Cisco Router Switch- No Power Problem Repaired

By on July 2, 2013

The complaint was no power. The moment I connected the AC supply there was no sound and was very quiet. If there is sound or the fan rotates for a while and stopped then I can roughly know where to check for the problem. I opened up the casing and saw only two boards i.e. the power supply and the main board as seen from the photo below:

cisco mainboard

cisco switch

cisco power supply

After that I tried to scan for any bulged electrolytic capacitor but all seems to be normal. Here is what I had observed:

1)      The power supply was quite after turning it On

2)     No bulged electrolytic capacitors

3)     No burnt components

From above, my mind suggested that:

1)      The main fuse could have an open circuit

2)     One of the secondary output diodes could have shorted

3)     No DC supply (VCC) or improper voltage to the power IC

4)     Bad component/s in the primary section. It could be an open start up resistor, shorted run DC diode, or even partial shorts in the transformer primary winding.

ciscopowersupplies

The first component that I have check was the main fuse and it was Ok. Then I powered On the set and check if there is any 300VDC across the main filter capacitor. Yes the 300VDC was present and this suggested that I should perform the voltage test on the VCC line of power IC. When I was done placing the probes on the VCC supply line and to the hot ground and powered On, I got about 3 volts which were abnormal.

cisco voltage testing

It should have at least 14 to 16 volts. Now I got the clue at where I should troubleshoot the fault. Here are the things that were in my mind again:

1)      The startup resistor could have gone up in resistance value.

2)     The filter capacitor for the VCC supply line to the power IC could have problem (bad ESR value).

3)      The power IC (UC3845) itself could have problem and pulled down the supply voltage

4)     The related component could have direct or minor short circuit that had pulled down the supply voltage.

Ok here is another round of checking. The startup resistor of 160K ohm checked to be within range. Next was the filter cap connected along the supply line. It was 10 uf 50 volt and checked to be good. Now left only power IC and the corresponding components. I desoldered the power IC VCC pin and powered On and the voltage was still 3 volts. This suggested that it has nothing to do with the power IC and the problem could be corresponding components.

ciscocircuitboard

I started to trace along the dc supply line and found that besides the supply going to the power IC (UC3845) it also goes to the VCC pin of LM393N. I took out my analog meter set to x100 and checked between the supply line and hot ground I found that it showed shorted reading. When I removed the VCC pin 8 of the LM393N IC the shorted reading was gone.

lm393 ic

lm393 schematic

So I took out the IC and checked between pin 8 (VCC) and pin 4 (Ground) and the reading was shorted as seen from the photo below. A good one should not show two similar readings when checking on the VCC and the ground pin.

cisco shorted ic

cisco good ic

A replacement of LM393 IC cured the fault. Ok I’m not sure what are the procedures you use to troubleshoot power supply with no power symptom but the procedure that I have just described can be applied to any type of power supply in the market. The power IC has to get the right supply voltage in order to function well.

cisco switch working

Now the above checking method is different if you are troubleshooting a power supply that has the standby power. The standby power supply has to work first to produce 5 volt to the mainboard before the MCU IC sends out an On signal to trigger the main power supply.

Hope you have enjoyed this repair article. Please give a support by clicking  on the social buttons below. Your feedback on the post is welcome. Please leave it in the comments. By the way if you have any good repair article that you want me to publish in this blog please do contact me HERE.

Happy Repairing

jestineyongtestingcomponents

 

 

 

 

 

Jestine Yong

Recommended Switch Mode Power Supplies Repair Ebook 

powersupplysmallClick Here now to become a professional in Switch Mode Power Supplies Repair

 

 

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78 Comments

  1. Waleed Rishmawi

    July 2, 2013 at 5:43 pm

    nice articl, nice photos. thanks for sharing. I wish I could know how to do the arrows on the pictures. what program are you using?

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 2, 2013 at 6:18 pm

      HI Waleed,

      Just use the Windows "Paint software" and it is easy to use.

