LG 47LN5700 Back light issue

[CANAM13]

Hello all,
I recently repaired this tv for a friend and replaced all the led strips from this unit, which I bought at shop Jimmy. TV fired up, looked great and ran fine for about 45 days.I ran the software update, turned down the backlight settings like recommended. But, the unit is dark with another backlight issue.
I've measured the voltages at the led output plug, i'm getting 78v. when I first turn on the set, voltage is at 130 and then continually decreases till it settles out and stays at 78v.

P/S board output voltages states - 100v row 47
110v row 56

Do I have another LED light/strip issue or should I be checking other voltages from the power supply?
any help would be greatly appreciated!
thanks

[Andrew F. Ali]

Re: LG 47LN5700 Back light issue

Looks like a LED Driver issue.

[budm]

Re: LG 47LN5700 Back light issue

That sounds like you have open circuit in the LED string, the rated Voltage on the board is with loads connected so if there is no load then output will rise.

[gliebisch]

Re: LG 47LN5700 Back light issue

I have the same problem with similar symptoms. 47LN5700. Board is EAX64905501 (2.3). Strips are LC470DUE SFR1. I should ask , though, whether there is any significance to the LED- test point (PS output connector) voltage. My LED+ voltages with or without strips connected rise to 100 volts initially and then bleed back to 78 volts. However, the LED- test points are both zero volts without a load, but with strips connected, ONE of the two LED- test points reads 19 volts and the other reads zero. With strips disconnected, both of the LED- pins read 0 volts. I am trying to determine whether to try replacing the power supply first, or conclude I need LED strips. One theory I have is that the 19 volts is a phantom voltage from the end of the string caused by the LED- terminal not seeing ground (i.e. a fault on the PS board). Right now these strips are unavailable and out of stock from any source I can find, but I can get a PS board.

[Andrew F. Ali]

Re: LG 47LN5700 Back light issue

You have an open LED string or strings....

[budm]

Re: LG 47LN5700 Back light issue

"the LED- test points are both zero volts without a load, but with strips connected, ONE of the two LED- test points reads 19 volts and the other reads zero."
The one that show 0 V on the LED - has open circuit.
L+ is the ANODE the LED string, boosted Voltage is fed to this pin.
L- is the Cathode return of the LED string.

[budm]

Re: LG 47LN5700 Back light issue

LG LN5000 series.
https://www.badcaps.net/forum/showth...ht=LG+47LN5790
19V is not the phantom Voltage, that 19V is the Vdrops of the PWM controlled MOSFET and the current sense resistor to maintain constant current through the LEDs,
The one that reads 0V is due to open circuit so there will be no Voltage on the return line (LED -).