LG 42LB5610 BackLight problem.

[HellasTechn1]

Good day to all !!!

Dear friends, i am now working on this LG 42LB5610 TV. When powered on everything looks normal but about 15 - 30 minutes later the backlight will shut down. Audio can be heared and the panel will display picture. Only the LED's shut down.
After i turn it off and then back on it does the same thing (works for a while etc...)
I am thinking that the led strips are fine since they do work even for 15-30 minutes.

Could you please provide me with guidelines to troubleshoot this problem like for example what measurements i should take ?

Thank you all so much !

[dick_barton]

Re: LG 42LB5610 BackLight problem.

Do you still have the Backlight On and DIM voltage when the backlights go off?

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

I have not taken any measurements while it is happening. So here is a start

I do need to identify the correct connector and pins for that first.

[dick_barton]

Re: LG 42LB5610 BackLight problem.

The other place to measure when you lose the backlights is at the led connector to see if the voltage has risen above what it says on the power supply circuit board. If that's the case then it would be looking like a led or led string connection problem.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

Ok. i have the readings.

With the TV turned on and while the backlight went off the connector reads the following:

Pin marking - reading

12V - 11.85V
24V - 24.10V
3.5V - 3.51V
PWR-ON - 3.31V
DEV-ON - 3.05V
P-DIM - 2.63V

Then i attached the leads of the dmm set to DCV, to the led power connector and i turned of the TV and back on to try to read the voltage with the leds on but after i turned it off and back on the backlight never came on.
Now it does not turn on at all.

could it be a shorted led situation ?

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

Also i noticed that now that the leds do not come on, the DMM set to DCV will not give me a steady reading but instead it changes rapidly. when i set it to ACV it reads 0.22V .

[dick_barton]

Re: LG 42LB5610 BackLight problem.

With the meter set to DC volts and black lead on the chassis can you measure the voltages on the led connector again.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

Before i read your last post i tried leaving it unpluged for a while and now it came back to the initial problem.

The led connector measures 180V when the leds are on.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

What do you think about the measurements ? PWR-ON and P-DIM look ok to me.
What is DEV-ON ?

[budm]

Re: LG 42LB5610 BackLight problem.

DEV-ON = BL-ON.
You have all the correct control Voltages.
"The led connector measures 180V when the leds are on." So the backlights are on now?

[budm]

Re: LG 42LB5610 BackLight problem.

"I am thinking that the led strips are fine since they do work even for 15-30 minutes. " Turn down the backlights level (not brightness) to as low as you can to see if the backlights will stay on longer.

[tw2005]

Re: LG 42LB5610 BackLight problem.

The chances are the LED strips are failing. Simply they burn out so I suspect as the strip heats up one is shorting. Affects a large % of LG LED TVs around this era of all sizes. I've had them simply not illuminate, illuminate briefly or repeatedly blink. The other issue is the lenses fall off.

They get to about 3 years and die but I've had examples with only 6000hrs and upwards. Most are 8000 - 10000hrs when they die.

Also I've tired using Super Glue (cyanoacrylate) and with the clear lenses the fumes leave a smoky residue affecting the light dispersion which is noticeable.

[tw2005]

Re: LG 42LB5610 BackLight problem.

These are the style of strips

LC420DUE FG

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

The latest sitrep is that the backlight does not stay on that long. they shut down within 1-2 minutes. i will try changing brightness to see.

Assuming that the psu is good and some led overheats and shorts, then how will i be able to locate it ?

[freakaftr8]

Re: LG 42LB5610 BackLight problem.

What I do is put on nice dark sunglasses in keeping eye on the led's. What it full capacity one may flicker meaning it's about to die out or a bad SMD soldering job. K this will throw power supply into a tizzy And eventually will shut down because of it.

Also you could have an overheating LED control circuits on the power supply

[budm]

Re: LG 42LB5610 BackLight problem.

"The latest sitrep is that the backlight does not stay on that long. they shut down within 1-2 minutes" Do you have video source connected? Did the TV actually go into standby after 2 minutes due to no video present?

[freakaftr8]

Re: LG 42LB5610 BackLight problem.

This is why I never throw the screen and diffusers back in the TV until I let the LED strips run and burn in for about 2 hours. I've made that mistake before.

This allows me to check heat dissipations and any occurrences that may happen before the panel goes back in and TV goes back in operation

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

"The latest sitrep is that the backlight does not stay on that long. they shut down within 1-2 minutes" Do you have video source connected? Did the TV actually go into standby after 2 minutes due to no video present?

Yes video source is on (aerial antenna) and the tv will not go to standby after the backlight turns of (audio and controls work normally).

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

What I do is put on nice dark sunglasses in keeping eye on the led's. What it full capacity one may flicker meaning it's about to die out or a bad SMD soldering job. K this will throw power supply into a tizzy And eventually will shut down because of it.

Also you could have an overheating LED control circuits on the power supply

sounds like a good practice.

[HellasTechn1]

Re: LG 42LB5610 BackLight problem.

i will try dimming the leds as much as possible like budm suggested after i get the remote control. For now i have tested the voltage to leds from power up untill they turn off.

Voltage at powerup went from 0V to 180V stays like that for about 30 seconts and then slowly droped to 175V - 173V untill it reached 155 volts where it spiked to 180V and then went 0V and leds turned off.

I think, since its a constant current source then i guess the voltage drop is probably to compensate for exess current draw, untill it reaches the lowest possible voltage (or highest possible current) and then shuts down.

Any thoughts ?