LG(50LB6000) TV No Picture/Audio, Backlights Lit

[Chee]

Hello Everyone,

I bought a 50" LG TV in July of 2014 model 50LB6000. About 2 weeks ago, I came home to find that the TV is no longer functional. The backlight will turn on when the power is connected, but neither the remote control or manual power button on the TV seem to have any response. The red LED light at the bottom is on. I have already tried replacing the entire power board, but the TV remains stuck in this backlight on state. I have tried both HDMI inputs and there is no response. I have also checked for visible damage(bulged caps, burnt IC's, broken connector pins, etc), but I do not see anything readily apparent.

My first guess is that the main board has an issue, but I'm not sure how to isolate the issue to either the main board or t-con board. I would like to isolate the issue to a specific board before I go about ordering a replacement. I have access to a multimeter(at home), scopes and soldering stations(at work if needed).

Ironically, my profession is an Electrical Test Engineer, but I can't actually isolate the issue much without any schematics of the boards themselves haha.

Regards,
Chee

[budm]

Re: LG(50LB6000) TV No Picture/Audio, Backlights Lit

If you do not have control over the TV using the remote or the TV buttons on the TV then more likly to be main board problem, at this points it looks like the PS-ON, and BL-ON commands are stuck on that is why the backlights are on which also means the power supply and the backlights circuits are OK.
You can try disconnecting the cable between the T-CON and the main board to see if you will have control on the TV again.
You can also try heating and cooling the main board, push the main board using plastic stick, tighten all the boards mounting screws.
Check the Voltage regulator ICs also.

[CapLeaker]

Re: LG(50LB6000) TV No Picture/Audio, Backlights Lit

Can we see high resolution, straight shot pictures of your boards?

[Chee]

Re: LG(50LB6000) TV No Picture/Audio, Backlights Lit

Thanks for your quick responses. I have attached some photos of the boards. I tried disconnecting the ribbon cable from the main board to t-con board, but the TV is still stuck in the same state. I have heard that heating the board up could fix some bad solder joints, but I was a little bit wary about sticking the board in an oven. I figured the plastic connector sockets would melt.

I will mention I did try heating up the main BGA chip on the main board with a heat gun, but that didn't seem to do anything.

Any schematics on where the regulators are and what voltages I should be seeing?

Thanks again,
Chee

[CapLeaker]

Re: LG(50LB6000) TV No Picture/Audio, Backlights Lit

In order for the TV to turn the backlights on without the main board attached, the PSU must have a pull up resistor.

On the first picture on the bottom there is a plug with wires going to the main board. We want you to measure the DC V of each pin (main board connected of course), and post the results with the corresponding pin out name.

5V STBY is always on, regardless the state of the TV. That will feed the main CPU and some circuits on the main board. When you turn the TV on, the main board sends a PSon and BLon back to the PSU (all voltage rails should be up and the backlights on).

In your case the LED is on, but it is not responsive to the remote or the buttons on the TV itself, I'd say your main IC (XD chip) on the main board is bad. It probably got hit by lightning through the HDMI.

[Chee]

Re: LG(50LB6000) TV No Picture/Audio, Backlights Lit

So when the main board is connected to the power supply these are the pinout voltages I read:

Pin1: GND->GND
Pin2: 12V->12V
Pin3: 12V->12V
Pin4: GND->GND
Pin5: 24V->24V
Pin6: GND->GND
Pin7: 3.5V->3.5V
Pin8: 3.5V->3.5V
Pin9: PWR-ON->3.5V
Pin10: GND->GND
Pin11: N.C->GND
Pin12: 12V->12V
Pin13: GND->GND
Pin14: 24V->24V
Pin15: P-DIM2->3.5V
Pin16: 3.5V->3.5V
Pin17: P-DIM1->0V
Pin18: DRV-ON->3.5V

[Chee]

Re: LG(50LB6000) TV No Picture/Audio, Backlights Lit

My next plan of action was to try baking the main board, if that doesn't work then order a new main board. But it looks like we are narrowing it down to that board as the issue so that's good news!

[Chee]

Re: LG(50LB6000) TV No Picture/Audio, Backlights Lit

Good news everyone, replacing the main board fixed the tv and it is back up and running. Would have been nice to figure out what was broken specifically, but $40 saved me some headache lol

[CapLeaker]

Re: LG(50LB6000) TV No Picture/Audio, Backlights Lit

I think the main IC is bad. The BGA is hard to replace, unless one has proper tools.
You could see if you find the data sheet of the ic, or a schematic of the main board and start probing