LG 50PA4500 - Sound with no picture

[mincind]

Turned on my two year old LG 50PA4500 unit a few weeks ago and discovered I had no picture, but I still had sound.

In doing some research, the consensus seemed to be that the power board and the buffers were likely not the culprit, but rather the Y-sustain board.

I ordered a Y-sustain board (was supposed new) for $22.00 and installed. I still have no picture - just sound.

I am not overly familiar with electronics (I am a chemical engineer by trade) - and do not own an oscillascope....but I do have a Fluke multimeter and am willing to take any picture or test any hypothesis the experts are willing to offer.

Any ideas on what to do next?

Thanks!

Dante Mincin

[tom66]

Re: LG 50PA4500 - Sound with no picture

In a dark room, can you see a faint glow from the screen at all?

Could you take some pictures of the boards inside the TV - one overall, and then one of every major circuit board.

[mincind]

Re: LG 50PA4500 - Sound with no picture

Tom66,

Thank you for such a timely response.

I put the television in a pitch black area and cannot detect even the faintest of a glow.

Please see photos of all major boards (and entire TV backplane) attached.

[mincind]

Re: LG 50PA4500 - Sound with no picture

By the way....while looking at the pictures, I noticed that the one board (I believe the z-sustain board) was unplugged from some earlier testing.

Plugged back in, but still not the faintest of glow.

[tom66]

Re: LG 50PA4500 - Sound with no picture

Ok, on this board:
https://www.badcaps.net/forum/attach...6&d=1419109570

There are jumper wires marked VS_TP and VA_TP.

Set your multimeter to the DC volts scale. Place the black probe on an exposed part of the chassis, for example the wall mounting brackets. Then touch the red probe against each of VS_TP and VA_TP. What do you measure? (The TV should be plugged in and switched on - i.e. where it would normally have sound.)

Compare these voltages to those printed on your "panel sticker"; it's a sticker with a load of voltages marked "LG Electronics". Middle top of chassis in this picture: https://www.badcaps.net/forum/attach...6&d=1419109570

[mincind]

Re: LG 50PA4500 - Sound with no picture

The VS segment is reading 204 and the spec says 200-206

The VA is reading 53 and the spec says 55

[tom66]

Re: LG 50PA4500 - Sound with no picture

Ok, this board:
https://www.badcaps.net/forum/attach...5&d=1419109549

There are two green resistors, center left near the white connections going to the Y-buffer, marked Vsc and -Vy.

For these, please put your meter probes across the device, and say what you measure on each. Also note if it seems to pulsate or is unstable.

[mincind]

Re: LG 50PA4500 - Sound with no picture

Sorry to be so dumb, but what am I measuring across the resistors - DC voltage as well?

Or am I measuring resistance in ohms or some other unit?

[mincind]

Re: LG 50PA4500 - Sound with no picture

Also - do I test with the TV on or off??

[tom66]

Re: LG 50PA4500 - Sound with no picture

Yes DC voltage, and yes the TV should be on.

[mincind]

Re: LG 50PA4500 - Sound with no picture

If to be checked with the set off....I am getting a steady 25.6 ohms on VSC and am getting "0" on the -Vy

[mincind]

Re: LG 50PA4500 - Sound with no picture

The Vsc is bouncing around between 1.8- 2.9 volts and the -Vy is bouncing between 36.7 - 40.2 volts

[tom66]

Re: LG 50PA4500 - Sound with no picture

OK, now can you unplug the top Y-buffer board. It's the top green board in the picture linked above. May be marked "YDRVTP".

This board is screwed in. You will have to unscrew it, then pull it GENTLY to the left, which will allow it to come loose. For now, do not disconnect the orange ribbons-- you only want to disconnect the connection between the Y-sustain (the orange board) and Y-buffer. Be very careful with the orange ribbons, do not tear or damage them as they cannot be replaced.

Leave the lower board connected too. You might have to disconnect the cable linking the two. To do so, gently lift up the latch and slide the cable out. Make sure to keep hold of it, ideally keep it attached to one board.

Needless to say, only make connection changes when the TV is OFF and UNPLUGGED...

Then plug the TV in.
- Do you get half a picture? (maybe be distorted or noisy)
- Do Vsc and -Vy stabilise, about 150V and -200V each, exact voltage on sticker.

[mincind]

Re: LG 50PA4500 - Sound with no picture

Tom66,

I need to head out for a holiday engagement in a few minutes, and this is a function that will take a little time and one which I need to be overly cautious.

I will need to delay performing this task until tomorrow morning.

Thank you so very much for your help so far....I WILL be back as soon as I can.

[mincind]

Re: LG 50PA4500 - Sound with no picture

Tom66,

After disconnecting the Y-buffer board, I did get an image (solid grey....but it did illuminate) on the lower half of the board.

The reading on Vsc stabilized (only moved 0.1 V) at 151.0 and the reading on -Vy stabilized at 190.2 V.

[tom66]

Re: LG 50PA4500 - Sound with no picture

Ok, I hope this is not another issue with LG's razor frame. As your top buffer is bad. There is a known issue with LG 50" PA5500 and PA6500 and some PM-series damaging the plasma panels due to the frame cutting into the delicate orange FPC ribbons. Design flaw. I had thought the 720p models were immune..

For the next test see the below video:-

https://YouTube.com/watch?v=51SZsfH60EY

If you need specific help on what connector to test post a close up shot of the connectors on your board. I need to be able to see the PCB traces.

[mincind]

Re: LG 50PA4500 - Sound with no picture

I watched the video (which I understand conceptually - but not sure I can identify the proper "common pin" to land the one static probe on) and disconnected the board.

Here are 3 photos of the board - 2 are segments of the board to be able to get closer and the one is the entire view of the board.

Thanks for the continued timely help!







Please let me know if these are close enough and clear enough to advise on which pins need to be landed.

Thanks again!

Dante

[tom66]

Re: LG 50PA4500 - Sound with no picture

Ok, in this case, it's any one of the pins on "P702", the top most white connector.

Pictures are very good, if only everyone else had such clear photos...!

[mincind]

Re: LG 50PA4500 - Sound with no picture

Thanks for the kind comment on quality of the photos...it is the very least that I can do and am very, very appreciative to persons like you who are so willing to help!

I checked all three ribbon connectors.....and suere enough, the top 8 - 10 pins on the top most connector indicate a short (continuity and beeping). All of the rest of the pins showed a readout of around 0.77.

I assume this means that I need a new upper Y-buffer board?

The good news - is I have two of these TVs. I use them as a mobile Disc Jockey top display slide shows and video montages at weddings - and such is why it critical for me to repair....because if one of the two dies, I need to but TWO new TVs that match.

I suppose I could swap the board with the working TV with this bad until to see if I regain functionality?

Thanks so much!

[mincind]

Re: LG 50PA4500 - Sound with no picture

Sorry for the lousy grammar - should have proof read before send...

should have read

The good news - is I have two of these TVs. I use them as a mobile Disc Jockey to display slide shows and video montages at weddings - and such is why it critical for me to repair....because if one of the two dies, I need to buy TWO new TVs that match.