TCL 65C715 backlight issue

[Cornito]

Hello team,

I'm back with another topic.
It's a 65C715 TCL tv with backlight issue.

But the problem is a bit strange.
When I took this tv from the marketplace backlight didn't work but picture was ok.
Back at home I checked with led tester if led are alive and I can find something different in term of voltage supply => nothing

Putting all back together the tv worked fine for weeks. => Contact issue??

Few moment later the tv has started to have backlight issue back, turning off and on worked sometimes and tv was working.
Then one day nothing.

Reading other threads I've seen that the C614 may be faulty even if not really visible.
I replaced it and tv was back to life for couple of days, then same issue.

Same maybe not really, the backlight flashs at strating up the tv, I've didn't noticed that before.

Ok putting apart another time the tv, I checked the PSU voltages, seems that PSU does into protection mode because a small time I can read a voltage before it drops to 0.
checked all led strips by the back connector, nothing special, all voltages are the same, 43V on outer strips and 33V on inner strips, the ones with two connectors.

So, as nothing was noticed, I've checked functionality and tv works well since.

As the tv is glued I'd like to avoid useless LCD panel removal to check led 1 by 1.



Do you have any idea of what can be the problem?

Do you know what is the part number for led strips, I can't find it online. I'd like to get new ones if needed before removing the glued LCD panel.

Thank you in advance

Damien

[Cornito]

New from battleline, since the last reassemble tv is running for 12 hours without issue.
One restart without issue.
Seems that there is a bad connexion that should be corrected now when I unplugged and plugged back all connectors.
fingers crossed

[CapLeaker]

Check the backlight brightness level and turn it down to 67%. If it stays on you got one led in the backlights failing.

[Cornito]

Hello Capleaker, could you explain me you can test it like that.
I'm at full brightness now, just deactivated the dimming of movement control because this is visible for me.

Regards

[EazyBone]

I still think it's backlights. When you say 33 and 43 do you mean voltage with multimeter or are you using led tester to test the leds. Which one to test them.

[Cornito]

Yep, i've tested the led strips and values are pretty similar, but on this model there are 14 connectors on the top and 6 in the middle of the panel and close to the botom of the screen.
43V on the strips without connector at the bottom and 33V on the other.

it worked several hours since, and each time moving the connector or doing nothing special than disassemble the TV put it back to life.

Looks like a bad contact.

Nevertheless, i'm not able to find the replacement LED strips for this TV, do you have an advice to find them out?

Screen is glued, i'd like to avoid the unassembling if useless

[CapLeaker]

Hello Capleaker, could you explain me you can test it like that.
I'm at full brightness now, just deactivated the dimming of movement control because this is visible for me.

Regards

That's the oldest trick in the playbook. Once you LED TV starts having back light problems, turn the brightness of the back light down between 50% and 67%. This way you can prolong the life of the TV a bit longer. Why this works? LED power supply is current controlled not voltage controlled, so the power supply can shut down instead of burning your house down. At a lower brightness level, the LEDs don't consume as much power and the LEDs won't get as warm either. For a LED that's on the fringe this trick may bring the bad LED back to life for a while. But it will fail at some point. That's why sometimes you can't find the bad LED or LED strip even not with a LED strip tester, because the tester won't run the strip on full tilt.

I run all my LED TV's and monitors back lights at 75% period. Never had a back light fail.

[Cornito]

Hello CapLeaker,

thank you for the explanation, it makes sense, i'll try this and i'll let you know.

Regards

[EazyBone]

This is a QLED now that I researched it.. so everything I said about our tvs don't apply.

I can't find the LEDs at all anywhere. Kind of odd.. but it looks like these strips run up and down? I don't understand your 43 v at connector 33 at other comment?

These connectors in picture you tested individually?

There should be like 12 of them that run up and down, you say there is extra at the bottom?

[Cornito]

Hi EazyBone,

To clarify a bit what I said, the "bottom" connector are the one in the red circles.
These connectors exist only on the 6 middle strips.
There are 14 strips in the whole TV, 8 without the middle or bottom connector, 6 with.
Or maybe there are 20 lines at the end 8 with 43V 6 with 33V, 6 with ?? (i didn't measure the voltage on the middle connector)


TV is in another house, i'll be able to check in 2 weeks the missing voltages.
I'll try to reduce the dimming to preserve the LEDS.

Regards

[Cornito]

Hello back,

So tv was working fine until another backlight issue.
It seems that PSU goes in protection mode, but why ?
It's a pitty that led strips seems not fixable, because of non availability and glued screen.

Any idea ?

[nomoresonys]

Well did you do as suggested in post # 3? maybe also check the solder joints on the bottom of the board, examples of bad joints: https://www.google.com/search?client...h=599&dpr=1.09

[Cornito]

Yep, i've set the brighness to a lower value.
Checked also the solder joints whitout find out a bad one.
Replaced the big cap

[nomoresonys]

Another good thing about that test is if the tv stays on longer or doesnt act up anymore after turning backlights down, then it's very likely bad/weak led/s as CapLeaker said. You say you replaced the big cap, why not replace those other 4 electrolytic caps, one or more may be weak, hard to tell without a tester or dmm capable of testing caps.

[Cornito]

I have another psu for this tv, that didn't solve the issue, so i'll replace all the caps to see if it fix the problem.
Still strange for run like a charm for weeks then stops, one time just desasemble and put back together all connector let it relive.
I'll have another rounnd looking on solder joint caps, then i'll should need to replace it

[Cornito]

Finally it seems i found the LED part number: GIC65LB100_3030F2.1D_V0.7_20200108




Found these on AliExpress :
https://fr.aliexpress.com/item/10050...|query_from%3A

https://fr.aliexpress.com/item/10050...|query_from%3A

On pictures i can now understand why i've got so many connector and same not at the same height on the same led strip.

I hope this can help somebody,

[Cornito]

Hello guys,

Back from the battlefield, in order to try to find out which LED strip is defective, I've used my LED tester.
Nothing seems really strange; all lines work perfectly with the same voltage requirement.

As with each time I test the LED strips, when I plug the TV back in, everything works well.
It seems like LED strips need to be unlocked before being able to let the PSU start correctly.

Furthermore, I can see the LED flash when the backlight is defective.

So it seems that it's an LED strip defect, even if I'm not able to find which one is causing the problem.

Any thoughts?

Regards,

Damien

[EazyBone]

So did you buy new LEDs and replace them?

[Cornito]

Nope i was in vacation and i'd like to confirm that it's really led strip issue.
Replacing is not si easy on glued 65" panel, and led strips are not so cheap to take one more risk.

[EazyBone]

Well you have two power boards. Chances are you don't have 2 defective power boards. Take a pic of the legend for PSU to main or it may be on main board. Potentially main board has a signal line not sending to turn on lights. Excuse if it's already posted but I don't see it.