LG 32″ LED TV No Backlight Repaired
This 32″ LED TV came in with the complaint of no backlight but have sound. The power LED light was also normal. Could see both the power and mainboard once the cover was removed.
If the LED backlight was not On but have good audio this could only suggest either no DC supply voltage going to the LED backlights through the LED output connector, open circuit in the LED driver area or even the entire LEDs have problem (very unlikely).
Below photo is the LED driver circuit and it needs 24 volt in order to function.
But there was no 24 volt output when I checked on the connector that liked up to the main power supply. This is very obvious the problem is in the power supply section.
With visual inspection check on the power supply section found one high voltage capacitor was burned and the outer layer was blown off!
I need to check the corresponding components (photo below) and found the FET that was link to the defective capacitor was shorted.
With the replacement of the FET and the capacitor I got back the 24 volt and to be exact it was 24.5 vdc. Assembled everything and powered On, I could see the display.
See the photos below:
Conclusion- Actually I would use visual inspection first before I start to troubleshoot the circuit board but I need to explain to you first about the important of checking the backlight output voltage which was 24 Vdc (but it was missing in the above case). In the future you should know what to expect from a 32″ LED TV backlight voltage. By the way, if you are an experience repairer and would like to know the problems and solutions of LED TV fast you can check out on Damon’s latest PDF list here. His previous 713 PDF list can be found in this link.
This article was prepared for you by Suranga Bandara who owns an Electronics repair shop in Anuradapura, Sri Lanka.
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https://jestineyong.com/mbrf10200ct-dual-schottky-diode-shorted-in-led-tv-repaired/
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HECTOR DELATORRE
November 25, 2020 at 11:50 pm
very informative, good article
Albert van Bemmelen
November 25, 2020 at 11:51 pm
I understand that you are saying that there is a simple rule to how many leds are used depending on the TV's screen size and so to the used needed backlight voltage. 32 inch (about 81,28 cm) is the diagonally measured screensize, and the led strips are normally always horizontal placed in a straight line. Leds need each about 3V DC. So 24,5V would be 8 leds on a stripe. Horizontal we measure about 73,7 cm for 8 leds to be placed. We have 7 spaces inbetween 8 leds and 2 spaces on the sides. 73,7 cm / 9 spaces gives a distance between 2 leds, and to the sides of both left and right side led, of about 8,18cm. So one led gives backlight to a square area of about 16,37cm2. And the next horizontal led strip is place at 8,18cm from the previous one. But of course we can also place 8 more leds in serie with the previous led strip parallel on the same 24,5V. So what rule we can conclude here is not clear this way. I previously never measured the distance from one led to another so that should clear up this question.
suranga bandana
November 27, 2020 at 12:14 am
Mr.Albert
24V for LED Back Ligth inverter Board.
Look at The Third Photo PCB .
Yogesh Panchal
November 26, 2020 at 3:29 pm
Good Repair! Suranga.
Parasuraman S
November 27, 2020 at 8:50 pm
Good Job there! You are an expert in electronic service and only professional work can come out from you! Thanks for sharing!
suranga bandana
November 29, 2020 at 10:06 pm
Thanks ..!
Mr Parasuraman.
nader bahrami
November 29, 2020 at 10:58 pm
Excellent expert colleague.
This is the best way to troubleshoot
Backlight voltage is generally present and the backlights are damaged.
Thanks for sharing your helpful experience
Doğan
December 16, 2020 at 5:30 pm
You are a very good mechanic. You are telling very well. Greetings from Turkey...