$2.2k Sony 70in 3D TV has uneven backlight, but why?

[iamdarkyoshi]

Picked up this TV on craigslist with a bad power supply, it shuts off after a few seconds.

Model KDL-70W850B

Its an awesome TV but with one massive issue...

The edge lit LCD is super uneven. It's lit from the sides, but something has gone wrong inside.


Apologies for office chair obstructing image

Its not failed LEDs or whatever since the TV has 4 wires going to the backlight driver, and unplugging one of them will make one half of one side stop lighting up. Unplugging the backlight driver entirely does not fix the powering off issue, so I'm pretty certain something is wrong with the PSU.

Obviously visual performance like this is unacceptable for a 2,200$ TV when its new, so does anyone know how it happened, and how I could fix it?

I really want to get this unit running in my livingrooom but this backlight makes me want to vomit...

[Kilrah]

It very likely is failed LEDs in the strips around the panel.

Hope it was cheap (more like free), that's not easy to replace.

[iamdarkyoshi]

It very likely is failed LEDs in the strips around the panel.

Hope it was cheap (more like free), that's not easy to replace.

Unless it has LEDs top and bottom that all failed entirely without the sides failing at all, and they also run off the existing 4 ribbon cables that go off to the sides, I'm not sure that's the case...

I've done LED strip replacement before, it's not too difficult, but in this case, all the LEDs appear to light, assuming it is just side illuminated.

[Kilrah]

Unless it has LEDs top and bottom that all failed entirely without the sides failing at all

It almost certainly is that, there isn't really any other way it could look like that. Strips are typically arranged in parallel groups of series strings, one dead LED takes out the whole series string but the other parallel legs continue working. Guess there are 2 dead LEDs, on in the top strip and one in the bottom one.
They likely didn't fail at the same time but the previous owner tolerated the situation long enough for a 2nd one to fail, then tolerated that long enough until something else also gave up.

Note that you can find the service manual for that TV in seconds, there are LED blink error codes when it turns off that might tell you about the power problem.

[Kilrah]

Looks like it has local dimming, so there's a number of individually controlled LEDs / groups of LEDs in a matrix instead of series/parallel strips. And an active controller board between them and what you unplugged from the power supply which thus becomes the most likely culprit, probably a driver has failed on that and it took out a whole portion of the outputs. Then it's "smart" and notices it at power up self diag, and the TV shuts off. See if you have the 6x blinking LED signaling a backlight issue.

[iamdarkyoshi]

Looks like it has local dimming, so there's a number of individually controlled LEDs / groups of LEDs in a matrix instead of series/parallel strips. And an active controller board between them and what you unplugged from the power supply which thus becomes the most likely culprit, probably a driver has failed on that and it took out a whole portion of the outputs. Then it's "smart" and notices it at power up self diag, and the TV shuts off. See if you have the 6x blinking LED signaling a backlight issue.

I get 4 blinks.

I looked up the LED strips the TV uses and it just appears to be 4 strips, each doing top left, bottom left, top right, or bottom right. The ribbon cable that connects to them looks the same as what I can see on my backlight driver board. Unplugging one fairly clearly will knock out half of one side of the TV's backlight.

I've worked on TVs with zoned backlight and I'm fairly sure this one doesn't use them, the ribbon cables running out of the driver have way too few pins to drive a matrix...

[Dacke]

This TV doesn't have local dimming,  but instead what Sony calls "frame dimming".   That is,  the entire edge lit backlight will increase or decrease in brightness as a whole,  depending on what's displayed.  I've worked on many TVs for a few years now but not this particular model.   The horizontal darkened bar on the top of the screen is fairly well-defined so I don't think it's a direct backlight issue.  Typically in my experience,  vertical bars with distortion = TCON issue is most likely or possible bad connection with FPC (flat flex) cables.   Well-defined horizontal bars = LCD panel issue most likely,  followed by TCON.  The repair manual for a model very close to this one (KDL-70W830B) states that 4 blinks is likely a LD (LED driver) board issue, LCD panel issue, or possibly a TCON board issue (in that order).    My guess is that with the symptoms on the display and the blink code,  either an LCD panel fault which is not repairable,  or a TCON board fault which should be fairly cheap and easy to replace.

