Power Surge Blew 12V Adaptor And PS IC In LG Monitor Model W1643C
The monitor and its adaptor were brought to me with the complaint that it was dead and that the service provider could not resolve it. My interest always aroused when others failed and I brought it home for checkup. The monitor had such a lot of dust inside that it took almost an hour for me to clean it up thoroughly. Evidently, the service provider did not even open! On checking up the mother board, I noticed that the 12V to 5V buck IC, APW7142 had a puncture on it.
I removed the IC using my blower and then cleaned the area and fixed a new IC as I had this in stock.
On checking the adaptor, it was dead though it had signs of opening. I used one of my adaptors and switched on the monitor. I found it to be working well:
Then turned my attention to the adaptor. It was found closed with glue, which I had to release using petrol and even then I had to apply a bit of force to detach it. I found that the DC wires were not connected, the Current surge preventing low Ohm resistor which also served as a fuse at the AC input was missing, one leg of the tank capacitor was cut etc. So, someone had tried to troubleshoot and rectify this. The capacitors at the output were found bulged.
My first step was always checking the health of the SMPS transformer using my self-made Ring tester, the result of which can be seen from the second picture above. Then replaced the bulgy capacitors. The tank capacitor and another 100uF/25V capacitor in the primary were dead short, which were also replaced. The rectifier diode was ok. I checked all other components in the primary and secondary for any defects and found them to be ok. The output, however was zero when the power was applied. Then checked whether the run DC was coming to the switching IC and found it to be present. Changed the TL431 and opto IC 817 though these were ok.
But nothing made the ACA working. Then removed the Mosfet and the switching IC, 63936p, a picture of which is shared below, as no datasheet was available:
I used a three wire module, upon which around 30V was coming in the output, which was quite abnormal as the adjusting pot in it was at minimum. Then removed that and used a 5 wire module which used the feedback circuit also and found that the output was dead. Something was wrong terribly. Output of 30V could come only if there is a partial short in the primary of SMPS transformer. I consulted my techie friends and they all said that SMPS transformer was the culprit. It was probably true as the ACA suffered such a voltage surge that shorted even the tank capacitor. So, my attempt to revive the ACA ended in utter fiasco.
I bought a new one from the market which was available for a cheap price and retained the defective ACA with me as I had already replaced a lot of components in that. Assembled the monitor back and applied power and fed a signal and allowed it to be on for sufficient time. Mission accomplished with satisfaction getting added to the collection. (For those who do not know about 3 wire and 5 wire module, I am providing links below where I had extensively covered the usages)
https://jestineyong.com/cracked-unknown-pwm-replaced-by-universal-module-in-pressure-monitor-smps/
https://jestineyong.com/beware-of-these-fake-pwm-ics/
This article was prepared for you by Parasuraman Subramanian from India. He is 76 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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Albert van Bemmelen
January 24, 2026 at 11:23 pm
Thanks again for sharing the information on how to use the universal power 'test' modules. This keeps showing the main ingredient of electrical difficulties that exist around fixing bad smps units.
Parasuraman S
January 25, 2026 at 7:55 am
Many thanks for your expert comments, dear Albert!
Mark J
January 25, 2026 at 3:53 am
Parasuraman good job on the repair. I like your self made ring tester.
Parasuraman S
January 25, 2026 at 7:56 am
Many thanks for your comments, dear Mark!
MP
January 26, 2026 at 11:30 am
I take it that whoever had earlier on done disconnections in the adapter was saving both the adapter and monitor from further damage as the fuse and rectifier diodes were okey. In the name of modern technology and being economical, down-sized electronic components may fall short of the robustness requirement, hence resulting in multiple systems failure in a component. I'm siting this example where both the primary and secondary operations fail in what looks to me like a small struggling adapter that runs a pressumably sixteen-inch monitor. From whatever angle, the two are a perfect mismatch. Besides, I'm sure the adapters swithing mosfet is not fitted to a heatsink, yet the mosfet works in an enclosure without cooling aeration. And, not many technicians ever take the trouble to replace smd capacitors, leave alone testing them for faults. My other opinion is we sometimes have to face the realities of cruel and rigid customers who only agree to a bargain that favours them, oh ! This man-eat-man world !
Parasuraman S
January 26, 2026 at 2:55 pm
Both the mosfet in the primary and double diode in the secondary are fit on separate heat sinks fit on sides. Many thanks for your analytical comments and explaining the facts of facts!
Muykit
January 26, 2026 at 1:58 pm
Its challenging for me when their is no datasheet for an IC (eg transistor/regulator or any multi-pins IC with circuits visually disappearing underneath) that lies in the way of a circuit to be examined. Even more challenging is when a circuit runs in and out of both sides of a boards surface. I'm afraid Engineer Parasuraman if you want to gain hapiness without attaining satisfaction anyway, you will have to practically replace everything single electronic component on the retained adapters circuit board. Well, such work makes me forget about the hard to deal with misers who are stingy about payments. But somehow they always pay tenfold for some other unrelated cause, in the normal course of events.
Parasuraman S
January 26, 2026 at 8:22 pm
Many thanks for your involved comments based on exposure and experience!
Waleed Rishmawi
January 26, 2026 at 2:39 pm
good job my friend on the repair. professional as alaways in fixing stuff and bringing them to life again. testing modules are very interesting and important step to diagnose repair and fxing stuff. thanks for sharing and have a blessed day
Parasuraman S
January 26, 2026 at 8:23 pm
Many thanks, dear friend! A lot more thanks for coming back to our dearest stream!
Yogesh Panchal
January 26, 2026 at 6:31 pm
Good Job!Sir.
Sometimes it seems that the component is made only with this design which is not seen in any other device like 6 pin SMD switching IC. May be available with some another marking codes.
Parasuraman S
January 27, 2026 at 2:53 pm
Yes, very true! Many thanks for your comments!