SMPS Of Running Display Failed Due To Defective PWM 3842 IC
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:
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:
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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