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SMPS Of Running Display Failed Due To Defective PWM 3842 IC

By on October 27, 2020









power supply repair

This SMPS was brought to me by my Techie Friend, who could not find time to troubleshoot this due to heavy load of work. This SMPS was of the Running Display installed in front of his shop. He brought only the board and not its cover and it was very clean, saving my time and efforts to a great extent. He had already replaced all the electrolytic capacitors on the board, but could not find the values of two burnt resistors in the run DC section.

As he was aware of my craze on troubleshooting SMPS, he made use of my weakness! (LOL) Anyhow, for people like us, any such chance is a boon and I went straight ahead in locating the fault. Another techie friend helped me giving the value of the two resistors fit in parallel. It was 10 Ohms. I replaced the resistors. A few pictures before we proceed further:

how to fix power supply

After replacing the resistors, I checked all the components in the primary and secondary side of the SMPS. Replaced the TL431 and A1015 transistors though I could not find anything wrong with it. 

The IC in the SMPS was found faulty when tested. So replaced it with a new one, on a Socket. Checked the opto coupler too and it passed the test. (Method used was connecting the digital Multimeter in ohms range to the Transistor side, and touching the LED side using an Analogue Multimeter in X1 Ohms range, observing the polarity. This will drive the LED and the transistor will conduct showing a low resistance, if opto is ok. ) Powered the SMPS and got the green LED lit on the board and output of 5V, for which I had to adjust the pot to make it accurate. Here are a few pictures:

how  to fix smps

how to repair and fix smps

Mission accomplished and satisfaction jumped into the collection bag!

This article was prepared for you by Parasuraman Subramanian from India. He is 70 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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  1. Albert van Bemmelen

    October 27, 2020 at 7:55 pm

    Good thing that your techie friend saved you a lot of time and effort by cleaned the board and replacing all capacitors. And also a good thing that there still are SMPS units that use this common PWM 3842 controller. Because now a lot of new and exotic parts are used which makes fixing them these new units very hard!

    • Parasuraman S

      October 27, 2020 at 11:43 pm

      Yes, now-a-days almost all SMPS use different PWMs, for many of which, we can't even get a datasheet in the web! Some have the numbers written at the bottom! Many thanks for your comments!

  2. Yogesh Panchal

    October 27, 2020 at 9:55 pm

    Good job! Sir,

    • Parasuraman S

      October 28, 2020 at 8:30 pm

      Many thanks!

  3. Tito Kanshulu

    December 22, 2020 at 1:24 pm

    I don't understand how you did the testing of the opto coupler,did you use both multimeters at the same time. Can you clearly explain boss.


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