LG TV sound no picture, boards replaced

[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

[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

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.

[nomoresonys]

Re: LG TV sound no picture, boards replaced

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

[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.

[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.

[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

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.

[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

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

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

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

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?

[budm]

Re: LG TV sound no picture, boards replaced

I re-read your post #10: When TV is turned on, "LG" flashes for a split second. That mean the backlights did flash on?
In your post #13: You did test the on the LED1+ and LED2+; you did leave balck probe on the chassis and Red probe on LED1 +and then turn on the TV, correct? The you did the same on the LED2+, correct? But you did not do two more test on the LED1-, or the LED2 - as requested with black probe still on the chassis.
My post #12: 4) You need to leave black probe on the chassis, keep red probe on each LED pin one at time then turn on the TV to see how high the Voltage jump up too and settle down to, repeat that on the other 3 LED connector pins. 4 tests total.

[voltman]

Re: LG TV sound no picture, boards replaced

New board came from ShopJimmy, I have soldering tools but if board is defective, shopjimmy will and should replace it. Voltage at main filter caps with TV on: 373V each

[budm]

Re: LG TV sound no picture, boards replaced

Main filter caps are the long brown caps rated 450VDC laying on the side.
Pictures of the new board? and where did you get the new board from?
You will be working in the deadly side so do not touch anything with your hands.
Do you have tools for soldering works?

[voltman]

Re: LG TV sound no picture, boards replaced

Please define or show me where or what main filter caps are. I am using a cheap craftsman digital multi meter, I will try to get something better to retest with.

[budm]

Re: LG TV sound no picture, boards replaced

If you look at the Voltage chart printed on the board, the LED power supply is rated at 58Vdc with current capacity of 550mA, but are only getting 20V without LED connector in place, so your readings do not look right.
What DCV do you have when measure right between the two legs of the main filter caps when TV is turned on?

[voltman]

Re: LG TV sound no picture, boards replaced

Pics of old board show something burned up. Board is coated with black residue from whatever burned up. You can see top of back panel in circled area where the smoke or residue went up. The 3 capacitors circled were coated with residue but appear normal. I do not know proper way to test them. I only found one component, I dont know what it is on the upper right corner of the board, it is blue in color. Either it burned up or the thing next to it (to my uneducated eyes looks like a transformer?) got really hot as one side of the blue thing is coated in thick black residue. The board at this area (upper right corner) also shows signs of heat and is stained.

The back side of this board shows black residue (from heat?) in the middle, but otherwise looks ok. To be clear both this board and the main board have been replaced. My assistant is taking notes lol

[voltman]

Re: LG TV sound no picture, boards replaced

1)

1) You can upload the pictures here, no need for hosting site.
2) Is this your board? https://www.shopjimmy.com/lg-cov3444...iver-board.htm
3) You have two LED strings, LED1 + (Anode) , LED1 - (Cathode), LED2 +, LED2- each pin of the LED connector is labeled, so your report does not mean much without the pin names.
4) You need to leave black probe on the chassis, keep red probe on each LED pin one at time then turn on the TV to see how high the Voltage jump up too and settle down to, repeat that on the other 3 LED connector pins.

1. Thanks! pics coming shortly
2.Yes. That is my power board
3. LED1+, LED2+ I corrected my posts
4. This is how I did it

Six pin connection from power supply board to LED's shows 20.5V at LED1+ when turned on then quickly drops to .02V and 19.5V at LED2+ when turned on then quickly drops to .03V. With six pin LED connector disconnected at LED1+ when turned on, shows 20.3V then quickly drops to .0V. LED2+ when turned on, shows 15.6V then quickly drops to .0V. The drop happens so fast it is hard to see on the meter.

Do you mean repeat the testing using the negative contacts on the plug instead of a chasis ground?


Thank you budm for your help