Return Of An LG LED TV With Backlight Problems After Four Months of Working
This LG TV returned to my workshop after working for about FOUR months, with the same complaint. It was taken delivery in December, 2020 and returned in April, 2021. Please see my earlier article: Backlight Replaced And LED Driver Bypassed In LG LED TV MODEL 32LF561D-TF | Electronics Repair And Technology News (jestineyong.com)
When I opened and checked, I got a burnt smell and located it to the double coil green LED Driver Board. Mosfet was found dead short. Used my LED Tester and found that the LED backlight was intact.
On consultation with my techie friends, I learnt that this type of board had the mosfet failure due to its poor quality. After removing the shorted mosfet, I tried using the original mosfet which was in the power supply board. But that too failed immediately on switch on, indicating an inherent defect in the board. Last time the supply voltage given was 13.20V from its power supply. Some Techies suggested me to use 24V as this board is designed to work on 24V and when the supply voltage is down, the strain for the mosfet increases to make up the difference.
As it sounded logical, I used a booster board and adjusted the output to exactly 24V, connected a new double coil green board, applied power and let it run. When I took measurements, I got 111.5V, which was slightly more by around 3.5V, as 18 LEDsX6V comes to only 108V. But when I measured the current, it was only 196mA. So, I thought it was safe.
Unfortunately, the TV stopped after around two and half hours of working, emitting burning smell around. I switched off the TV and checked what went wrong. The mosfet of the double coil green board had burnt and it was dead short. It had got stuck to its base and I had a lot of difficulty in removing it, and finally it ripped off the copper pad along with it! So, the experiment failed and I was back to square one.
After vociferous consultations with my techie friends, they suggested different boards. I tried a black one and another yellow one with double coil. The black one had its own power supply, but did not deliver enough voltage and the yellow one was drawing around one ampere current, which the LG power supply board could not deliver. Moreover, the Yellow board works on 24V, and I had to use the booster board too. Here, let us have a look at these black and yellow boards:
Luckily the backlight was still intact surviving all adverse situations! Anyhow, after another series of consultations, one supplier, who was also a technician, suggested a new board that had just come to the market. This had its own power supply; voltage/current could be adjusted and looked sturdy. Let us have a close look at it:
The supplier-cum-techie suggested me to short the 100mA and connect the board. I gave 230V AC in tapping it from the LG power supply board input, wired the on/off (enable pin) and ground. I forgot to mention that the adjust connection had suffered some internal fault and whenever that was connected even in my previous attempts, the board failed to start. The driver output just dropped drastically.
So, there were some unknown problems there, which I anyhow neglected as it was not absolutely necessary. Then wired the output to the LED connector located in the power supply board, like before. After switching on, when I checked, the voltage was 107 and the current drawn was 142mA. So, it looked like was the solution. I also fixed a 12V mini SMPS fan on top of the mosfet, even though it was only warm, and quite normal.
I allowed the TV to run for two days before I ventured to call the customer for picking it up. The picture clarity was really superb!
Here is a picture of the burnt boards and parts:
Thus this marathon innings came to a victorious end, adding intense satisfaction getting added to its collections. In this way, I learnt quite a lot about the intricate problems that can crop up, even though the job looked simple! Well, well, the learning never ends and even when we die, we learn how to discard this body! (LOL)
This article was prepared for you by Parasuraman Subramanian from India. He is 72 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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You may check on his previous repair article below:https://jestineyong.com/defective-smps-ttransformer-chopper-found-in-ibell-dvd-player-model-ibl3288/