      Jestine

      Likes(3)Dislikes(0)
  2. Sethu ramachandran

    July 2, 2013 at 7:37 pm

    Dear jestine ,

    Thanks for sharing the same ..

    your observations and the approach are good and this is the straight method of solving the problem .

    long live jestine and keep posting service informations.

    with love,
    Sethu

    Likes(2)Dislikes(0)
  3. bbaale

    July 2, 2013 at 7:43 pm

    Dear Jestine, I just admire the way you test you equipment, however correct me if am wrong, I thought the resistance between pin 4 and 8 should be higher and not lower isnt it?

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 2, 2013 at 8:40 pm

      HI Bbaale,

      Thanks for pointing to me the problem. I have just amended it.

      Jestine

      Likes(0)Dislikes(0)
  4. Tin

    July 2, 2013 at 7:51 pm

    Thanks for everything you do. On your site you can really find good advices for repairing the electronics.

    best regards, tpongra

    Likes(0)Dislikes(0)
  5. moshe

    July 2, 2013 at 7:52 pm

    great articles....this and the compaq repair!!
    *waleed - how did you realise that the fuses (that look like diodes) in your compaq repair were fuses?
    And do you have a picture of them?

    Likes(0)Dislikes(0)
    • Waleed Rishmawi

      July 3, 2013 at 7:47 pm

      sorry I do not have a closeup photos of them..they look like diodes but when testing with the meter it did not give me the reading of a diode. they are fuse because, one side of it was getting voltage and the next part was getting nothing..Also I have seen the same parts in differnt computers..

      Likes(0)Dislikes(0)
  6. Bradley

    July 2, 2013 at 7:59 pm

    Nice article.Thank you for sharing with others.

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 2, 2013 at 8:40 pm

      To all,
      You are welcome.

      Jestine

      Likes(0)Dislikes(0)
  7. Andre Gopee

    July 2, 2013 at 10:26 pm

    Jestine, you make these thing look so simple. I guess that it comes with experience.

    Andre

    Likes(0)Dislikes(0)
  8. shihab kably

    July 3, 2013 at 12:15 am

    As a network engineer i would like to thank you for saving this wonderful switch.wonderful equipment + wonderful repairer = wonderful job done

    Likes(0)Dislikes(0)
  9. cresente v. raza

    July 3, 2013 at 12:50 am

    Dear Jestine,

    Your articles really help enriched the knowledge of electronic enthusiasts all over the world. I hope you will come up with an article in the future featuring electronic application on automotive such trouble
    shooting electroic fuel-injection system and CDI (capacitive discharge
    ignition).
    Thank you ang more power!

    Cresente

    Likes(0)Dislikes(0)
  10. Barry Witz

    July 3, 2013 at 12:53 am

    I enjoy your professional approach as well as your presentations which are so very well done.

    Likes(0)Dislikes(0)
  11. danny

    July 3, 2013 at 1:11 am

    It's very nice article, Thank you so much sir jestine On your site that you are always giving a good learning for electronics repairings.

    regards
    Danny-KSA

    Likes(0)Dislikes(0)
  12. HABARUREMA IVAN

    July 3, 2013 at 1:33 am

    Dear Jestine yong, thank you so much for the repairing tips you give me. i like them so much.i learn a lot from the tips give me and i request again for one of the eBook. i have not yet get money for the books but i will get with time. thank you

    Likes(0)Dislikes(0)
  13. Balaji

    July 3, 2013 at 1:45 am

    Hi Jestine,

    Nice and very valuable Information. Great work...

    Likes(0)Dislikes(0)
  14. Tyrone.

    July 3, 2013 at 1:48 am

    Very good article Jestine,keep the good work up and God bless you
    for sharing with us,thank you very much.

    Likes(0)Dislikes(0)
  15. Elucious

    July 3, 2013 at 3:39 am

    Hi mr yong, thanks for the useful articles l have been receiving every month from you. l've got a question for you. can a blue esr meter be used to measure smd caps? if yes what is are the common values for good caps? if no would you supply me with a smart tweeter and how much if @ all you can supply?

