No Power In 3Com Baseline Switch 2226 Plus
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.
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.
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.
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
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rohit
July 9, 2012 at 9:25 am
hi jestine good article with clear pictures.
rohit
July 9, 2012 at 9:28 am
the smd diode that u found shorted,is there any code written on it?
Jestine Yong
July 9, 2012 at 9:52 am
HI Rohit,
Thanks. The smd diode does not have any code printed on it.
Jestine
Stanley
July 9, 2012 at 9:56 am
Nice articles on repairing a network switch power supply...
From your experience, are "most" of the switch mode power suply have similar functions but using different components ?
By the way, the brand name is 3Com and not 3 Com... there is no space between the 3 and Com
Jestine Yong
July 9, 2012 at 10:06 am
HI Stanley,
Yes the power supply is to produce DC output. Different power supplies using different type of power ic, power fet and etc. Thanks for the telling me about the 3com
Jestine
sim adm
July 9, 2012 at 9:59 am
Good job Jestine on this one,
I also remember you mentionned , in one of your articles , that SMD diodes rarely fail!!
Jestine Yong
July 9, 2012 at 10:01 am
HI Sim,
Thanks, rarely fail does not mean it would not fail but then again it depends on what type of equipment design.
Jestine
Bernie Scott
July 9, 2012 at 10:00 am
Very good article. Read your articles all the time and pick up some good tips from doing so. Keep up the good work. I mostly repair music equipment such as PA gear, reel to reels and any equipment that musicians would use...lots of vacuum tube stuff......guitar amps, effects units, lighting equipment, keyboards, etc.....etc.......looking forward t your next article....
Cheers ,Bernie
Jestine Yong
July 9, 2012 at 10:09 am
HI Bernie,
Thanks! How's repair business in the music industry?
Jestine
Richard
July 9, 2012 at 10:00 am
Very good troubleshooting Jestine. I enjoy all your articles. I am 75 years old and retired but am still working on friends, relatives, and neighbors electronics for the experience more than the money. I still enjoy seeing a TV get a picture, or a radio getting sound after working on it.
Jestine Yong
July 9, 2012 at 10:08 am
HI Richard,
Glad you liked my articles. Hats off to you even at this age you are still working!
Jestine
kamal
July 9, 2012 at 10:05 am
Very good explanation, mr jestine, you always helping our worldt from being flood with electronics junk. Try u calculate how many equipment restore. Good job man. We are same..safe our world
Jestine Yong
July 9, 2012 at 10:07 am
HI Kamal.
Thanks, already repaired thousands since the day one I joined this line.
Jestine
anand
July 9, 2012 at 10:17 am
Hi,
It would be much clear, if you can draw schematic of affected section and then explain.
Jestine Yong
July 9, 2012 at 10:20 am
HI Anand,
I wish i have the extra time. Repairing, writing, taking photos and blogging already taken tons of hours.
Jestine
Lal Kahandawa
July 9, 2012 at 10:24 am
It's a good article . good job Justine.
The steps you have taken use full to every body.
Tana
July 9, 2012 at 10:24 am
Dear Jestine,
That fault finding is a good tuturial . You have shown how to systematically eliminate components in the no power networking system .This shows your highly skilled knowldge in electronics. I hope one day I will become as good as you.
Thank you
Tana
Dave Adcock
July 9, 2012 at 10:27 am
Hi Justine.
Don't understand the "..It turned out to be working well after burning in for a few days..", what needs burning in?
Surly the 1N4148 doesn't need to 'burn in' - does it?
Dave
Jestine Yong
July 9, 2012 at 9:31 pm
Hi Dave,
Burning in means to test the whole system (power supply and the mainboard) and also the component that I have replaced to make sure it really works. We do not want customer to send the equipment back to us after a day or two. Not only this equipment you need to burn in, you also need to burn in test for all other equipment too.
Thanks.
Jestine
prashanth kumar
July 9, 2012 at 10:32 am
Dear Jestine,
a very good article with crystal clear explanation.
Please leave similar type of articles in my mailbox.
Thank you so much Jestine,
Regards,
Prashanth Kumar
peter djuh
July 9, 2012 at 11:07 am
thanks for this wounderful and interesting article sir. sir, i have a problem of determining the values of SMD components especially capacitors and zener diodes
Jestine Yong
July 11, 2012 at 5:20 am
Hi Peter,
Actually it is tough to determine the value of smd capacitor and zener diodes if they are shorted and do not have schematic or any bard for comparison.
Jestine
Bernie Scott
July 9, 2012 at 11:28 am
Jestine...You are welcome. I never realised how much music equipment was out there to repair...and where I am there is nobody else doing it as far as I know. I get a mixture of everything from vacuum tube gear to solid state to SMD stuff....sometimes the equipment could have all three...so it keeps you on your toes and having a good supply of technical books and websites is actually a necessity.Thank god for the internet......I read your articles all the time...very informative...keep it up....
