mystery lcd problem

[efwiofoi]

i have 40 inch lcd, when i got the tv one backlight was cracked, now replaced.
the backlights are working perfectly but the lower left of the screen is darker then the rest, its like the brightness or contrast is turned down in this area. at times it looks fine but in darker scenes it sometimes looks much darker than the rest. not a defined area just dark emanating from the corner. the affected area seems to radiate in and out as the screen brightness changes. I'm 100% its not the backlight, the diffuser, pressure on the panel, or anything like that. it seems to be the panel itself. its possible the tv was dropped. but I've never seen a panel do this before only sharp lines, cracks, physical damage that is obvious. its like the voltage is lower in this area or something. can anyone explain whats going on?

[tom66]

Re: mystery lcd problem

Can we see a picture of the fault?
It could be permanent "mura" effect. Though that is rare on 40" panels. It is common on 32", 37" and 42" Chi Mei and Chunghwa panels.

[efwiofoi]

Re: mystery lcd problem

the bottom left seems to flicker at times, and looking closely the pixles appear to be moving/scrolling. it looks like it must be the panel, but an unusual problem.

[tom66]

Re: mystery lcd problem

I can't see any other part causing that. Looks like mura to me. What is model of the LCD panel inside and of the TV?

[efwiofoi]

Re: mystery lcd problem

its a samsung lta400ha07. its like some sort of clock signal has been lost to that corner? maybe the edge is slightly cracked but i didn't see anything when i changed the bulb. not that noticeable when watching but not good

[tom66]

Re: mystery lcd problem

Mura effect
https://www.google.co.uk/search?q=lc...do%3B960%3B720

Various types of mura can occur.

If some signal were getting lost, it would affect a block region. The tab drivers drive rectangular regions of the panel, so you might get a whole block that's blank or in error in some way, but never a random region like that.

[efwiofoi]

Re: mystery lcd problem

i would say its electrical rather than physical, the area looks more pixelated. maybe like only half of the pixels are turned on on certain colours. if you look really close its like the pixels are vibrating/dancing. but the area affected is not clearly defined it seems dependent on brightness.

[tom66]

Re: mystery lcd problem

LCD panels drive the pixels alternately, it's called odd-even addressing and helps reduce flicker, maybe this is what you are seeing? If you look at any working LCD and move your head you should see pixels flicker. I have had mura before which made that flicker more obvious.

[efwiofoi]

Re: mystery lcd problem

I'm wondering if theres a chance it could be a main board problem? i changed the t con trying to diagnose the backlight so i know its not that(i swapped the spare one back to check). the bottom left definately flickers more than the rest of the screen and the pixels look different, hard to explain without seeing it but its like the signal to that area is affected/of lower quality.

[tom66]

Re: mystery lcd problem

There is no way a main board or t-con board could cause such a fault.

Main boards process the video signal as a whole, faults typically render themselves as repeating error patterns which is -rarely- fixed to any obvious grid because they don't usually process the video as blocks but rather as pixel after pixel. T-cons rarely store video information (very old ones do) so any errors they cause affect one of their outputs such as a repeating vertical pattern. They also often process each subpixel as a separate pixel unlike the main board which processes 3 subpixels as one pixel (R-G-B: three subpixels...)

It's almost certainly panel. Regional defects nearly almost always are. There's a small chance that cleaning the diffusion layers could sort it, but from what you've described, you did this already? When I say panel fault, I consider the whole LCD assembly to be the panel (backlight + diffuser + LCD + drivers + frame) to be the "panel".

[freakaftr8]

Re: mystery lcd problem

you say you replaced a back light bulb I would think more or less you have a problem with the filter and the polarizer behind the LCD panel maybe something is not sitting right

[efwiofoi]

Re: mystery lcd problem

I'm sure its the panel also(the actual lcd)
it has an effect where light areas in the middle are affecting the area outside them to the edge of the screen, in this pic the white line that comes off the corner of the menu box, only the defective area has this not the rest of the screen.



what ever the exact cause ill put it down to being unfixable.

[efwiofoi]

Re: mystery lcd problem

you say you replaced a back light bulb I would think more or less you have a problem with the filter and the polarizer behind the LCD panel maybe something is not sitting right

thats what it looks like, but im sure its the panel itself
something to do with the way the pixels are driven.

[tom66]

Re: mystery lcd problem

Could be excessive pressure. I have seen that sometimes.

[efwiofoi]

Re: mystery lcd problem

i checked the mounting all is as it should be even powered it up without the metal bezel on the panel. pressure imbalanced within the glass due to impact maybe?