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 3, 2013 at 2:52 pm

      Hi Elucious,

      Yes it can check the SMD ecaps and the value is depends on the e-caps. Sorry I do not supply smart tweezer, you can try advancedevices.com

      Jestine

      Likes(0)Dislikes(0)
  16. adel

    July 3, 2013 at 4:35 am

    thank you for helpus with great article.

    Likes(0)Dislikes(0)
  17. Nick

    July 3, 2013 at 12:19 pm

    It was very informative topic about repairing cisco switch ,have a nice day and long live.

    Likes(0)Dislikes(0)
  18. Henry Thomas

    July 3, 2013 at 2:18 pm

    Hi Jestine,

    I enjoy reading your repair tips,it makes me better.also i enjoy the repair tips that waleed gave they are quite inspiring.they also help me a great deal to make my repair work faster.

    Thanks.

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 3, 2013 at 2:52 pm

      Hi Henry,

      You are welcome and thanks to all for the support.

      Jestine

      Likes(0)Dislikes(0)
  19. mark

    July 3, 2013 at 6:17 pm

    NEED TROUBLE SHOOTING FOR DELL MODEL W2600 ASAP!POWERS UP WILL PLAY OK 4HOURS OK U TURN OFF STARTS BLUE POWER LED THEN TURNS OFF WILL NOT POWER-UP!

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 5, 2013 at 9:48 am

      Hi mark,

      Check if the output voltages good or not even can't power up. If no output voltages then possible the start up resistor could have problem. Keep me update on this.

      Jestine

      Likes(0)Dislikes(0)
    • MARK

      July 13, 2013 at 1:39 am

      HOW DO I TROUBLE SHOOT THIS PROBLEM WHERE START-UP RESISTOR

      Likes(0)Dislikes(0)
      • Jestine Yong

        July 13, 2013 at 11:53 am

        Hi Mark,
        The start up resistor is 160K ohm as seen form the photo

        Jestine

        Likes(0)Dislikes(0)
  20. Radhakrishna kommireddi

    July 3, 2013 at 7:58 pm

    Thank you sir, good Fault find, trace the fault nice. one request sir easy faulty finding crt/lcd manual your suggest. please mail sir with Indian price.

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 3, 2013 at 9:06 pm

      Hi Radhakrishna,

      Please visit the link below:

      http://www.jestineyong.com/resources-2/

      Scroll to LCD and CRT section as these are recommended ebooks. The price of the ebook can be found in the website that you want to click.

      Jestine

      Likes(0)Dislikes(0)
  21. Henry

    July 3, 2013 at 8:37 pm

    HI Jestine,
    Very good article presenting nice and logical methodology to diagnose no-power symptoms in a power supply board of Cisco Switch. Or in any power supply for that matter. I really like the way you have described the sequential steps and checks.
    I have a follow up question here - How did you assume that pin 8 (Vcc) and pin 4 (Gnd) of LM393N chip are shorted? Your analog meter (shown in the picture) indicates about 10 ohms on x100 range (eg it reads about 1000 ohms). How could I know that the measured 1000 ohms is actually too low or almost shorten condition?
    I would much appreciate your further clarification. Thanks.
    Still enjoying your articles.

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 3, 2013 at 9:04 pm

      HI Henry,

      The analog meter when set to lower range (x1, x10 and x100) when measuring between the gnd and vcc should not show similar reading on either way. It should be one higher ohm and the other measurement will be lower ohm. In fact when I tested it on board between the supply line and the ground it already had the same reading both ways. If the IC is direct short then I should not be getting any DC voltage. Since I'd got 3 volt I know something is partial short and not direct short.

      Jestine

      Likes(0)Dislikes(0)
  22. Titos

    July 4, 2013 at 12:40 am

    Hi Jestine

    This is a good stratergy to fault find power supplies, keep up the good work

    Likes(0)Dislikes(0)
  23. star34

    July 4, 2013 at 4:37 am

    HI Justine,
    tanks for thé us full articles

    Likes(0)Dislikes(0)
  24. Nimal

    July 4, 2013 at 2:05 pm

    Very good, detailed repair information. Thank you very much.

