LG TV sound no picture, boards replaced

[budm]

Re: LG TV sound no picture, boards replaced

Correct and correct. I assume at least some of the LED's must flash or I would not see the "LG" this occurs only for a split second when turning TV on. It is not very bright but again, I can see the "LG" for a split second.

I will test with negative probe on chasis ground and positive probe to LED1- and LED2- turning TV on after probes are in place. Will try to get this tonight if not by morning. Thank you again for all of the help.

Does the 373V at the main filter caps sound normal?

That means the LED+'s must have gone up more than 20V but your meter may be too slow to catch the peak reading.
The LED1-, 2- readings may give a clue.
The PFC Boosted Voltage, 375VDC, is fine.

[voltman]

Re: LG TV sound no picture, boards replaced

These tests done with six pin connector leads to LED's disconnected, negative probe on chassis ground. Probes in place before turning TV on. Then turning TV on for reading.

LED1- 0V
LED2- 0V
same results when repeating test with six pin connector connected

These tests done with six pin connector leads to LED's disconnected, negative probe on chassis ground. Probes in place before turning TV on. Then turning TV on for reading.

LED1+ 41.2V dropping to 0V
LED2+ 7.9V dropping to 0V






the voltage when TV is turned on ramps up so fast and drops off almost instantaneous. It does not stay on the meter long enough to read. I have to video record the meter and play it back on slow to see the reading. I think it may be too fast for my meter to register it accurately. Every time I do this test I get different results on both LED1+ and LED2+ the 7.9V above is the lowest I have seen while the 41.2V is the highest. I have seen voltages of 41.2V, 36V 15.V 20.3V, 26V before dropping to 0V all with the six pin connector disconnected.

These tests done with six pin connector leads to LED's connected, negative probe on chassis ground. Probes in place before turning TV on. Then turning TV on for reading.

LED1+ 45.1V dropping to 0V
LED2+ 27.9V dropping to 0V

lol this is funny, I typed the above about my meter not being fast enough before your post came through and you said my meter may be too slow to catch the peak readings. I believe this is the case.

[budm]

Re: LG TV sound no picture, boards replaced

I expect the LED1- and 2- to show some Voltage when TV is turn on and when the Backlights flash on for seconds, you are getting 0V withe LED's connector in place.
But right now the LED string #2 looks suspicious due to only showing 27.9V, it may mean that LED string #2 may have some shorted out LED's so it loads down the power supply down to 27.9V and causes the protection circuit to kick in.
This is where the LED tester comes in handy.

[voltman]

Re: LG TV sound no picture, boards replaced

My meter may not be detecting the voltage peaks accurately. I am confused as to why LED1- LED2- would show voltage. I thought they were grounds to the LED's?

I do not have a LED tester but I do have an old Knight regulated power supply. Could I use this to LED1+ and LED2+ to confirm back lights?

What do you think about the old board? The heat marks and black residue? Im guessing something went bad and maybe voltage spiked and blew some LED's out?

[budm]

Re: LG TV sound no picture, boards replaced

In the typical LED driver the LED1-, 2- do not go to ground directly, they connect to ground via PWM controlled MOSFETs and current sensing resistors to ground to maintain constant current through the LED strings. LED's are driven by constant current power supply, not by regulated Voltage power supply.
So set your power supply to about 40V for starter and limit the current to about 100 ma (you must limit the current) then connect it to each LED string (connector removed from the board of course) to be tested.
BTW, this TV is re-badge Skyworth TV, selling at higher price because it has LG tag on it.

[jjanssen7]

Re: LG TV sound no picture, boards replaced

From what I see in the first picture posted, it looks like soot from burned LEDS (actual fire soot) has come up the back of the TV and stained everything.

[budm]

Re: LG TV sound no picture, boards replaced

From what I see in the first picture posted, it looks like soot from burned LEDS (actual fire soot) has come up the back of the TV and stained everything.

https://www.badcaps.net/forum/attach...9&d=1569457791
I think you are right! I just look at that picture again and see dark stain far to the right of the LCD panel far away from the board. It does not look good.

[nomoresonys]

Re: LG TV sound no picture, boards replaced

Might be time to dig down to the led strips and see what's going on, if you do be very careful handling the panel, if it breaks game over, check out several youtube videos for how to if needed.

