TCL 65S517 No Backlight, Picture and Sound OK

[nomoresonys]

Re: TCL 65S517 No Backlight, Picture and Sound OK

are you getting ALL expected voltages on the connector from powerboard to mainboard?

[nomoresonys]

Re: TCL 65S517 No Backlight, Picture and Sound OK

test it in standby, leave probe in place, power up tv, report voltages both in standby and powered on.

[nomoresonys]

Re: TCL 65S517 No Backlight, Picture and Sound OK

example of testing there: https://www.youtube.com/watch?v=XH9GfkGQX1I

[nomoresonys]

Re: TCL 65S517 No Backlight, Picture and Sound OK

best to start at start of thread, post #1, do the tests, if you have trouble ask here.

[dskall]

Re: TCL 65S517 No Backlight, Picture and Sound OK

If you look at screen with flashlight and can see a picture, you have bad backlights.

[MichaelPitts]

Re: TCL 65S517 No Backlight, Picture and Sound OK

I tested all of the connector pins and the OZ IC voltages on and off. I get the exact same readings as OP. Same behavior as well, where it lights the backlights for milliseconds, then immediately shuts them off. I'm hoping the OZ chip is bad or perhaps one of the components supporting it.

With the LED connector unplugged, the PSU still does not supply power to the LED connector, expect for a brief moment. I've seen in a YT video that when LEDs are bad, unplugging them from the PSU should allow the PSU to output the voltage it normally would...at least on an LG. I not certain if perhaps the lack of connected LED strips will also cause this PSU to kill power to the connector.

With the LED connector still disconnected, I stood the set up and powered each strip independently using the LED tester. There are 4 vertical strips. Each strip appears to be fine. I do not see any dim or dead spots.

One thing I found surprising, measuring resistance for each strip...each one of them is greater than 20M ohm (as high as my meter goes). I tried pos/neg each way. But they do all light up.

I'm trying to avoid removing the screen, leaving that as the last resort. I've ordered a new PSU since it was less than $50 shipped. It's on backorder though and won't be here for awhile. If that doesn't bring it to life then I'll dig behind the screen. I have some suction cups used to hang glass mirrors...hopefully they'll help if/when it comes to that. Not very optimistic I can take the screen off without breaking it given the number of stories of broken screen I've read.

[nomoresonys]

Re: TCL 65S517 No Backlight, Picture and Sound OK

do Buds test in post #69, see what you get, I think that test using multimeter can be more conclusive than using a backlight tester sometimes.

[MichaelPitts]

Re: TCL 65S517 No Backlight, Picture and Sound OK

I did that test. Had no voltage but hoped it was due to a bad chip on the power board. Turns out it was not the power board. I got a new one and it didn't fix the problem. So it has to be one or more bad LEDs.

I just bought a new Samsung 70" to replace this one. On the fence about trying to repair this TCL or not. Part of me wants to give it a shot for the heck of it. Also considering selling it as "needs repair" or for spare parts. Seems a shame to throw it out when everything but the backlighting is good.

[MichaelPitts]

Re: TCL 65S517 No Backlight, Picture and Sound OK

So good news and bad news.

Good news: The 65S517 does not need to have the LCD removed to access the backlight LED strips. This is an edge lit panel and the two strips are on a detachable metal bar on the bottom.

I removed this and tested the strips. Each and every LED lit up nice, bright and consistent with all of the other LEDs. Well, that's strange. So I re-connect the strips to the TV and power it on. The strips light up and remain lit up the entire time. WTH? This is using all of the original parts. Original PS and LED strips.

So I figure I must've jostled something which was intermittent. Cold Solder joint, bad cable connection...something.

Against my better judgement I re-assemble the TV with all original parts.

Bad news: The backlight stops working at all again.

I hate intermittents! I haven't dug back into yet because even though the LCD doesn't need to be detached from the frame, getting that bar off does take some time and about 50 tiny screws. I guess I'll put the new strips in anyway since I've purchased them...even though the original strips appear to be perfectly fine.

I guess this is why I haven't found any other posts about replacing the backlight in a 65S517. They must not fail very often.

Kudos to TCL for at least making these accessible without removing the LCD.

[MichaelPitts]

I eventually figured out that one of the two strips was shorting to the aluminum bar it's attached to. When that aluminum bar touches the frame, it shorts, the PS detects this and turns off the LED power. I measured a 180v potential difference between the mounting bar and the main case! Gave me a nice shock once too.

I had already ordered replacement LED strips, which I did not use since the original were all lighting up when I had it apart. So I removed the strip which was shorting to the mounting bar, cleaned up the glue and put one of the new strips in. Verified it was not shorting and voilà, it now works! Fixed after sitting for about two years (in frustration)