Cisco Router Switch- No Power Problem Repaired
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:
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.
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.
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.
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.
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.
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.
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
Jestine Yong
Recommended Switch Mode Power Supplies Repair Ebook
Click Here now to become a professional in Switch Mode Power Supplies Repair
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Leave a Reply
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?
Jestine Yong
July 2, 2013 at 6:18 pm
HI Waleed,
Just use the Windows "Paint software" and it is easy to use.
Jestine
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
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?
Jestine Yong
July 2, 2013 at 8:40 pm
HI Bbaale,
Thanks for pointing to me the problem. I have just amended it.
Jestine
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
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?
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..
Bradley
July 2, 2013 at 7:59 pm
Nice article.Thank you for sharing with others.
Jestine Yong
July 2, 2013 at 8:40 pm
To all,
You are welcome.
Jestine
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
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
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
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.
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
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
Balaji
July 3, 2013 at 1:45 am
Hi Jestine,
Nice and very valuable Information. Great work...
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.
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?
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
adel
July 3, 2013 at 4:35 am
thank you for helpus with great article.
Nick
July 3, 2013 at 12:19 pm
It was very informative topic about repairing cisco switch ,have a nice day and long live.
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.
Jestine Yong
July 3, 2013 at 2:52 pm
Hi Henry,
You are welcome and thanks to all for the support.
Jestine
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!
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
MARK
July 13, 2013 at 1:39 am
HOW DO I TROUBLE SHOOT THIS PROBLEM WHERE START-UP RESISTOR
Jestine Yong
July 13, 2013 at 11:53 am
Hi Mark,
The start up resistor is 160K ohm as seen form the photo
Jestine
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.
Jestine Yong
July 3, 2013 at 9:06 pm
Hi Radhakrishna,
Please visit the link below:
https://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
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.
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
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
star34
July 4, 2013 at 4:37 am
HI Justine,
tanks for thé us full articles
Nimal
July 4, 2013 at 2:05 pm
Very good, detailed repair information. Thank you very much.
M.Hatim
July 5, 2013 at 2:42 am
Hi Jestine
thank you for the good articles
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,
Jestine Yong
July 5, 2013 at 9:47 am
Hi Mansour,
Thanks.
Jestine
Jamaloodeen
July 6, 2013 at 2:59 pm
Hello Jestine
A professional and technical approach to servicing .Thanks.
Richard
July 7, 2013 at 2:29 am
Mr Young , Thanks very much for your PRACTICAL training articles. I really enjoy them
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)
Jestine Yong
July 7, 2013 at 3:15 pm
Hi Johm
You are welcome and thanks to all!
Jestine
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.
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
balamurali
July 7, 2013 at 11:42 pm
Very good article,thank u sir
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 ……
Jestine Yong
July 8, 2013 at 9:41 am
Hi Farhan,
You are welcome and thanks to all.
Jestine
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..
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
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.
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.
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
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.
Jestine Yong
July 11, 2013 at 9:51 am
Hi Prince,
You are welcome and I have sent an email to you.
Jestine
raymundo saura
July 11, 2013 at 5:55 pm
thanks for the info thanks and GOD bless
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.
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
Jestine Yong
July 25, 2013 at 8:28 pm
Hi John,
You are welcome.
Jestine
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
Jestine Yong
August 3, 2013 at 4:11 pm
Hi Darie,
You are welcome.
Jestine
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
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
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
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
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
Jestine Yong
October 28, 2013 at 8:47 pm
HI Anand,
Thanks.
Jestine
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
Jestine Yong
April 7, 2014 at 10:24 am
HI Felix.
What sound was that? Where is this sound from?
Jestine
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
Jestine Yong
April 7, 2014 at 8:29 pm
No power? Do you mean the system was having a no power symptom?
Jestine
Felix
April 7, 2014 at 9:26 pm
YES. NO POWER
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
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
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
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.
Jestine Yong
June 29, 2017 at 5:17 pm
Hi Adesanya,
You are welcome.
Jestine
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 ..
Jestine Yong
July 7, 2017 at 8:50 am
Hi Afrim,
Good to hear that.
Jestine
Eric Ostrike
September 20, 2017 at 10:26 pm
I have a Cisco 3560CG-8 that when powered up it lights up then goes out. Nothing else happens. I am hoping it is just the power supply and will use your troubleshooting steps to see if I can figure it out.
emanuele
July 29, 2019 at 12:52 am
Hello mate, could you tell me where is the inline POE Controller on WS-C3750-48PS?
Thanks!
Jhon Herrera
January 5, 2021 at 6:44 pm
hi nice tutorial,
how we can repair a AIR-AP1832I-A-K9 Due to an electrical short it did not work anymore
Jestine Yong
January 7, 2021 at 3:22 pm
Hi Jhon,
You need to open it up and troubleshoot to find out the problem.
Jestine