Fungus Attack And Inverter IC Failure Found In LG 32″ LCD TV
This LCD TV was brought to me by a personally unknown customer (another one from the mouth to mouth publicity) with the complaint that the sound was ok, but no display with a dark screen. As he had brought it to my home when I was on my duty at the Hospital, my wife contacted me over phone seeking permission to accept it or not. After speaking with the guys who brought the TV, I decided to accept it and told them to write the name and telephone number on the backside. As a precaution, I never accept any sets from unknown people, and never usually allow my wife to deal with them. First of all, we need to know the fault as described by them face to face and secondly, we have to switch on the set (wherever possible) in front of them, apart from the visual inspection for any damages. This screening and filtering has brought in only very genuine and decent customers to me so far.
I switched on the TV on my return from the hospital and saw that the complaint reported was genuine. I opened the set and cleaned it thoroughly. When I spoke to the customer, I also learnt that the set was not taken to someone else. That was true, as there was a lot of dust even inside the screw holes. Dismantled the boards and brought it to my work table, as my table was not having enough space to keep the panel flat on it. On visual inspection, I found moisture and fungus at several places on the Power Supply Board and also bad solder joints:
Did a thorough cleaning of the PCB and patched up all dry solders, by retouching the board from one end to the other. Just have a look, how the PCB was after cleaning and retouch:
After ensuring that all the voltage outputs are ok, I reconnected the power supply to the board and switched on. But the complaint remained same. Observed that the backlight was not lighting up. On checking the inverter board, which was separate in this TV, it was getting the 24V B+ and 4.65 V on/off , indicating that all was well up to that point.
I removed the cover of the inverter PCB to check what IC was used and whether it was getting the B+ on its pin. It was OZ9966SN SMD IC. You can have a look at the board and the components:
The last picture marked as LED1 is an indicator, which lights up if the inverter is functioning. This was in off position when I checked up, though the IC was getting the voltages, indicating every possibility of a defective IC. I got the replacement for the IC OZ9966SN, and my techie friend helped me resolder it properly. Another techie friend suggested a method to check the inverter board using a 15W incandescent lamp. The steps are (1) Supply 24V to the pin marked at the input. (2) Tap 3.9V from it using a 10K drop resistor and 3.9V Zener. (3) Connect this to the on/off point. (4) Use a 10K resistor to pin 15 of the IC to deactivate protection. So I made the temporary connections of these on the board and applied 24V from an Adaptor. The bulb lit properly, and the LED on the board was also lit, indicating that the inverter board was working. Just see the snaps:
I am providing readings of normal voltages on the inverter board: The three readings given in bracket are (1) Standby (2) ON (When inverter IC was defective) and (3) when backlight was functioning:
Status (0.6, 0.6, 0), VBR-B (0, 0.79, 0.79), On/Off (0, 4.67, 4.67), VBR-A (3.35, 1.6, 1.6), Ground 2 & 1 (0, 0, 0), VBL 2 & VBL1 (0, 24.65, 24.39)
Dismantled the components and connected the board directly to the TV. The backlight was on this time and the LED on the board remained on for a few seconds and then went off, probably that’s how it should function. The remote control was defective and after I opened it and cleaned it well, it started working well. The problem now was no display. Checked the voltages on the Tcon Board and found it to be 0. Checked all outputs from the SMPS at the input to the main board and found 12V missing. So, disconnected and dismantled the boards and removed the SMPS board again and checked what went wrong. After combing the area several times and a minute scrutiny of the 12V path, I could see a drop of solder that had got stuck between two pins of an SMD transistor. Removed it and checked whether the transistor had failed. It was found to be good. Connected the board and tried and this time the display was on. Fed a Video and checked and found the result was too good! Just for your information, I am also giving the snap of the main board along with the picture of the result:
Sorry, I forgot to click a few more snaps of other stages explained.
Another job completed fully satisfactorily, and got attracted to the bulging bag!
This article was prepared for you by Parasuraman Subramanian from India. He is 69 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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