[nomoresonys]

Re: LG TV sound no picture, boards replaced

Not your exact model but example there: https://www.youtube.com/watch?v=s4wNc3hF4JI

[voltman]

Re: LG TV sound no picture, boards replaced

Thanks everyone. I have been trying to learn all I can about removing the screen and trying to do everything possible to not have to do that if not necessary. I am collecting as many videos as I can on pulling the screen. It seems suction cups are the way to go for this?

I know about the thin film and being careful there just not sure yet about pulling the screen out. The flash I get when turning the TV on only lasts about a tenth of a second, just long enough to see the "LG" I thought the black soot was strange being that nothing seems burnt on the boards. I did not know that LEDs can burn up like this.

[neilc6]

Re: LG TV sound no picture, boards replaced

That rebadged LG model may be trickier to remove the panel. The boards flex connected to the panel may be double sided taped to the back chassis and the panel itself could be double sided taped to the bezel. An exacto knife will help separate it. Also lots of screws holding the bezel to the chassis. Just did a 42LN5700 but video recorded the flash with the panel off and on to show you can diagnose it by videoing it and going through the video frame by frame to see which leds are out.

[voltman]

Re: LG TV sound no picture, boards replaced

neilc6, Are you saying its possible to see the LED's that are out with the panel still mounted by video taping the screen? If so, I will do this next.

I seen the screws all at the bottom, did not see any on the sides or top

[voltman]

Re: LG TV sound no picture, boards replaced

Tried video taping the split second flash. It is really fast. The "LG" and "lifes good" (not anymore) does not look right. Is this possibly what only a few working LED's might look like when flashed? These are stills captured at the best frame of video.

[neilc6]

Re: LG TV sound no picture, boards replaced

You need to video the flash with the panel removed. See my blog post here

http://techhometravel.com/lg-ln-seri...ed-backlights/

[nomoresonys]

Re: LG TV sound no picture, boards replaced

I'd strip it down to leds, it's a 43, shouldn't be a problem if you take it slow, it's usually the big screens that break. Being rebadged chinese tho, odds are the bga will go to hell shortly, you may have a money pit there.

[voltman]

Re: LG TV sound no picture, boards replaced

Very informative neilc6, thank you. Nomoresonys, I think you might be right about the money pit. Been watching the videos and feel confident about the tear down now. Question. I know something had to burn up. If the reflector is burnt and or any of the layers of diffusion are burnt or stained, is the set still repairable? Will a slight burn mark effect the picture quality? What is and is not acceptable here? Cannot believe I researched before buying and got stuck with this junk. TV is only 22 months old!

[voltman]

Re: LG TV sound no picture, boards replaced

I have an update on my LG4300

I removed the screen, many thanks to nomoresonys, the video you gave me was very helpful. I was shocked to not find anything burned up. The wiring and all the LED's look pristine. I powered up the set without the screen and the top two rows of LED's are out. So only the bottom 18 are working. Nothing visibly notable about the top 18. Is it common for two whole rows to go out at once? Whatever burned up has to be on the power board or main board. Both have been replaced

I understand that when any LED's burn out the power will shut down and you get nothing or rather no picture just sound. Powering up the set without the screen per neilc6 suggestion has shown me which ones are out. I intend to test the bad strips to confirm. Your thoughts, suggestions, and how to procede from here would be much appreciated.

[neilc6]

Re: LG TV sound no picture, boards replaced

You have an open LED in the top half causing the whole string not to light up. Tougher to isolate but you can use a multimeter in diode mode and check every LED or swap each non lit strip into the working string. Can replace only the bad strip but all the LEDs have wear and could fail in the future. You need to turn down the backlight level on all inputs and all smart apps after repair. Top and bottom strings are probably wired separately so the backlights flashed from only the bottom string. I think I have a full set of strips for this model (from new cracked screen tv) for this model on ebay (check the part number printed on the strip).

[nomoresonys]

Re: LG TV sound no picture, boards replaced

Set there for 29 bucks, see if that's a match: https://www.ebay.com/itm/LG-LED-Stri....c100010.m2109

[dick_barton]

Re: LG TV sound no picture, boards replaced

I was shocked to not find anything burned up. The wiring and all the LED's look pristine.

But something has definately burnt out with all the soot. It's even on the Varistor by the mains input where it says HOT on the upper side of the power supply.