LG 50" 50PK550 - Sound, no image

[itachi]

Hello fine people. I have this TV giving me issues.

Symptoms:
-Powers on with no image but can hear tuner static
-No OSD
-As it powers on, the screen comes on to a light grey for a couple seconds then changes to a faintly noticeable grey. Like the raster is there but no image.


What I've done:
-Verified high voltages. Va 60, Vs 203, Vsc -190, 150, 105
-Verified lower voltages like +5 and +17 from the PSU. 1.8, 3.3, 5 on CTRL board
-Pulled the upper Y-Buffer board out. This makes the top half of the display completely black instead of that faint grey.
-Probed a "waveform" pad on the upper Y buffer board. Waveforms are present
-Removed the CTRL board which caused the panel to be completely dark. Also the waveforms I probed earlier were gone.


I'd like to rule out the PSU as all voltages coming off it seem correct. I want it to be the CTRL board since its cheap and connected to everything but it is sending out signals to the Y board. Maybe this implies the mainboard (input board) is not sending the CTRL board a good video signal? But I want to think the mainboard is working since if there's sound, it must be decoding a sound signal so the main IC would be operational.

So I'm kind of stuck at this point.

[tom66]

Re: LG 50" 50PK550 - Sound, no image

Have you tried test patterns on the control board?

You need to jumper the two points marked EXT_AUTO_GEN and disconnect the LVDS data cable between the main board and control board. Then power the TV up. A series of test patterns should appear:
https://www.youtube.com/watch?v=9LcqLM5TRHk

If so, then only the main board is bad.

Check the test patterns carefully for defects like stuck lines or severe maldischarge.

[itachi]

Re: LG 50" 50PK550 - Sound, no image

Hmm I'm hsving some troubles locating that on the board.

[itachi]

Re: LG 50" 50PK550 - Sound, no image

Oh wait it's not labeled but based on other variants of this board I'm assuming its the vias coming off R51 and R52 in the lower right, top side.

[tvtimmy]

Re: LG 50" 50PK550 - Sound, no image

Yep, those are the ones.

[itachi]

Re: LG 50" 50PK550 - Sound, no image

Hey it works, test patterns look good. Thank you both. So mainboard it is then. It has serial port on it...I want to connect to it and see if its spitting out any info. Also might see if I can possibly troubleshoot it. Would you know of anything to look for on these boards or is it better to just replace it?

[tvtimmy]

Re: LG 50" 50PK550 - Sound, no image

Unless you can re-ball the main bga chip I would try to find another one.

[diif]

Re: LG 50" 50PK550 - Sound, no image

I've got away with just a reflow on these. I've not had one back yet.

[goldbird78]

Re: LG 50" 50PK550 - Sound, no image

If you are sure it is the main board with the HDMI connections you can try the oven trick. I have a 60pk750 and it worked for me. It is a short term fix until you can find a reasonably priced replacement. See link below, bottom of page 2.

https://www.badcaps.net/forum/showth...t=49836&page=2

[itachi]

Re: LG 50" 50PK550 - Sound, no image

That was indeed the fault. Replaced the mainboard and its good to go. Thank you good people.