Cheers,
Bernie
Jestine Yong
July 11, 2012 at 5:21 am
Hi Bernie,
Thanks for the update and glad that you liked the articles.
Jestine
michael sathe
July 9, 2012 at 11:31 am
Dear Jestine,
Thanks for the Information
michael sathe
Mumbai
Suman lal
July 9, 2012 at 11:35 am
Jestine sir,
Thank you for posting this article, I,m new in this field. Sir i hope you will help me and give me a support for my job.
thanks by suman lal
Grant
July 9, 2012 at 11:44 am
GREAT repair job as usual 🙂
Saj
July 9, 2012 at 11:44 am
Check all semicondutors first. Transistors then diodes, then ics, then resistors, then caps. Glass diodes have a high failure rate. Leakage especially.
Jestine Yong
July 9, 2012 at 9:27 pm
Hi Saj,
Thanks for the comment.
Jestine
Aboagye
July 9, 2012 at 11:50 am
Hi Jestine,
I wish everyone could take his time to produce article with good pictures like yours. I always enjoy reading your books. Good work done.
kim seguido
July 9, 2012 at 12:12 pm
Thanks Jestine,
nice one, its a big help to technical lovers...
kim seguido
Anoop
July 9, 2012 at 12:48 pm
This is a good article, Thanks Jestine.
James Stoffel
July 9, 2012 at 12:52 pm
Very good article. And, I like the new layout of your website.
Carlos
July 9, 2012 at 12:56 pm
excellent, great article,i hope to buy all your ebooks soon, thanks
Jestine Yong
July 9, 2012 at 9:26 pm
Hi Carlos,
Thanks for the support!
Jestine
mutahir
July 9, 2012 at 1:11 pm
very good, jestine to helping .so the smd ic also dimage
Jestine Yong
July 9, 2012 at 9:25 pm
Hi Mutahir,
The SMD IC is good only the leaky SMD diode problem.
Jestine
bogie
July 9, 2012 at 1:14 pm
As a beginner,i'm having a hard time thinking whats the voltage value of zener diode mostly if i donnot have a diagram. Did you just guees the 1N4148? Can you share the idea?
Jestine Yong
July 11, 2012 at 5:18 am
Hi Bogie,
Since the board marking is "D" thus it represents a diode. In fact 1n4148 was a test and it turn out to be working fine. Without the zener diode code and schematic and if the zener diode is shorted then there will be a hard time to finding out the fault.
Jestine
Juan Martinez
July 9, 2012 at 1:18 pm
buena clase de electronica
ARMEND
July 9, 2012 at 1:27 pm
Very profesional troubleshoting and allso very googd article
Rodney Symister
July 9, 2012 at 1:33 pm
Hi Mr Yong,
I really enjoy your article each time,ever since from a young boy,I have a passion for electronics,I am now 42 in construction and still have that passion. I seem to have this natural talent for fixing things,but never get a proper training,I manage 60% of the time though to locate and fixed. The tools I work with now,a cheap multimeter,small soldering iron,screwdrivers,solder sucker and pliers.
Presently I am working on a 13" samsung Tv, in the 90's with the long backs and having some problems.The cable wire accidentally touch something at the back of the tv ,and shorts it out. Each time I put new fuse in,it got blown. Can you tell me what to check,so I can locate the problem?.
Jestine Yong
July 11, 2012 at 3:10 am
Hi Rodney,
Nice to hear about your passion. Blown fuse usually cause by shorted semiconductor. You may have to check the bridge rectifier, power fet or even direct replace the power ic. If possible use the series light bulb trick to tackle the problem and save on the fuse. You can google more for series light bulb trick.
Jestine
Ruel Corniffe
July 9, 2012 at 1:49 pm
Thank you Mr. Jestine Yong
very good article, will help me with my troubleshooting of network equipment.
Merlin Marquardt
July 9, 2012 at 1:56 pm
Very nice!
Ray
July 9, 2012 at 2:00 pm
Very interesting. Good to read about repairs to different types of equipment.
lalit shrivastava
July 9, 2012 at 2:01 pm
Hi! Jestine, I've been28 years in the field of repairing electronic items, but have never seen anyone so dedicated, and unselfish in sharing his talent like YOU...inspite of such a busy schedule,,thanx,,,your articles really help me out a lot,,,
Richard
July 9, 2012 at 2:10 pm
Thanks a lot what you've mentioned today happened to me,Tedelex crt TV was no power.I've tried testing many components but was no lucky,but after replacing ZD C18Ph everything worked fine.anyway let me thank you about the article,it was great.by Richard
bashar hamdan
July 9, 2012 at 2:15 pm
Realy, Nice article and thank for your newsletter and Thank you for your good perseverance
have a nice day
mhmtcay
July 9, 2012 at 2:28 pm
Want to know how best regards mists lcd repair information
Jestine Yong
July 9, 2012 at 9:37 pm
Hi Mhmtcay,
Could you elaborate because I don't get your meaning?
