LG 43UM6910PUA backlight on, but no picture

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Hello,

An LG 43UM6910PUA was donated to me. The backlights turn on and the voltages on the power supply are good. The mainboard appears to leave standby but the LCD stays off.

Disconnecting the right-side LVDS ribbon cable turns one half of the LCD black. Now if I measure voltage at the AVDD test pad on the connected left driver board, I get 17V.

If I re-connect the right LVDS cable, then the AVDD test pad on both of the driver boards goes to 0V. If I only connect the right-side LVDS cable, then the right driver board's AVDD test pad is still 0V.

With my + probe on the LOCK test pad (the only pad above the LVDS connector in the blurry pic [sorry]) on the faulty right-side driver board, I can see the voltage jump to ~3V and then AVDD goes to 0V. The ~3V is only an estimation since my multimeter isn't fast enough to measure it.

I tried taping off the CLK pins, they're the left-most 4 pins on the LVDS connector (again in the blurry pic [sorry]) but no difference.

Would appreciate any further guidance to dig deeper in this.

Thank you!

[dskall]

Re: LG 43UM6910PUA backlight on, but no picture

Check caps on right side for shorts.

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Re: LG 43UM6910PUA backlight on, but no picture

I checked all the capacitors and no shorts. All of the resistors appear fine too.

I checked for shorts at the test points as well and those appeared fine too.

[dskall]

Re: LG 43UM6910PUA backlight on, but no picture

Do you lose any other voltages?

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Re: LG 43UM6910PUA backlight on, but no picture

Yeah, all of the voltages on the test pads of this suspicious driver board go to 0V after power on.

The other driver board (with the suspicious one disconnected) have voltages present on ODD/EVEN/STV/VSS/CLK1..6.

[lotas]

Re: LG 43UM6910PUA backlight on, but no picture

Check the resistance at these test points (CLK...) against ground (ground) on a non-working bar, they should not be low and short circuit.
If there is low resistance on any of these lines, then the controller on the T-con blocks the power to the panel strips.

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Re: LG 43UM6910PUA backlight on, but no picture

Thanks, here are the resistances:

CLK1..6 = 14.7MOhm
VSS = 1kOhm
STV = 17MOhm
EVEN = 14.7MOhm
ODD = 14.7MOhm

The resistances on both boards are the same

[dskall]

Re: LG 43UM6910PUA backlight on, but no picture

Is that with ribbons hooked up?
VGH, VGL, VCOM voltage?

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Re: LG 43UM6910PUA backlight on, but no picture

My apologies, I tested the resistance with the ribbon cables attached. With the cables disconnected, I get OL on both good and suspect driver boards with my ground attached to the respective driver board.

With only the GOOD driver board connected these are its voltages when the backlight turns on:

AVDD 17V
VCOM 5.87V
HAVDD 7.85V
VCC 1.83V

For the suspect driver board voltages, they're all 0V when the backlight turns on.

Also, I don't see VGH or VGL test pads... are they aliased?

[dskall]

Re: LG 43UM6910PUA backlight on, but no picture

One IC produces most of these voltages. Have to think one of the outputs is being loaded down. VBH, VGL, AVDD, HAVDD could have a short.
Would need better pictures to find test points.

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Re: LG 43UM6910PUA backlight on, but no picture

Here are some closer shots of the driver board.

The LVDS cables are coming from the main board as opposed to a separate TCON board -- there's a photo of that set up in my first post.

Thanks everyone for the help so far!

[dskall]

Re: LG 43UM6910PUA backlight on, but no picture

I don't see the area on your blurred picture on bottom set of pictures.

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Re: LG 43UM6910PUA backlight on, but no picture

Hmmm. Which blurred area you referring to?

The pictures are of the driver board PCB so it's narrow -- anything blurred is in the background. The blue/cyan is just a towel on my table.

Apologies if I misunderstood!

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Re: LG 43UM6910PUA backlight on, but no picture

Here are two more pictures with the ICs on the LCD PCBs. Please let me know if you need more!

[dskall]

Re: LG 43UM6910PUA backlight on, but no picture

Third picture at top. What picture would that be on bottom pictures?

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Re: LG 43UM6910PUA backlight on, but no picture

Ahhh, I see -- the third picture in the top is actually a picture of the GOOD driver board. The good board and suspect board are mirror images of each other.

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Re: LG 43UM6910PUA backlight on, but no picture

FYI I taped up about 15 pins and got a semi-stable picture. I reduced the number of taped pins by 1 repeatedly until I got a picture that looked good to me. At the end of the procedure, I had stopped at about 11 pins.

Not quite the fix that I was hoping for, but spending hours beeping out caps and resistors, and measuring resistance, and then comparing to the other driver board led to my wits end when everything matched up.

Good enough for a garage TV!

Thanks for the help everyone!

[nomoresonys]

Re: LG 43UM6910PUA backlight on, but no picture

Nice work, getting that crap just right can be a real pain.

[nomoresonys]

Re: LG 43UM6910PUA backlight on, but no picture

Good video there: https://www.youtube.com/watch?v=pfrRhw7-ZII

[ReeceyBurger123]

Re: LG 43UM6910PUA backlight on, but no picture

Connect to the hdmi and see if you can move the tv to that input and play sound, this is my method for ruling out main boards these days.