not complicating our thinking, we all seem to go into some kind of negative trance when we hear Switched Mode Power Supply.
    Many thanks for your regular emails and great repair guides, they are always something to look forward to.
    Kind Regards,

  14. Albert Kitchen

    July 10, 2012 at 1:54 am

    Hi Jestine,
    Thanks once again for a splendid informative article,  Where do you find the time.
    Keeping it simple is always best,and sets us on the right road to success.
    Kind regards and God bless Albert.

    • Jestine Yong

      July 10, 2012 at 2:15 am

      Hi Albert,

      If I don't have repair class I will do the repair work.


  15. Luciano F.T. Khware

    July 10, 2012 at 2:04 am

    Greetings Jestine,
    i like your sharing spirit, keep it up!! i have really learnt something from this article. i have headache in tracing these small faulty componets in SMPS but your approach has given me a good lesson.
    Stay blessed.

  16. dilip tankaria

    July 10, 2012 at 2:57 am

    jesting yong,
    thanks, it is easy to learn troubleshooting as you have describe it very nicely with is like doing all in front of me.

    • Jestine Yong

      July 10, 2012 at 3:54 am

      Hi Dilip,

      Yes making sure all my readers could easily follow.


  17. Nimal

    July 10, 2012 at 3:13 am

    Good day to you Mr Jestine.  Thanks for the valuable repair tips.  

  18. thulani

    July 10, 2012 at 3:27 am

    only a diodie stopped the whole psu,so you can replace an smd component with a normal component as long the power rating is the same?

    • Jestine Yong

      July 10, 2012 at 3:53 am

      Hi Thulani,

      Yes, but in my case it was a test because I can't seem to get the original data of the diode.


  19. larbi g

    July 10, 2012 at 3:46 am

    he jestinne thinks for this lessen j was very hapy because j had a seme  problem with a swith where the ic were shorted but j have n t repleced it because j das n t find it

  20. Wayne

    July 10, 2012 at 6:07 am

    i am looking for service info on wharfedale ,teac and logic lcd tv faults
    can you help

    • Jestine Yong

      July 10, 2012 at 7:54 am

      HI Wayne,

      Sorry I do not keep schematic. Try


  21. leo

    July 10, 2012 at 6:29 am

    Thanks for the reply. Also what if the board is multilayered. Can you still desoulder the component without damaging the layers of the board ?

    • Jestine Yong

      July 10, 2012 at 7:48 am

      HI Leo,

      Yes it still can be done. Soldering on top removing components will not affect the underneath layer.


  22. Gurpreet

    July 10, 2012 at 6:32 am

    Hello Mr. Yong
    Your tips are really valuable for all of us.  I only pray you grow more and live healthy life.
    Best of luck

  23. Adil

    July 10, 2012 at 6:52 am

    Thanks Jestine for all your good repair  tips.

  24. Richard

    July 10, 2012 at 7:07 am

    Hi Jestine,
    I follow your work with much interest. I have just one comment - that I never throw away old boards, which have proven to be a very useful source of replacement parts. For example, SMD components such as the diode above. Anyway, with an internet link many of the SMD parts can be identified through tables.
    Keep up the brilliant work.

    • Jestine Yong

      July 10, 2012 at 7:51 am

      HI Richard,

      You are right. I will still keep some of the junk boards in my technical room.


  25. Raghu

    July 10, 2012 at 7:46 am

    Hi Jestine,
    Thanks for sharing this tip. Really it's a good and helpful tip.

  26. Jhun

    July 10, 2012 at 8:27 am

    Well detailed troubleshooting and accuracy in diagnosing Mr. Jestine.....BRAVO!!!You are very excellent in component and board troubleshooting...I salute you!

  27. Thomas Leveille

    July 10, 2012 at 8:53 am

    Good tip for any kind of power supply

  28. marchano

    July 10, 2012 at 9:36 am

    hi Jestine,
    i like the way you share your knowledge with us. people like you will never born again. your ebooks and reports have learned me a lot.
    thanks teacher,

  29. robert gonzales

    July 10, 2012 at 9:59 am

    thanks for another technique u shared mr. yong ur the best teacher in electronics soon i order again another interesting  e books to you about power suplly thanks angd more power god bless.

  30. gerald factor

    July 10, 2012 at 10:01 am

    Thanks jestine for this information.. more power and godblesss.

  31. sushanta halder

    July 10, 2012 at 10:21 am

    really you have done a great job. sooooo many thanks for this post me and I think all repairar woul be thakfull for this post.we really want to learn how to Identify the bad parts in power supply please keep posting that type of article.
    thak you very much

  32. Henry Wilson

    July 10, 2012 at 10:35 am

    Jestine.  I have been receiving your tips on repair and troubleshooting for a long time.  You're technical writing and photos are getting better with each repair job.  I first got your Monitor repair electronic book a few years ago to repair a Dell monitor.  Found the problem and the monitor has been working good ever since.  Getting some equipment apart is a major job sometimes just to look inside.  Your info on the 3Com switch power supply is very good.   Thanks for sharing and putting all the time and effort into making it easier for the novice trying to do repairs on electronic equipment.  May God bless you in your efforts and meet all your needs.
    God Bless,

    • Jestine Yong

      July 10, 2012 at 9:57 pm

      Hi Henry,

      Thanks for the support on my ebook and glad that you liked the repair article.


