LG 43LH510V Backlight problem

[unimatrix93]

Hello everyone,

I'm trying to fix a small LG TV with wierd issue.

So I check that the display is OK by fashing a lamp at the screen. LCD is okay OSD is showing up.
Now since the BL is dark, I disconneted the BL from the mainboard and I injected voltage to it with my blacklight tester.
The BL turns on, the tester shows around 80V. So the led strips are okay it seems.

Now I started to suspect the BL driver IC which is a MP3378.
I have a donor board (dead cpu) with the same IC, so I swapped the driver IC but it did not solve the issue.

Some measurements:
I read around 18.86V at the black light connector. With the BL connected and disconnected too.

I checked the surrounding parts around the driver but I cannot find anything out of ordinary

What do you guys think?😊

(Ignore the red and blue wire at the bottom of the left picture, its my benchtop PSU.)

[the Benninger]

You could discard the source by jumping on it with a resistor, if it works you could discard the main board , I once saw it but I couldn't get information I'm sorry I hope this video takes you to the right path : https://www.youtube.com/watch?v=6hQOiN4-XPk

[Diah]

check pin 16 on the MP3378 if there are <5V

[yokoono]

Even if the LEDs light up with the tester, you have faulty LEDs. Replace the strips.

[sickboy]

Even if the LEDs light up with the tester, you have faulty LEDs. Replace the strips.

Agreed. The tester only supplies a limited current into the backlights. Some backlights will only fail under full current from their proper LED driver circuit.
You could also check for a voltage spike at the LED output plug on the power supply as you turn the TV on.

[EazyBone]

I have yet to find a set of LEDs that my led tester said were good and then turn on the TV and the LEDs don't work. Always been another problem not the LEDs. Only repaired about 100 sets of LEDs though.

What is the legend for PSU to main board? There is a BL signal pin sometimes that needs voltage in order for power board to turn on BLs

[Diah]

I have yet to find a set of LEDs that my led tester said were good and then turn on the TV and the LEDs don't work. Always been another problem not the LEDs. Only repaired about 100 sets of LEDs though.

What is the legend for PSU to main board? There is a BL signal pin sometimes that needs voltage in order for power board to turn on BLs

this set doesn't have any power boards , it use external adapter AC-DC 19V. the signal you are referring go direct to the IC MP3378 , which it build in step-up circuit to feed LEDs, the MP3378 also have inside VOP to monitor the currents on LEDs if there are short or drop down then it will switch the step-up circuit off.

[EazyBone]

this set doesn't have any power boards , it use external adapter AC-DC 19V. the signal you are referring go direct to the IC MP3378 , which it build in step-up circuit to feed LEDs, the MP3378 also have inside VOP to monitor the currents on LEDs if there are short or drop down then it will switch the step-up circuit off.

Word.

[unimatrix93]

check pin 16 on the MP3378 if there are <5V

i measure 12v on pin 16.

[Diah]

i measure 12v on pin 16.

12V !!!!! the IC operate under 5V so the specification said... however LG use LED with zener protect diode build in.... you need to take the screen apart and test LEDs one by one with diode mode..

[lotas]

12V !!!!! the IC operate under 5V .

Everything is correct on 16 pin and should be 12v (output power on 16 pin is set by a resistor divider).

[EazyBone]

Everything is correct on 16 pin and should be 12v (output power on 16 pin is set by a resistor divider).

Where do you get these schematics from?

[lotas]

Where do you get these schematics from?

From the Internet and repair forums.

[EazyBone]

From the Internet and repair forums.

Well that narrows it down lol

[unimatrix93]

Turns out you guys were right about the backlight.

I ditched my BL tester and used my bech powersupply with 20V/0.60A.

Out of the three LED stripes the middle one had a faulty LED which only showed it's fault with the lab PSU, not with the tester.

Now I fixed the BL, but I wonder if I could increase the longevity of the LEDs bz decreaseing the current it supplyied by the TV board.

I'm not sure about which resistors should I remove. I cannot measure voltage on R525/8/6/4, so i don't think they are part of the backlight circuit.
What do you guys think?

[lotas]

Increase the resistance of R508 a little.

[unimatrix93]

Increase the resistance of R508 a little.

Thank you,

I replaced R508 (5.6kohm) with a 8kOhm resistor.
Current draw dropped from around 900mA to 700mA.