LG 42LE4500 PSU Replaced Backlight Failing Again

[Repair_dane]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

Right thanks everyone. I'll see what he wants to do and if he just replaces the tv I'll buy it off him and repair it myself. Probably was caused by the leds being driven too hard in the first place as you say and the firmware was probably never updated on the tv either since he got it. I think the backlight was always on 100 too but it had to be the first time it was failing otherwise it kept flickering between dim and full brightness even though it was down which I suspect was the uneven load on the driver board. It's still going to cost over 100 to fix it but it's still about a third of the price of buying an equivalent TV or actually less considering an LG like that would probably cost near 500 still.

If you don't have an LED tester, you can try and connect some 9 volt batteries in series. Assume every LED needs 3V. Sum up the numbers and connect the amount of batteries you need. Then use it as a tester, but be careful, since you are building up some voltage even though it is only batteries

[Repair_dane]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

Some stuff is slow, other stuff is arriving in 2 weeks.

True. But covid-19 is definately causing some delays

[nomoresonys]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

Have yet to see anything get to the U.S. in less than 2 months from china.

[JonathanAnon]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

Took out the LED strips. Took a long time to dig them out.



4 x strips with 52 LEDs, and each LED is ~.720V drop.
So, 52 x 0.720 = 37.44V for one strip, and I'm assuming the 65.9V output from the driver powers two of the strips in series.

I measured all the LEDs individually, none are shorted, but some have a >.8V drop.

[budm]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

0.720v drops, that cannot be right at all. How did you go about testing the Vf of each LED?

[R_J]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

I suspect each strip is divided in two, so 26 led's X 3V 78volts, the power supply outputs 89V/0.96A, and the control circuit likely accounts for the extra 11v. I believe there are 8 led control lines on this led driver board.

[JonathanAnon]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again



These are so small I couldn't even see the + and - on the two pins on the left. I measured the pins in diode mode and the multimeter is seeing a diode, the cathode of that diode being on the + pin.

My multimeter will not measure a 3V diode, it only goes up to 1V I think. I have a power supply here that I can set to 3V and limit the current to 10ma, so I could test the diodes by putting the pins across the + and -

[JonathanAnon]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

I forgot I had one of these.



Tested all the LEDs individually and they all light up. So maybe it is the driver board.

[dick_barton]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

What is it? Battery holder and leads but what voltage out?

[JonathanAnon]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

What is it? Battery holder and leads but what voltage out?

2 x 1.5V AAA batteries in series = 3V
I went through all 52 LEDs on all four strips and they all light up.

[JonathanAnon]

Re: LG 42LE4500 PSU Replaced Backlight Failing Again

Well things have just got interesting. I did what I should have done in the first place, pulled out the LED power cables and wired it up on the desk. When I power on the TV, I see the following before the LED strips shut down completely



So I said "okay I DO have a dodgy strip". BUT, when I swop the four backlight cables, the fault follows the cable. It seems very unlikely that the backlight cable has been damaged but I will try and somehow work out some replacement wires for testing.