    Likes(0)Dislikes(0)
  25. M.Hatim

    July 5, 2013 at 2:42 am

    Hi Jestine
    thank you for the good articles

    Likes(0)Dislikes(0)
  26. M Mansour

    July 5, 2013 at 6:07 am

    Thank you Mr. Yong for the precious information you and your colleagues provide every time, sharing information is like sharing mindes, it enriches our knowledge and strengthen our repiar skills. We are proud of you.
    Best regards,

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 5, 2013 at 9:47 am

      Hi Mansour,

      Thanks.

      Jestine

      Likes(0)Dislikes(0)
  27. Jamaloodeen

    July 6, 2013 at 2:59 pm

    Hello Jestine
    A professional and technical approach to servicing .Thanks.

    Likes(0)Dislikes(0)
  28. Richard

    July 7, 2013 at 2:29 am

    Mr Young , Thanks very much for your PRACTICAL training articles. I really enjoy them

    Likes(0)Dislikes(0)
  29. john

    July 7, 2013 at 7:29 am

    a great lesson on using process of elimination and knowledge of components, great for all involved in electronics, I am really glad that I became a member of this great site,and am picking up great advice which can help bring pennies in ,at this hard financial period. wish all members good health and success regards Celestica(john)

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 7, 2013 at 3:15 pm

      Hi Johm

      You are welcome and thanks to all!

      Jestine

      Likes(0)Dislikes(0)
  30. peter djuh

    July 7, 2013 at 8:40 pm

    Thank you very much for this inspiring article Mr Jestine.I'm just a local technician because i didn't study electronics in school but with these your articles i am able to repair different types of electronics.Electronics is my passion but i have a problem, i wanted to study it in school but they are no electronic training schools in my country,so i need your advice sir.

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 8, 2013 at 9:43 am

      Hi Peter,

      In this case you can get the ebooks that is related to your repair field. If you want to repair LCD TV then get the LCD TV repair ebook and the support from the author. With this way you can fast to become a LCD TV repair expert. Next you can venture into some other equipment.

      Jestine

      Likes(0)Dislikes(0)
  31. balamurali

    July 7, 2013 at 11:42 pm

    Very good article,thank u sir

    Likes(0)Dislikes(0)
  32. FARHAN KHAN

    July 8, 2013 at 2:44 am

    Dear Yong,
    I have been reading your repair articles, you and your team are quite inspiring. Please, kindly accept this expression of my cordial thanks to you ……

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 8, 2013 at 9:41 am

      Hi Farhan,
      You are welcome and thanks to all.

      Jestine

      Likes(0)Dislikes(0)
  33. billy

    July 8, 2013 at 12:30 pm

    Thanks for the repair article, i always enjoy your site. I purchased your switch mode power supply e-book and I use it as a reference all the time.I would recommend it to anyone in electronics repair.I will be sending some repair articles to you as I myself am a electronics tech.i generally repair a lot of RF stuff ie, ham radio and cb radio
    but I have repaired everything from microwaves to TV's,vcr dvd,and industrial controls. thanks again I wish you and yours the best..

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 8, 2013 at 2:04 pm

      Hi Billy,

      You are welcome and already amended. Yes you are encourage to send me any repair articles.

      Jestine

      Likes(0)Dislikes(0)
  34. Arniel Gapol

    July 8, 2013 at 1:55 pm

    That IC is very common in any power supply of electronics appliances.
    I rememembered that I also encountered that IC in DVD. Thanks a lot for keeping sending article that is really help me in repairing.

    Likes(0)Dislikes(0)
  35. Ljuba Jovanovic

    July 10, 2013 at 9:35 am

    Dear Jestine,
    I am very pleased with yours articles. Your ligical findings are very usefull and valuable for me. I always admire to yours experienced and simple solutions, I really enjoyed reading them. I especially like the complete way you display the methods of solving different kind of problems.

