LG plasma PDP50T3, with sound, no picture

[Sraros]

Hello people, I have the am TV which was brought to me by a friend.

How it came:
When plugging the TV, the power board relay was clicking on and off not letting tv to get power. By pulling out the cables I found short at Z-SUS. I run through it and found a diode shorted, 2 out of 3 IGBTs shorted and an optocoupler on the back of the board damaged (most probably from the technician he gave it too...)
Now after repairing these I did not find something else at least to my knowledge.

Now I plug the TV, I get red light and the TV is on standby. Opening it with the remote, the TV turns on, I have sound but I get no picture at all. Another thing is if I come close to the boards I hear from the Y-SUS and the power board, a constant clicking sound like every 0,3 second which I can bearly hear. The T-CON board green LED flashes four times with a pattern of flashes:
short, short, long, short and again.

Voltages:
TV on standby:
STBY: 3,5V

TV on:
Vs: 205V (rated between 201-207)
Va: 53V (rated 55V)
M5V: 5,12V (rated 5,1)

M_ON: 3,29V
RL_ON: 2,53V
STBY: 5,16V
5,1V: 5,16V
17V: 16,94V

Any indication on what to check next, what to do and what it can be?

[tom66]

Re: LG plasma PDP50T3, with sound, no picture

Can you supply a picture of the panel sticker, which has Vs/Va and other voltages printed on it.

The slight ticking would make me suspect a short on the Vsc supply that feeds the Y-buffer board.

[Sraros]

Re: LG plasma PDP50T3, with sound, no picture

Hello, thanks for having a look. Here are the photos. I forgot to upload them...
I attached a photo of the whole back of TV. A few things that are weird are one cable from the Z-buffer to the panel is missing although it seems it was always like that, no welding or sth visible and from the T-con and PSU there are at each a cable socket but I didn't find any cables hanging. I'm thinking if the so-called repairman forgot sth :P

[tw2005]

Re: LG plasma PDP50T3, with sound, no picture

The cabling is normal, missing ribbon normal and the logic control led , also nothing to worry.

On the ZSUS, was the pico fuse ok, any burnt or open cct resistors like those low ohm green high wattage flameproofs?

all I can add at this point

[tom66]

Re: LG plasma PDP50T3, with sound, no picture

We still need a pic of the panel sticker, top centre of the TV so we can tell you what -Vy and Vsc need to be

[Sraros]

Re: LG plasma PDP50T3, with sound, no picture

Here you go

[tom66]

Re: LG plasma PDP50T3, with sound, no picture

OK so
-Vy is -193V
Vsc is 143V

Unfortunately I do not have an exact SM for your 50T3 panel, but 50R3 is from a similar generation but is 1080p Full HD vs 720p WXGA. See page 142

[Sraros]

Re: LG plasma PDP50T3, with sound, no picture

Tom you are spot on.
Vsc is trembling between 0,5 and 5V and Vy is going between 85-96V

[tom66]

Re: LG plasma PDP50T3, with sound, no picture

You probably have a shorted buffer (scan driver) board then.

Disconnect the top buffer including the ribbon between bottom and top boards. You may leave the ribbons attached to the plasma panel.

Check Vsc and -Vy.

Repeat test with bottom buffer.

This will allow you to identify the failed driver board.

Whichever causes Vsc to come up when removed is the shorted board.

You may get half a picture or partial discharge on the screen when you do this.

Also -Vy should come back up when Vsc short is removed.

If it is still low when both boards are removed there is another issue, probably with the Y-sustain board.

[Sraros]

Re: LG plasma PDP50T3, with sound, no picture

Sorry but can you point out based on the first picture of the whole TV which to remove?

If I undrestood correct, remove the Y buffer boards one by one?

[Sraros]

Re: LG plasma PDP50T3, with sound, no picture

I rushed so much to do the steps you pointed out, I missed to see in both Y buffer boards there is a chip burnt. I can confirm both are shorted by chexking some SMD caps. So with both buffer boards removed I get
Vy: 194,7
Vsc: 144,4
And I measured also on the X-SUS
Vzb: 90V (like sticker rated)

However I still got no sign at all of partial image with the Y-SUS board connected and only Y buffer boards removed... Does this mean even if I order new buffer boards I might have to go deeper like main board or is it in some TVs normal?

Edit: I will try to remove the burnt SMD chips to see if the shorts fix

[tom66]

Re: LG plasma PDP50T3, with sound, no picture

The buffer boards / scan drivers are the boards on the left hand side of the TV when looking from the back, and they are connected to the Y-sustain board (the board that you measured -Vy and Vsc on). They are connected to the plasma panel via orange ribbons.

You need to remove their mounting screws, and the ribbon cable that links one to the other. Remove the top board, power the TV up, then check the -Vy and Vsc and see if something appears on the panel (may be random picture noise, may be half of a picture, it depends on the design). Power off TV, then re-connect the top board and disconnect the bottom board. Again check -Vy and Vsc and check to see if something appears on the panel.

[tom66]

Re: LG plasma PDP50T3, with sound, no picture

If the buffer ICs are both burnt on both boards that could be an issue with the Y-sustain. You will not get any discharge with both boards removed because the Y-sustain connection is vital to get any image. With Z-sustain removed you usually get discharge through panel capacitance, but it is very weak (dim, poor discharge)

[Sraros]

Re: LG plasma PDP50T3, with sound, no picture

It's one chip out of 3 for the top and 1 out of 4 for the bottom. Also the voltages of the Y-SUS are ok when I remove them. It can still be sth else that fries the chips?

If all three boards have to be replaced I think it will be very costly and I ll leave it. At this point of time I cannot find a good price for the bottom Y buffer already...

[tom66]

Re: LG plasma PDP50T3, with sound, no picture

I can't be sure. You'll want to check the panel is not cracked

[Sraros]

Re: LG plasma PDP50T3, with sound, no picture

Hello again. So I had ordered and received today the chips of the Y-buffer. I replaced the burnt chip successfully on the bottom buffer board and put it back in but still no picture.
Vsc: 144,5V and Vy: 194,5V stable.
Va: 54,7V at bottom buffer
Vzb: 88V

So I just found also..At the control board P102 connector (coming from Z-SUS) point 14 and 15 mark 15V but I measure 1,5V

[tom66]

Re: LG plasma PDP50T3, with sound, no picture

I can't remember for certain on these sets but I think the 15V for the Z-sustain comes from the Y-sustain, so it could be a problem with the Y-sustain not generating the 15V.

[Sraros]

Re: LG plasma PDP50T3, with sound, no picture

The 1,5V instead of 15V I am measuring at the control board are at both ends of the y-sus cable and the z-sus cable. Is it coming in from y-sus to control board and then go out to z-sus? How is this exactly working?
I uploaded a close photo of the control board too

[tom66]

Re: LG plasma PDP50T3, with sound, no picture

I think the Y-sustain generates the 15V from the transformer on the Y-sustain board. It could be something wrong with that part of the circuit though I can't be certain.

[Sraros]

Re: LG plasma PDP50T3, with sound, no picture

Hello again, I replaced the Y-board and at first got nothing. It seems also the previous technician didn't connect somehow correctly the Z-buffer board, so after attaching the ribbons correct again the TV works fine now
Thanks for your help Tom. I appreciate!