[iamdarkyoshi]

This TV doesn't have local dimming,  but instead what Sony calls "frame dimming".   That is,  the entire edge lit backlight will increase or decrease in brightness as a whole,  depending on what's displayed.  I've worked on many TVs for a few years now but not this particular model.   The horizontal darkened bar on the top of the screen is fairly well-defined so I don't think it's a direct backlight issue.  Typically in my experience,  vertical bars with distortion = TCON issue is most likely or possible bad connection with FPC (flat flex) cables.   Well-defined horizontal bars = LCD panel issue most likely,  followed by TCON.  The repair manual for a model very close to this one (KDL-70W830B) states that 4 blinks is likely a LD (LED driver) board issue, LCD panel issue, or possibly a TCON board issue (in that order).    My guess is that with the symptoms on the display and the blink code,  either an LCD panel fault which is not repairable,  or a TCON board fault which should be fairly cheap and easy to replace.

LCD displaying a black image still looks excessively bright on the edges and dim in the middle. I'm not home right now but I'll bet unplugging the LCD and just running the backlight gets me the same results, assuming it'll let me power on with no LCD connected.

4 blinks is likely a backlight driver issue, so I've ordered a new one and a new PSU.

I've found someone on ebay with a cracked panel so push comes to shove, I'll drive to go get it and use the backlight and diffuser from it, but I'd really like to avoid doing so if at all possible.

[iamdarkyoshi]

This TV doesn't have local dimming,  but instead what Sony calls "frame dimming".   That is,  the entire edge lit backlight will increase or decrease in brightness as a whole,  depending on what's displayed.  I've worked on many TVs for a few years now but not this particular model.   The horizontal darkened bar on the top of the screen is fairly well-defined so I don't think it's a direct backlight issue.  Typically in my experience,  vertical bars with distortion = TCON issue is most likely or possible bad connection with FPC (flat flex) cables.   Well-defined horizontal bars = LCD panel issue most likely,  followed by TCON.  The repair manual for a model very close to this one (KDL-70W830B) states that 4 blinks is likely a LD (LED driver) board issue, LCD panel issue, or possibly a TCON board issue (in that order).    My guess is that with the symptoms on the display and the blink code,  either an LCD panel fault which is not repairable,  or a TCON board fault which should be fairly cheap and easy to replace.

So I ran the TV with the LCD/TCON entirely disconnected, so we'd see just the backlight through a black LCD.



So with no active LCD, we still have uneven backlight.

But here's the strange thing... The TV stays on with the LCD unplugged. No longer shuts off.

So I reconnected the TCON board to the mainboard, and it shut off again.

Tried again with JUST the TCON, but unplugged the LCD from TCON, and it still shuts off. So I guess our issue lies at the TCON board for the shutdown problems, but still doesn't explain the backlight issues.



I ordered a new PSU well before I did these tests, should I cancel it and order a new TCON board instead? Or should I wait for the PSU to come in so we can rule it out as an issue?

Once I get this damn thing to stay on, I'm taking it apart to tackle the backlight. I've taken plenty of TVs apart before, shouldn't be too difficult, just need to find a 70in table and blanket...

[iMo]

Years back I got $1.8k Sammy with uneven edge lit backlight (LEDs at all 4 edges). Horrible..
They told me "with time it will improve" .
I insisted on a replacement of the unit and I got a new one with somehow "better" distributed backlight (but still uneven).
Frankly, after about 9y it really improved a little bit.

It is about the LEDs used, imho.
 
