LG47LN5700 With no image

[davefigueroa]

Hello!:
My name is Dave Figueroa, i am from Santiago de Chile (Por si alguien habla en español...)
Well, my problem is the following:
Someday, I turned on my TV and I did not get any pictures, but I had sound.
I started to search in google this problem and I arrived in a "X" forum saying that the LED's are broken... and I was like this
... BUT...
I visited a friend that helped me, he changed the broken LED's. I was very happy in that moment, but when I turned on the TV I did not have image. (All this happened in February)
The last weekend, I changed the BD9483 and the 2 x 4NF20L IC's and nothing happened
Yesterday, I started to review the TV and I found a new broken LED. I changed that but there still no picture.
The TV, have 6 LED rails. When I turn on the TV the LED's blinks and I view the following:
First Rail-------------------------- ON
Scn Rail-------------------------- OFF \
Thrd Rail-------------------------- OFF } Light up little
Frth Rail-------------------------- OFF /
Fifh Rail-------------------------- ON
Sith Rail-------------------------- ON
I started to measure the LED OUTPUT RAILS of the PSU and I got the following:
I put the positive cable of the multimeter in the channels and the negative cable in GND, with no LED's connected in the PSU

• CH1 + and GND = 163V to 77V
• CH1 - and GND = 28V
• CH2 + and GND 163V to 77V
• CH2 - and GND = 28V

I put the positive cable of the multimeter in the channels and the negative cable in GND with LED's connected in the PSU

• CH1 + and GND = 78V
• CH1 - and GND = 1V
• CH2 + and GND 78V
• CH2 - and GND = 1V

The PSU Model is: EAX64905501
What can I do in this case?
Maybe is a broken capacitor? (there is 4 x 33uF 200V capacitors near of the LED outputs and there is not continuity through them)
PLEASE HELP ME!
Thanks in advance

[tmwalsh]

Re: LG47LN5700 With no image

daveFiguera:I put the positive cable of the multimeter in the channels and the negative cable in GND, with no LED's connected in the PSU

CH1 + and GND = 163V to 77V
CH1 - and GND = 28V
CH2 + and GND 163V to 77V
CH2 - and GND = 28V

I put the positive cable of the multimeter in the channels and the negative cable in GND with LED's connected in the PSU

CH1 + and GND = 78V
CH1 - and GND = 1V
CH2 + and GND 78V
CH2 - and GND = 1V


If you monitor ch1 + & ch2 + when you apply power, if you note a peak voltage then a slow drift back to the 78V noted, I that is the common symptom of failing LEDs. The circuit monitors current, I think, and upon sensing over-current, it shuts down. In most cases you'll get a momentary flash of light and then darkness.
tom

[budm]

Re: LG47LN5700 With no image

You have open circuit is the LED string.

[tvsteve2]

Re: LG47LN5700 With no image

The LED's fail on this model Shopjimmy sells complete sets of LEDs to keep them from failing LG has a firmware upgrade the MUST be done that limits the maximum current to the backlights! also you should turn down all the customer backlight settings in the menu to about 60-80% of max (at max the LEDs overheat!)

[tvsteve2]

Re: LG47LN5700 With no image

They seem out of stock at Shopjimmy but LG has them here is Shopjimmys listing:
LG LC470DUE-SFU1 Replacement L.E.D. Strips - 12 Strips (SEE NOTES)


$64.95

Strips manufactured to replace originals used in LG LED panel LC470DUE-SFU1.

PLEASE NOTE: Most LED TV models use a couple of different LED strip variations. Please confirm that your TV matches one of the TV version/serial number combinations shown before ordering.

IMPORTANT: The LED strips you receive will have either printed part numbers 6916L-1259A, 6916L-1260A, 6916L-1261A and 6916L-1262A or 6916L-1174A, 6916L-1175A, 6916L-1176A and 6916L-1177A. These two sets are completely interchangeable.

IMPORTANT: If you are replacing LEDs in an LG TV, it is recommended that you install the latest TV software after installation to minimize the risk of repeat LED failure. On Smart TV models, please update software from the Setup menu. For non-Smart TV models, please visit LG's TV support website for update instructions. Turning down the "backlight" or "brightness" setting on your TV may also help to prolong LED life.

[Trevtron]

Re: LG47LN5700 With no image

Just found this old thread.
I have a similar set (LN575S) with similar symptoms (brief flash of backlight on start-up), but I don't see any image using the 'flashlight' method, so it's kind of hard to know if the set is indeed running. I also noticed an even shorter flash of the backlight on switch-off (to standby).
I do get the 100+ volts peak on both LED+ o/p pins on switch-on, dropping slowly to around 75V.
I would like to connect dummy loads to the 2 LED outputs to confirm one way or the other whether it's LED failure or PSU. Any idea as to what resistance would be required?

[Trevtron]

Re: LG47LN5700 With no image

OK, quick update: Image IS present using 'flashlight method'. I even managed to see enough to select an input & confirm (with audio) that the set is indeed running.
A more careful check revealed the two L+/ L- readings are different so I think I can safely assume it's (another!) LED issue.
Disconnecting one string then the other, I was able to establish which string was giving the flash.
As there was nothing to lose, I tried powering up the LED's from an external power supply, well two in series actually, as each only goes up to 31V! One 'string' drew 500mA at 30V with no output, the other string didn't draw any current at all even at 62V. That one is the string that gives the flash so I guess it wants a bit more juice.
I will continue to play with it.

Edit: Got the good string to fire up at 70V, 80V gives a current draw of 100mA. TV running with 3 external supplies driving the backlight. Very noticeable dark band from about 1/5 to 3/5 down the screen.
Firmware was out-of-date (04.24.08) current version is 04.28.14. I gather from other threads that the firmware was revised to reduce the strain on the LED's so I guess it was too late for this one.