Bulged Caps And Bad SMD PWM Found In Two Dead LG 12v Adapters
These two AC Adapters belong to the hospital where I work. (By the way, I have resumed my visits every alternative day, after doing online support from house for the past few months!) Both were reported to be dead. On opening Sl. No. 036208, I could see a bulgy 6.8mfd/400V Cap, parallel to which was another 3.3mfd/400V. I removed both and fixed a 10mfd/450V cap in its place.
As this ACA had undergone replacement of other electrolytic caps and retouching solder joints about a few months back, switched on and found it to be working fine. (pictures above). Refixed the cover and checked again:
Opened the other and did not find any bulgy caps. The fuse was intact. ESR of one 10mfd/50V cap in the primary section showed ESR as 27 and value as 3.3. So, replaced it. Checked all other components and also did the ring testing. No defects were found.
Applied power and did not get any output. Checked the VCC of the six pin SMD PWM marked as AAHBM and found it to be dropping indicating a possible failure.
So removed the Mosfet and connected a three wire universal SMPS module in its place and applied power. I got 12V, indicating that high voltage section primary and outputs were functioning well. (Those of you who do not know about this universal module can see this link : https://jestineyong.com/smps-transformer-live-tester-and-servicing-crt-tv/ )
So, the next possibility is a defect in the SMPS transformer or connected components in the primary run DC section. In order to check that, I connected a five wire universal module. (I think Jestine Yong has published a book on use of this module) This module uses STR DMO0565. Kindly see following link for datasheet: DM0565 pdf, DM0565 description, DM0565 datasheets, DM0565 view ::: ALLDATASHEET ::: This module has Yellow (Drain), Black (ground), Red (VCC), White (Feedback from Optocoupler), and Green (B+) .
I connected the wires in its proper place and switched on and got 12V, indicating that the SMD PWM indeed was the culprit. As a replacement was not available, drilled holes on the PCB and wired the module from the top. Retouched all the solder joints.
Housed the module after wrapping it with tape. Checked and found it to be working very well, when I connected an Induction Cooker 12V Fan as load.
(As this is used for Attendance Register device, the power consumption might not be high requiring a heat zinc to be provided for the STR.)
Fixed the cover. Missions accomplished and double satisfaction jumped into the bag happily!
This article was prepared for you by Parasuraman Subramanian from India. He is 71 years old and has more than 30 years’ experience in handling antique equipment like Valve Radio, Amps, Reel Tape Recorders and currently studying latest tech-classes conducted by Kerala State Electronics Technicians’ Association. He has done graduation in BBA degree, private diploma in Radio Engineering and retired as MD of a USA company. Presently working as Consultant to Hospital and other institutions.
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