    And of course I have a question: Is it posssible to simplify (better say - indipendent) testing PWM IC in switching power supply without output voltages (output filtered part) and comparator IC and in that way testing smaller number of suspicious components. Is it possible to cutoff the output part and comparator's componets of SMPS and replace them with any prooved good accessible electronics just to enable independent testing PWM and Comparator IC( from my experrience I'm verry unefficiant in that part of troubleshooting and sometimes I waste my valuable time).
    Or I've missed something, please just give me advice and suggestions

    Thanks a lot in advance.

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 11, 2013 at 9:55 am

      Hi Ljuba,

      If you have the an external circuit board then you can test those ICS out of circuit. This means just remove the ic and plug into your own made circuit board. With this method you do not need to test the input or even output voltage.

      Jestine

      Likes(0)Dislikes(0)
  36. prince apol

    July 11, 2013 at 12:39 am

    Hello Jestin,
    Thats wonderful! Thanks for everything and please, keep these letters coming coz they have inspired me. The problem is that i live in Africa and i get problems buying e books because many of us here do not use credit cards.
    How can you help me because i need your books.

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 11, 2013 at 9:51 am

      Hi Prince,

      You are welcome and I have sent an email to you.

      Jestine

      Likes(0)Dislikes(0)
  37. raymundo saura

    July 11, 2013 at 5:55 pm

    thanks for the info thanks and GOD bless

    Likes(0)Dislikes(0)
  38. Uzair

    July 12, 2013 at 6:08 pm

    Hi Jestine,

    I really liked your article, it was really a great help for all of us.

    Likes(0)Dislikes(0)
  39. john

    July 25, 2013 at 2:05 pm

    Hi Jestine,
    Your such a great help to all electronic hobbyist,
    Thank you and more power.

    john

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 25, 2013 at 8:28 pm

      Hi John,

      You are welcome.

      Jestine

      Likes(0)Dislikes(0)
  40. Darie

    August 2, 2013 at 9:55 am

    Nobody, nobody but you! Nobody nobody but you! Thank you for posting the valuable repair information on troubleshooting smps. It lets me know how an expert to repair such a complicated circuit in the proper way. It definitely upgrade our technique in repairing.
    Darie from Hong Kong

    Likes(0)Dislikes(0)
    • Jestine Yong

      August 3, 2013 at 4:11 pm

      Hi Darie,

      You are welcome.

      Jestine

      Likes(0)Dislikes(0)
  41. karlo

    August 6, 2013 at 3:16 pm

    Hi Jestine,

    Did you try a digital meter set to DIODE test and do the same procedure with your analog? The said test will display below 0.6V or almost zero in either direction. A good one will display 0.6V and no reading in the other direction. This is what I have explained in my previous comments about the DIODE setting in digital meters particularly the FLUKE.

    The ~0.6V is the protection diode in every IC pins in reference to the ground pin. However, you cannot find this in datasheets. This is how we check the open/short in a semiconductor manufacturing setting before other test parameters are performed.

    Karlo

    Likes(0)Dislikes(0)
    • Jestine Yong

      August 6, 2013 at 8:56 pm

      Hi Karlo,

      I forgot to test it and the set already returned to customer. Will test it next time if come across problem with ICs.

      Jestine

      Likes(0)Dislikes(0)
  42. kamel

    September 29, 2013 at 9:00 am

    Hi, thanks a lot for all these valuable trouble-shootings i want to ask you a favor, i have an LG MC-049B and i am trying to repair it but i am stuck with the regulator SRT-W6754 i'm not sure if it's faulty or not i did the dummy load trick and the bulb didn't lit so i assumed that the problem is in the primary side i checked the vcc of the regulator and it was 13.7 VDC whereas it should be 18 VDC i lifted up the vcc leg and it was almost 74 VDC i don't know what to think so please help me out thanks in advance.
    all the best

    Likes(0)Dislikes(0)
    • Jestine Yong

      September 30, 2013 at 10:56 am

      Hi kamel,

      In your case, if you have checked all the components to be good then the only way to test it is to direct replace the power ic and retest. Make sure also the secondary diodes are good.

