Samsung LE40A699 picture problem

[Eisenhorn]

Hello all!
I am having a problem with mentioned LCD TV. This is my first LCD TV that I am repairing. Before I try to fix something that is already working I would like to ask for some help. The screen has some horizontal lines and the right side is very bright. Everything else works. The voltages on the power supply PCB are max 5% lower than stated on the PCB and all capacitors are normal, at least visual.
Any advice would be great!

Thank you,
David

[tom66]

Re: Samsung LE40A699 picture problem

Never seen that. But it looks like a bad panel to me. Like a diffuser has failed. If you feed the screen a fully white image (may be done from the service menu, or a computer) does the right hand side appear much brighter than the working parts?

[Eisenhorn]

Re: Samsung LE40A699 picture problem

There is no difference in brightness... I tried it with service menu...
When I was taking it apart, to see if the connectors from T-con to LCD panel are faulty, it suddenly started working, but the next time when I turned it on it was the same again... And I can't reproduce the working screen...

[tom66]

Re: Samsung LE40A699 picture problem

It may be tab bonds, but I have never seen that before.

[Eisenhorn]

Re: Samsung LE40A699 picture problem

The diffuser is OK, because that on the right side of the screen are lines, some of them brighter than the other. If I press on tab bonds, nothing happens. What about some ICs on the tab bonds? They get quite warm.
Oh, and I talked to the owner of the TV and he said, that after a while, the screen just went black blank.

[Eisenhorn]

Re: Samsung LE40A699 picture problem

More update. If I unplug one of four connectors that go to lcd panel, turn on TV, turn TV off, and then plug that connector back, and turn on TV, then the picture is almost 100% good. Maybe some problem with power supply on T-con board?

[tom66]

Re: Samsung LE40A699 picture problem

The ICs themselves can be defective. I would also recommend checking the VON, VOFF and AVDD voltages on the T-con board.

[Eisenhorn]

Re: Samsung LE40A699 picture problem

Von is 17.3V, Voff is -7.4V, AVDD is 15.28. I measured some test points and the voltages on T-con board are ok.
Any other suggestions?

[tom66]

Re: Samsung LE40A699 picture problem

Von is a little low but probably normal; it should be closer to 25V. Does this T-con have two ribbons to the LCD panel? If so, check if it rises with one side unplugged.

[Eisenhorn]

Re: Samsung LE40A699 picture problem

Voltages are almost the same, plugged or unplugged. I managed to get it in working state again, voltages are still like I measued before.

[tom66]

Re: Samsung LE40A699 picture problem

Hmm, when in working state, if you apply slight pressure to the tab bonds, does it go wrong again?

[Eisenhorn]

Re: Samsung LE40A699 picture problem

No, the TV picture doesn't react if you touch the tab bonds. All together seems like a little mistery to me...

[tom66]

Re: Samsung LE40A699 picture problem

Have you tried heating up parts when it's working, or cooling them down?

[Eisenhorn]

Re: Samsung LE40A699 picture problem

Sorry, I don't have any heat gun nor cooling spray, but from what I can see so far, heat has no influence on working or not.

[tom66]

Re: Samsung LE40A699 picture problem

Hairdryer? Every TV tech needs a hairdryer .

[Eisenhorn]

Re: Samsung LE40A699 picture problem

Well I didn't think on that, I don't know how, but I didn't. But still, it has no influence. Now I tried to play something on screen, and there are vertical colored lines, depending on the current picture on screen, also, the right side is darker or brighter, depending on picture. If it is in "almost working condition" there are also vertical lines, just not that obvious. Should I try to replace T-con board?

[tom66]

Re: Samsung LE40A699 picture problem

I don't think it is the T-con, as the T-con doesn't usually cause those problems. It could be the T-con but I would be surprised.

[Eisenhorn]

Re: Samsung LE40A699 picture problem

Then at least I can try and replace the capacitors. It all sounds like a poor initialization to me, so T-con, main board, or PSU capacitors. I just hope it isn't main board.

[tom66]

Re: Samsung LE40A699 picture problem

The only voltage important to the t-con from the PSU is the 5V or 13V, is that good?

[Eisenhorn]

Re: Samsung LE40A699 picture problem

5.05V and 12.75V. Unfortunately I don't have my oscilloscope right now, so I can't check how stable it is.