LG 49UH668V Blank screen

[Runlinc]

Tv backlights not coming on:-
Power on - Standby red light on (red)
Press start button - Red standby light flashes 3 times then goes off. No backlights on at anytime during start up - no picture on screen with backlights off. I have checked all backlights in situ using 6v supply across test points on backlight strip, all light up.

Power Supply voltages at Led plug and P201 plug when connected to main board.

Stand by - Led + 59.8v P201 — MS 0v P-Dim1 1.9v
Led - 40.6v. GND 0v GND. 0v
Led + 59.8v (13.2v) 7.8v (13.2v) 7.8v
Led - 41.6v (13.2v) 7.8v (13.2v) 7.8v
GND 0v (13.2v) 7.8v
PWR-ON 0v P-Dim2 1.9v

Power on Led + 87.3v P201 — MS 0v P-Dim1 0v
Led - 66.9v GND 0v GND 0v
Led + 87.3v (13.2v) 13.1v (13.2v) 13.1v
Led - 70.0v (13.2v) 13.1v (13.2v) 13.1v
GND 0v (13.2v) 13.1v
PWR-ON 3.3v P-Dim2 0v

The voltage rise at the Led is smooth and stable. I cannot see anything obviously wrong on the boards. Please see attached photos.

[alfatv]

Re: LG 49UH668V Blank screen

You have to explain better what those numbers mean.
And why GND is not always zero, where do you connect negative terminal of multimeter?

[Runlinc]

Re: LG 49UH668V Blank screen

Thanks for your reply.
When I have uploaded it has condensed all the figures together without any spacing so I will try again.
All readings are with negative probe ground on the chassis.

I have annotated the photos with the voltage readings at Standby and after switching the set on. Hope this is ok.

[dick_barton]

Re: LG 49UH668V Blank screen

I'm not sure what MS is but it could be the backlight control signal input.
Do the backlights come on if you disconnect the mainboard and power up?

[Runlinc]

Re: LG 49UH668V Blank screen

I cannot find out what MS is either.
If I disconnect the mainboard and just run the power supply the backlights don’t come on.
With the main board connected there is 3.3v at PWR-ON.
I checked the voltage at the LED plug on the PS board during switching on and there is no voltage surge, only a stable rise to 87v

[dick_barton]

Re: LG 49UH668V Blank screen

What is the power supply board numbers?

Edit.
I've just noticed the numbers are available from your photos in post #1

I believe the MS is the backlight on signal but I would wait to see what others think.

[Diah]

Re: LG 49UH668V Blank screen

yours photo https://www.badcaps.net/forum/attach...7&d=1618309849
you have 2 groups LED backlights, each groups had same amount of LED units, LED- as your measurements at one groups had different of (-4V = 1 LED units defect / short / Open currents forward ziner diode )

the way you tested LED BL are wrong. because each LED units had build in ziner diode for currents forward, which only with Digital meter Diode option to exam it.

[Runlinc]

Re: LG 49UH668V Blank screen

Thank you for your reply.
How would I be able to isolate which led's are failing with a multimeter ?

[Diah]

Re: LG 49UH668V Blank screen

1- if you take the screen out, you still can power on the TV and you will see at start very dim fast the led will come on.. you may see which on will be off or...
2- with digital meter at diode mode. each led units if you test it will lit on dim and if you reverse the probe - + you will have ziner diode reading between 0.76 - 0.54 . open ziner will read nothings. short ziner will you have 0 with beep tone.

[alfatv]

Re: LG 49UH668V Blank screen

Is backlight flashing at all during power-up?
You may need to unplug TV for 1 hour or so first.
And measure the voltages between LED+ and LED- while powering on. See if there is voltage spike.

[Runlinc]

Re: LG 49UH668V Blank screen

Took out the screen & diffusers - power on tv - no backlights flashed or lit up.
Isolated each Led rail.
Multimeter on diode setting - 17 of the 32 led's lit up, multimeter reading 1
The 15 Led's that didn't light up had multimeter readings of between .2 & 1.8 and the same readings reversing the probes.
Measuring the test points at the end of the strips the reading was .725 on all 4 strips.
Hope this makes sense.
Thanks for all your help.

[nomoresonys]

Re: LG 49UH668V Blank screen

Pretty sure that's bad backlights, best policy is to replace all strips.

[Runlinc]

Re: LG 49UH668V Blank screen

Changing all the backlights is not an economically viable for me I'm afraid.
Is there any way to replace individual LED's as these appear much smaller than any others I have seen. The light strips are 6916L-2453A
I would like proof the other parts of the set are ok, before I buy a full set of backlights
Many thanks for your help

[Diah]

Re: LG 49UH668V Blank screen

you are still have error / wrong measurements. this cant be 15 LEDs get defect at once.

upload photo on the whole back of the set showing all boards how they connected together.

upload high resolution photos on the strips close to the sockets and to one of the LED to see the printed line on the strings how many you have.

[neilc6]

Re: LG 49UH668V Blank screen

Do you have 4 rows of strips? These are the style that also turn blue if you run them at max and super tiny. They are only $20 - $30 USD on Aliexpress and you may find similar pricing on ebay.

If backlights don't come on or flash, they are your first suspect.

[Runlinc]

Re: LG 49UH668V Blank screen

Photos attached as requested - LED's are 1mm square and show a blue light.
4 rows of 8 led's.
Hope photos are ok

[Diah]

Re: LG 49UH668V Blank screen

i can see you have 1 LED unit remarked with X didn't work..!!! i would suggest you to follow neilc6 found on aliexpress to replace all at once.

[Runlinc]

Re: LG 49UH668V Blank screen

Aliexpress has an 88% bad rating on Trustpilot in the UK !!!!!!!!

[Diah]

Re: LG 49UH668V Blank screen

Aliexpress has an 88% bad rating on Trustpilot in the UK !!!!!!!!

all what you find on Ebay are bought via Aliexpress / Alibaba then sold in high price... only Chinese produce them, if you still want to think on this negative rate then use candle without smoke 2 will be enough behind the screen and enjoy watching TV ..LOL

[nomoresonys]

Re: LG 49UH668V Blank screen

^^^^