  33. ifa

    July 10, 2012 at 10:36 am

    Helpful tip, you're doing grate. thank you

  34. Igor Levin

    July 10, 2012 at 11:21 am

    Dear Jestine,
    Your procedure is really good, but I don't understand one thing. Why not to test diodes for votage on them?
    Most multimeters have such a function. And test diodes on the board is possible, you have to take them off only if mesurement seems to be wrong.
    Thank you.

    • Jestine Yong

      July 10, 2012 at 8:30 pm

      Hi Igor,

      Good question-since I do not have the schematic thus I'm not sure what will be the right voltage. Thus removing it out to check it off board is much faster and accurate.


  35. leonard williams

    July 10, 2012 at 1:27 pm

    hi jestin  good tip for those who is repairing smps  thanks you

  36. kraipantip

    July 10, 2012 at 1:36 pm

    Thanks for your flawless article! 
    Your are my inspiration!
    God bless you.

  37. Justice

    July 10, 2012 at 1:42 pm

    Thank you so much Jestine, it is always a pleasure to read your articles with so much information. Keep doing this world-wide good work.

  38. Waleed Rishmawi

    July 10, 2012 at 3:51 pm

    good article. I really enjoyed it;
    I have worked on similar equipment but the fault was the capacitors. after relacing all of them, the machine was brought to life.

    • Jestine Yong

      July 10, 2012 at 8:26 pm

      Hi Waleed,

      Do you mean SMD capacitors?


  39. Andre Gopee

    July 10, 2012 at 6:40 pm

    Jestine, great article as usual. I am always interested in your experiences they teach me alot. Thanks again for the articles, keep them comming. 

  40. Froy Zurita

    July 10, 2012 at 8:00 pm

    Thanks Mr. Yong to share your experiencies, this article es very valuable for everybody who is involved in elctronics.

  41. John Cunningham

    July 10, 2012 at 8:26 pm

    It is refreshing to have a blog for technicians that address a variety of subjects and repair tips that are impossible to find at this level. I admire the dedication and hard work you put into every subject no matter how remote with the zeal and detail of true professional. It is only fitting that your efforts do not go unnoticed even if some of us remain silent, I thank you Jestine for your support and insight in the field of electronics that continues to inspire me to be a better technician.

    • Jestine Yong

      July 10, 2012 at 8:27 pm

      Hi John,

      Thanks for the compliment.


  42. Minhaz

    July 10, 2012 at 10:00 pm

    Dear Jestine,
    That was a wanderful article !!! Thanks for the post and keep going my friend.

  43. Pramod.K.P

    July 11, 2012 at 12:10 am

    Hi Justine Yong.... Greetings from India 
    I really like to appreciate the way of your precentation ...Great Job 
    And the faulty capacitors is a real fact....if there is any leakage in Elecrolytic capacitors , [most commonly using 100mf in power driver filter ] its better to replace the same blindly to save time
    God Bless
    From God's own country......Kerala

  44. kelvin chambula

    July 11, 2012 at 2:00 am

    thank jestine for this wanderful thing.
    jestine what about the line on screen of led tv and lcd?. where could be the problem coming from. could it be t-con or main board?

    • Jestine Yong

      July 11, 2012 at 5:08 am

      Hi Kelvin,

      it can be the t-con or the lcd panel.


  45. Peter

    July 11, 2012 at 2:33 am

    Hi Jestine,
    Thanks for the post, its very good.

  46. rajesh

    July 11, 2012 at 3:07 am

    great technicle point open mind then no short circuit there found problem .please upload more smps idea for common for all smps .PLEASE REGARD FOR THIS to convert 5v supply to 12v, 18v ,24v which value shifted on tl431 on secondary and how 2 ampeare convert to 5amp can increse adjust   all about smps please guide bigbro.THANKS

    • Jestine Yong

      July 11, 2012 at 5:12 am

      Hi Rajesh,

      I do not have those information and you need to do your research about boost circuit or even dc-dc converter circuit.


  47. Waleed Rishmawi

    July 11, 2012 at 4:13 am

    no, normal capacitors in the power supply. they had hight ESR reading.

  48. kate

    July 11, 2012 at 4:15 am

    Great stuff chief nice to read you again.

  49. Andrew munandi

    July 11, 2012 at 6:46 am

    keep on the great work

  50. fabzkie

    July 11, 2012 at 7:47 am

    Hi Sir,
    I like to thank you the way you share your knowledge with us. Your eBooks and reports have learned me a lot. More power sir and keep it up as a good technician and a good teacher to us as well...
    Thanks teacher,


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