PS: the "uneven" backlight issue was/is not visible under normal TV operation here, only in the setup as the yours above - with the backlight through the "black screen"..
I think you have to live with that

[EEEnthusiast]

The problem lies in the diffuser design of the TV which is unable to distribute the light evenly. If they had the LED strips lined up horizontally instead of vertically, things would have been a bit better as the distance to the center would get reduced and the fade is less noticeable. Try to align the diffuser and the LED strip against each other to check if that improves. Edge LED bleeding on large displays is a known factor on many models. The manufacturer's just don't want to acknowledge this.
You could replace the entire side LED and the diffuser and then replace with backlit LEDs and a suitable diffuser. This should solve the problem.

[Kilrah]

Have you even looked at the picture?
There is a defect, it obviously wasn't like that when new.

[Dacke]

So I ran the TV with the LCD/TCON entirely disconnected, so we'd see just the backlight through a black LCD.

So with no active LCD, we still have uneven backlight.

But here's the strange thing... The TV stays on with the LCD unplugged. No longer shuts off.

So I reconnected the TCON board to the mainboard, and it shut off again.

Tried again with JUST the TCON, but unplugged the LCD from TCON, and it still shuts off. So I guess our issue lies at the TCON board for the shutdown problems, but still doesn't explain the backlight issues.

I ordered a new PSU well before I did these tests, should I cancel it and order a new TCON board instead? Or should I wait for the PSU to come in so we can rule it out as an issue?

Once I get this damn thing to stay on, I'm taking it apart to tackle the backlight. I've taken plenty of TVs apart before, shouldn't be too difficult, just need to find a 70in table and blanket...

The shutdowns seem to point to a TCON board issue, yes.  Quite a few of Sony's TVs have fairly large heatsinks on the ASICs of the TCON board (not sure about yours),  they get hot and it's not an uncommon failure point.  There could be nothing wrong with the PSU,  most of the boards in these TVs (LD, TUS, TCON, MCU, PSU) communicate with each other,  if that communication link is broken,  or there is a fault signal or an over current / over voltage condition from another board,  it can trigger the PSU to shutdown.   I'd say try replacing the TCON board,  keep the extra PSU for now if you need it,  hopefully they have a return policy if you don't.  Deal with the back light afterward sure,  but before you go taking the entire TV apart to get at it, check the back light power level in the menu and then move to the LD board if needed.

[ChunkyPastaSauce]

Diagram 7 in section 2 shows some power connections related to backlight led control, might be worth checking

https://servicemanuals.us/sony/tv/kdl-60w850b-kdl-70w850b.html?limit=4&start=8

[ratbonesxx]

Hi iamdarkyoshi, I'm having the same issue with the uneven lighting, just wondering if you were able to fix it?

[Ironmendez86]

I have kdl-70w850b it all started with same problem as your tv....it had that black shadow on middle of screen....i replace main and t-con and update main board now it has no backligth....i wonder if you fix your tv and what other pfoblems you found about this problem.

[Mr544]

Hi guys! I’m new here. I have a Sony kdl-70w830b with the same exact issue as your imdarkyoshi. I found the tv and it is in excellent shape. Originally when I plugged it in the led would flash 3x. Traced it to the main board, replaced the board and it came back to life. No more led codes. So I installed the tv on a wall mount and now I realize I am having the same issue as some of you guys, meaning that the left and right side edges are brighter than the rest of the picture. Have any of you resolved this problem. Thanks

[masessum]

Hello, if you're still around, the problem with the TV is the diffuser...there are four strips, two on the left edge, and two on the right edge...somehow, the hard diffusing light pipe sheet, that the strips shine into the edge of, has deteriorated or distorted...I worked on one of these, same model, and there's nothing burned, darkened, or otherwise visibly bad about the diffuser, it's just no longer transmitting the light all the way through.  I spent several hours replacing all four strips, even though the old ones still looked and shined bright like new.  Sony junk, I guess.