Backlight Replaced And LED Driver Bypassed In LG LED TV MODEL 32LF561D-TF
This LG LED TV was brought to me with the complaint that there was no picture, but sound was ok. After observing the Covid Protocol, opened the set and cleaned it as first step.
Powering the TV, I checked the voltage to the LED and found it to be 217.8V DC, which indicated that there was no load. So, disconnected and removed the Mother, Power Supply Boards and speakers from the rear after removing the connection to the T-Con Board. Disconnected the strip going to the panel, removed the T-Con and the frame protecting the two cofs of the panel.
Removed the screws of the frame holding the panel. Then gently slid out the panel, very carefully handling it not to cause a crack or damage, allowing it to rest on its PVC frame and moved it to a bed nearby.
Removed the acrylic sheet, polarization film etc. and then disengaged the reflector holding plastic hooks. As I noticed visible burnt marks on a few LEDs on all the strips, removed the three strips which had altogether 18 LEDs with large size domes. Bought three strips of same type from the market and fixed these. We need to peel the old strip carefully with its sticky tape under, clean the base, and then fix the new strip in its place ensuring the connector is in the same place like before. Now let us see the following pictures:
Then I re-fixed the panel with all its associated parts. As advised by my techie friends, I added a 1.7 Ohm 2 W resistor in series to the 2.3 Ohm resistor at the source of the mosfet to control the current. Since the LEDs bought from the market may not be as good as the original company’s, this is a common practice being followed by my friends. Then reconnected the boards from behind, without fixing these, and applied power. Checked the voltages to the LED and found it to be around 140V, which was definitely wrong, as 6V LED x 18 should take only around 108 Volts. Then I knew that the LED driver circuit was over driving due to some fault somewhere, because of which the original backlight had blown.
I tried even adjusting the brightness, contrast and backlight control to safe levels. But by that time the resistors as well as the mosfet got heated up and smoke was coming from the resistor. So, disconnected and waited for the components to become cool. Then obtained a circuit diagram of the LED driver using IC BD559 and thoroughly checked all the components in the circuit. Everything was fine. So, it could be a problem with the IC itself. Even the mosfet was ok. As the customer was in a hurry, I bought a ready to use LED driver and wired it to the PS Board. (13.2V supply, ground, LED adjust and Driver on) and then wired the LED drive output to the PCB, cutting the tracks coming from the LED drive on the board. Applied power and noticed that the backlight was on, with a voltage of around 179.1V without load and 103.5V on load. This was safe enough to provide a longer life.
This LED driver actually adjusts itself to the required voltage and current sensing the load. So, no hassles at all! So, fixed the PS board back and also fixed the new board on one of its screws. Kept an old mobile cover underneath and applied Fevibond. Now the connected pictures:
Allowed the set to run for a few hours and then fixed the back cover. Called the customer and informed him to collect it. Here are a couple of pictures to provide a finale, without forgetting to add the satisfaction to its collection bag!
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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