Servicing Laptop Adapter – Key Model 7150
This AC ‘ARAPTER’ (LOL) was one of the three power supplies that I had collected from my technician friend, about which I had mentioned in my earlier articles. This was dead and on opening, I could find that it was another victim of high voltage surge!
Using the techniques given by Jestine Yong’s world famous SMPS service book, I located the defective parts very easily: Bridge Rectifier dead short, Mosfet dead short, two fast switching diodes dead short, ESR of electrolytic capacitors out of permissible range, and a lot of dry solders! But fuse was in tact (very surprising)! Replaced all these components and did a retouch of all solder joints and cleaned the PCB thoroughly. In the following picture, the LEDs are not lit (it is the reflection of the flash from my camera) (The adapter is not yet connected to power)
As I could not find anything else defective in my cold test, I connected a bulb in place of the Fuse and tested. The lamp behaved perfectly, i.e., momentary light up and switch off. But there was no output. Checked the oscillation using Oscilloscope, by connecting the earth to hot ground and keeping the probe on top of the SMPS TX. Could not find any oscillation. So, discharged the high voltage capacitor, and as suggested by Jestine Yong, replaced the PWM IC number UC3842 and I replaced one resistor which looked getting heated up and another resistor which was slightly out of range. Repeated my test and this time the lamp remained lit! So, switched off immediately and checked the Mosfet. It was short.
Then I knew that some other component is also failing on load! Checked the secondary part once again. This time the opto-coupler was also short! To be on the safe side, I replaced all the fixed capacitors as well along with the shorted components. Since there was no other component found to be defective, I retested the Adaptor, without removing the safety lamp. The output was present as the LED lit up, along with my face! So, replaced the fuse and tested it again, it worked perfectly well. The output voltage was also intact. (See the first LED in picture) (I had to provide a sleeve to the fuse, as I saw some random arcing between the AC input pin at the rear of the AC socket and metal body of the fuse as these were close!!!!)
Fit the PCB back in its case and switched it on. Left it on for several hours connected to a 12V stepper motor, without any problem.
These are the components that were replaced in this multi-voltage selectable adaptor:
Heaved a sigh of relief, and wiped off imaginary sweat from my forehead!
This article was prepared for you by Parasuraman Subramanian from India. He is 66 years old and has more than 30 years’ experience in handling antiques equipment Valve Radio, Amps, Reel Tape Recorders and currently studying latest techs classes conduct by Kerala State Electronics Technicians’ Association. He was a BBA graduate, retired as MD of a USA company.
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You may check on his previous repair article below:
https://jestineyong.com/servicing-onida-slim-300-crt-tv/
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Albert van Bemmelen
April 14, 2017 at 2:20 am
Thank you for this very interesting universal Power Supply repair Parasuraman!
Again you had to replace a lot of components that were defect or out of spec. Like very often the fuse was still intact, most likely because there wasn't any oscillation or electrical activity possible anymore. Peculiar still was that the primary Bridge Rectifier was dead short, Mosfet dead short, and two fast switching diodes were dead short. While the primary fuse kept intact is a little mystery indeed.
Henrique Jorge Guimarães Ulbrich
April 14, 2017 at 2:54 am
Indeed, a good "harvest" in this AC "Arapter" (LOL too!). Thanks for sharing your knowledge.
angelo
April 14, 2017 at 4:49 am
dear sir
is there any way to figure out wether a ceramic capacitor is bad or good on visual inspection
or by touching it ?
thank you
Parasuraman S
April 14, 2017 at 9:54 am
Well, if you find the capacitor is dark, rather symptoms of heating up, better to replace it. I would not recommend 'touching', unless it is in secondary, low voltage area! Please use a temperature probe of the Digital Multimeter to check whether the cap is getting heated up! If you have an analogue multimeter, remove the suspected capacitor, select a very high ohms range and check the cap. You should not get any reading. Sometimes the needle will kick to right and go back to left, which is an indication of good cap. Though it is advisable to have a capacitance meter as these are nowadays available at cheaper rates, it is not a fool-proof test, as the cap might breakdown at voltage built-up. For that, you may need high voltage capacitor tester, which is a costly instrument! (Breakdown tester)
Yogesh Panchal
April 15, 2017 at 5:30 pm
Good job sir!
Robert Calk
April 15, 2017 at 6:14 pm
Good job, Parasuraman.
Paris Azis
April 16, 2017 at 1:59 am
Good job,Parasuraman
In such a large scale repair,if I were in your shoes, I would have replaced that fuse as well. I have bitter experience about this issue. Many times, when repairing similar large scale failures, I performed catastrophic tests on the fuse itself just because it had survived. I can assure you dear Parasuraman that all surprisingly they were no fuses at all in the sense we use the term! They opened far beyond their rated current offering no protection at all as they should supposedly do!
Parasuraman S
April 17, 2017 at 6:13 pm
You are very right! Something that I overlooked! Shall be more careful next time! Many, many thanks for your keen observation!
reza
April 16, 2017 at 12:28 pm
Hi Parasuraman Subramanian
very well
You were admirable hard work
thank u
Humberto
April 19, 2017 at 11:41 am
Hi Parasuraman, you have done a great repair. Keep on and share with us your experiencies.
Ulises Aguilar Pazzani
May 1, 2017 at 12:41 pm
MR Subramanian great fix Sir
Ulises Aguilar Pazzani
May 1, 2017 at 12:44 pm
MR Subramanian can i ask for a favor I was checking some of You information Sir , how do you diagnose on a transformer microwave?