Jestine
Toni Kassab
July 9, 2012 at 2:31 pm
Excellent article. As always, very detailed descriptions and very good resolution photos. Thanks Mr justine. I hope to receive more similar articles.
moshe
July 9, 2012 at 3:09 pm
Great article Jestine....as always.
Somewhere i read that you need to put a load on a SMPS in order to test various aspects of it.
So in this case what would be an adequate load?
moshe
Jestine Yong
July 9, 2012 at 9:15 pm
Hi Moshe,
Yes certain power supplies need a load to run properly but this one does not need a load.
Jestine
Liviu
July 9, 2012 at 3:37 pm
Perseverence is the mother of succes. Good job. Congratulations.
leo
July 9, 2012 at 3:53 pm
Hello. How did you get the component out? Did you cut the leads or desoulder.
Leo
Jestine Yong
July 9, 2012 at 9:11 pm
Hi Leo,
I just solder it out using my Hakko 981 solder gun
Jestine
Charlie Bennetch
July 9, 2012 at 4:00 pm
Jestine--- would you think about an article on modifying switching supplies for different output voltages? I have a box full of old 15 amp server switching p/s that I would love to convert from 12 volts out to 13.2 volts output---to use as power supplies for ham radio use. ( It's just that 12 volts is a little too low for modern tranceivers. ) Maybe some other guys out there would also like to be able to do this. But I lack your knowledge of this technology, and I could use some help!
I always read your articles! Keep up the good work!
Jestine Yong
July 10, 2012 at 11:49 pm
Hi Charlie,
Thanks for the suggestion but it will not be easy to increase the voltage unless there is a boost circuit is use. If there is a ready made boost circuit then it will be easy to modify it. I suggest that you try it out yourself to see if it can be done or not. Once successful you will have a great satisfaction.
Jestine
Harry, Sr.
July 9, 2012 at 4:21 pm
Hi Jestine,
Very good, well edited, & organized blog! You're definitely a Professional!
I wonder if that bad diode was a Shottky or a regular silicon diode, but, I guess it dosen't matter if the
1N4148 does the job.
By the way, if you modify the antenna on an X'MAS Light tester (made in China), you can detect where AC voltage is, or is not, trace AC lines inside walls & check radiation from CF light bulbs, etc. I found that my power lines have dirty power coming into my house, & it gets dirtier as I use more CF lights! And, a portable AM Radio will confirm that other dirty signals are present that may cause health problems, like headaches, sore muscles, foggy vision, etc.! These testers have a hex inverter & 3 LR44 batteries, & are very sensitive.
Harry
Jestine Yong
July 10, 2012 at 10:06 pm
Hi Harry,
No clue on that diode that's why i used 1n4148 as a test but it turned out to be working well. Thanks for sharing about the dirty power lines. Not long ago I did saw a video on similar tester that can detect such problem.
Jestine
Amir Mukhtar
July 9, 2012 at 4:27 pm
Sir, jestine
Nice Article abount on 3 CoM networking switch device. its common problem but hard to recover sometime in switches. Iam very glad to read above article. Because i got more knowledge after read.
thanks you and God Bless you a Lot.
Amir Mukhtar Ashrafi
KEMS
July 9, 2012 at 5:24 pm
thanx for d tip
ikechukwu
July 9, 2012 at 5:33 pm
master,
nice attjcle, love what you are dojng, by the way how did you know that the diode is a signal diode and not a zener diode as there was no mark on it ? thanks and keep it up.
Jestine Yong
July 9, 2012 at 9:13 pm
Hi ikechukwu,
The board marking is "D" and not "ZD" thus I tried out the signal diode.
Jestine
Alois Napitalai
July 9, 2012 at 6:00 pm
Hi Justine
I like the article,you have given some ints to check out some faulty d-link switches lying around in our department.
Alois
sokhom
July 9, 2012 at 6:26 pm
Very good explanation, Mr jestine, you always helping our world from being flood with electronics junk. Try u calculate how many equipment restore. Good job man. We are same..safe our world.
antonio
July 9, 2012 at 6:30 pm
hi ,excelent ,concise and practic sequence,thanks......
beh
July 9, 2012 at 6:44 pm
HI MR YONG
thanks for this very beautiful article .
beh
Daniel
July 9, 2012 at 7:13 pm
Hi Jestine, once again a fabulous article, very clear and concise. I would love to sit in and learn in one of your workshops.
Take care
Regards
Dan
Willie
July 9, 2012 at 8:18 pm
Hi Jestine, thanks for another good tips in repairing i know you spent time on doing it, but you still have time to share it to us. Thank's a lot more power.