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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