      Jestine

      Likes(0)Dislikes(0)
  43. Anand Palkar

    October 28, 2013 at 12:26 pm

    Hi Jestine,

    Nice article .

    I really appreciate your good work. The approach is wonderful and the learning curve is rapid when you read such articles. The secret is pure hard work and study. The book published by you (Troubleshooting and repairing SMPS )explains all types of powersupplies found in electronic components. The key is we need to understand the block diagrams and study it thoroughly such that whenever we open the Powersupply ckt we should be able to visualize understand and identify the parts and its functions correctly. Then we are in a position to make tests of voltage, resistance, capacitance , ESR etc and to reach the suspected component. Thats why I say that the article is wonderful because we need to understand the process of fault finding.

    Best Regards

    Anand Palkar

    Likes(0)Dislikes(0)
    • Jestine Yong

      October 28, 2013 at 8:47 pm

      HI Anand,

      Thanks.

      Jestine

      Likes(0)Dislikes(0)
  44. Felix

    April 7, 2014 at 12:54 am

    Hello Jestine

    Thanks for sharing your knowledge and skills. We have fix
    similar power supply by applying this technique.
    Please like you mention on this page what if FANS on the
    board are functional and there is sound on the unit for a while.what do you do?

    Regards
    Felix

    Likes(0)Dislikes(0)
    • Jestine Yong

      April 7, 2014 at 10:24 am

      HI Felix.

      What sound was that? Where is this sound from?

      Jestine

      Likes(0)Dislikes(0)
      • Felix

        April 7, 2014 at 4:26 pm

        Yes the FANS on the main equipment are on but no power supply to the equipment itself.
        I guess the sound is from the equipment itself not the power supply.

        Regards
        Felix

        Likes(0)Dislikes(0)
        • Jestine Yong

          April 7, 2014 at 8:29 pm

          No power? Do you mean the system was having a no power symptom?

          Jestine

          Likes(0)Dislikes(0)
          • Felix

            April 7, 2014 at 9:26 pm

            YES. NO POWER

            Likes(0)Dislikes(0)
            • Jestine Yong

              April 8, 2014 at 10:29 am

              HI Felix,

              Have you checked if there is any output voltages from the power supply? If no output at all this means you need to concentrate on checking the components in the power supply. It can be bad e-caps, open resistor, shorted power fet and etc.

              Jestine

              Likes(0)Dislikes(0)
  45. jorge

    July 1, 2016 at 7:12 am

    Thanks Mr Yong, very good information for those who work with those kind equipment.

    ¿have you seen a switch with fancooler ok (on) but no post or led turned on (and no console message at all)?

    Regards

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 1, 2016 at 1:23 pm

      Hi Jorge,
      You are welcome. Do you mean the router internal fan was working but front power LED did not light up?

      Jestine

      Likes(0)Dislikes(0)
  46. Adesanya Olalekan

    June 29, 2017 at 3:37 am

    Waoooo!......I can't believe this wonderful article about electronic power repair has bee up there since 2013 and I just stumbled on it today. We replaced 3 cisco switches in our office within a month and I decided to fix one today. When online, Google and came across this. Finally I found the culprit but it's a very small IC and I doubt if I can it in the store. I had to go and get a complete power circuit for DVD and I worked it. It's working fine now. Thanks.

    Likes(0)Dislikes(0)
    • Jestine Yong

      June 29, 2017 at 5:17 pm

      Hi Adesanya,

      You are welcome.

      Jestine

      Likes(0)Dislikes(0)
  47. Afrim

    July 7, 2017 at 2:54 am

    Greetings Jestine yong I have built a ring meter with this ic lm393, this has made you a lot of help for me has helped many thanks ..

    Likes(0)Dislikes(0)
    • Jestine Yong

      July 7, 2017 at 8:50 am

      Hi Afrim,

      Good to hear that.

      Jestine

      Likes(0)Dislikes(0)

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