TCL 75S421 has sound but no backlight not even a flash

[EazyBone]

Hello easybone, take time to test it during days.
I've this on mine, worked perfectly some days and then I had the issue back, just unplugged and plugged back the connectors solved the issue. And few hours, or restart later, the issue came back.
No way to fix it on my side, broke the panel trying to remove it

Interesting. The voltages seem correct and the LEDs. I also upgraded the capacitor to a higher one. Time will tell

[Cornito]

Hello easybone, take time to test it during days.
I've this on mine, worked perfectly some days and then I had the issue back, just unplugged and plugged back the connectors solved the issue. And few hours, or restart later, the issue came back.
No way to fix it on my side, broke the panel trying to remove it

[EazyBone]

Got me a 75s425 and had picture with no backlight. Tested backlights and all working fine. Tested voltages at backlight and one line was .9 and other .18. remembered this thread and pulled it up. Replaced the cap mentioned above with a higher uf from another board. All working correct again. Extremely easy fix. Thought I'd add to this. Thx to whoever figured it out.

[mecha1166]

I found a scrap power supply that came from a set with no backlights. Cap looked normal, but replaced it anyway. Tried it in this set, backlights but still cuts off. So not helping on my tv, but wanted to let everyone know the backlight problem can be fixed with the capacitor. The capacitor can look good but still be bad!

[mecha1166]

Hi! I have one of these tvs here. Problem is it plays about 5 to 20 mintutes, picture is fine. Then picture goes blank but backlights stay on. I can shut off tv with power button. Unplug tv 15 minutes and reset, it will come back on another 5-20 minutes and lose video again but backlights stay on. I have lready tried main & tcon. Any ideas? Panel itself or power supply? The backlights should be good, as they stay on.

[Cornito]

Part number of the led strips, I missed to finish my sentense

[Cornito]

Hello,

I have the same issue on a 65c715 TCL, I've replaced the c614 cap and tv returned to life.
But 24h after same issue.

Is there anyone who knows the reference or part number of the 65c715 TCL.
I'd like to avoid double dismantling.

Regards

[ChattFix]

Here's the bad cap. It does read open with ohm meter on highest setting and reads unknown or damaged component on the component analyzer. Usually in ohms with a working cap you will see some low resistance that climbs to infinate once the cap is charged.
I can't figure out what manufacturer makes this part. I don't recognize the symbol and can't find it in the list (see attached )
how can I tell what the capacitance should be? I would like to find the datasheet but without the manufacturer I would go off another manufacturers datasheet which may not be the same?
I Think the 333 means 33 nanofarads...from looking at other manufacturers markings.
I get putting in a 2kv replacement for future proofing the tv but what of the capacitance? Do I have to get it exact ?

I have the same issue, what did you end up replacing it with? I have a bunch of other boards I might be able to pull from but wanted to check with you.

[nomoresonys]

Yes .033 uf, 33 nanofarads, see if you have something close and give it a try.

[triplefour]

Here's the bad cap. It does read open with ohm meter on highest setting and reads unknown or damaged component on the component analyzer. Usually in ohms with a working cap you will see some low resistance that climbs to infinate once the cap is charged.
I can't figure out what manufacturer makes this part. I don't recognize the symbol and can't find it in the list (see attached )
how can I tell what the capacitance should be? I would like to find the datasheet but without the manufacturer I would go off another manufacturers datasheet which may not be the same?
I Think the 333 means 33 nanofarads...from looking at other manufacturers markings.
I get putting in a 2kv replacement for future proofing the tv but what of the capacitance? Do I have to get it exact ?

[triplefour]

Holidays indeed. I still haven't been able to find the time to replace that capacitor. I did have a look around my scrap parts yesterday and may have found some donor candidates. Ohms reading on multimeter is not the best test for capacitors. Need a component analyzer. Which I do have. Will post more soon.

[EazyBone]

Yea, they may work still don't disassemble it again, just test if they work. If all connections work, then maybe rj or whoever posted earlier can chime in again. It is holiday weekend so

[evanroheff21]

I do have an LED tester, and the individual connectors are accessible from the back pretty easily. I'll give that a shot. I'll probably buy a new/used PSU to replace this one as I've about reached the depth of my troubleshooting skills at this point.

[EazyBone]

You can also get faulty LEDs that burn out immediately that can be the same problem. I've had it happen once, replaced all, tested them. Plugged in for first time, blew 3 of them in same strip. Some reason I was sent 2 extra strips in same order, put them back in and fine.

Do you have a LED tester and can retest the new ones you put in without taking it all apart?

And maybe rj can chime back in because, I only researched this problem I haven't had one myself yet.

[evanroheff21]

Hmm,
I replaced the C614A capacitor with an equivalent capacitor (1000V, 333J marking) that I pulled off of an older Samsung (UN65NU7100F model) power supply (BN44-00932B). It looks the same aside from being blue instead of gray. After reinstalling into the TV and powering up, I'm getting the exact same symptoms (no backlight, short initial flash when very first connected/powered. Is it possible that my replacement capacitor is also failed in the same way? Forgive my ignorance, I'm testing these with a digital multimeter set to the ohms/resistance setting. This new (blue) one from off the Samsung board seemed to also read 0L on the meter, but I just assumed I was doing something wrong. I guess I'm second guessing that evaluation. I'll see if I can find a 3rd one and test. Any other ideas as to what I might be overlooking? Here is an album with full res images of my board and the two capacitors, both old (gray) and new (blue). I appreciate the help!

https://photos.app.goo.gl/YmJ4zwXfZ5U9xUmY6

[evanroheff21]

Thanks Eazy. I removed the capacitor from the board today and it does indeed read as open, as R_J said was a common failure up above. For reference, the information on the side of this specific capacitor are "MMKP82", 1KV 333 J 1910. It is indeed the capacitor at board location C614A. There's no visual external indication (bulging, etc.) that the capacitor has failed.

[EazyBone]

Evan it's most likely going to fix it. Common failure, should of tried that first. Go to AliExpress and buy a capacitor or eBay one. Amazon also if you want it fast probably. Need to solder in, a used board will have same problem eventually. Who knows how long

[evanroheff21]

I also am in the process of repairing a very similar TCL. Mine is the 75S425, but from what I can tell they use the same power supply. I replaced all LED strips today, as many of my previous repairs of various TVs have almost always boiled down to that, however my set is still dead in the water. Flashlight test shows the Roku boot up screen and I get key-press sounds, so mainboard is good it would seem. triplefour please do post back if you're able to locate a suitable replacement capacitor for c614a, and if swapping that out does indeed fix the problem. I've noticed that if I have my set fully disconnected from the wall, and I plug it in, I get maybe 0.5 or 1 second of lighted TCL logo on the screen very briefly. I may buy a replacement board off eBay or somewhere, but it'd be great to repair it for less if possible.

[nomoresonys]

Re: TCL 75S421 has sound but no backlight not even a flash

Sure, see if you have one rated a little higher in voltage, might last longer than that one did, so if it is 1k then maybe throw a 2k in there.

[triplefour]

Re: TCL 75S421 has sound but no backlight not even a flash

Hi, this power supply has a common fault that causes no backlight. Normally capacitor c614a fails. It is near where the screen printed board number is on the circuit board. If you remove the capacitor you will feel on the side it is no longer flat and bulged out a bit. I have repaired many of these power supplies with this fault. Hope this helps.

wow thank you, and thank everyone else for their timely and insightful responses. this forum is getting better by the day! i will definitely be looking this board over for these issues. the replacement board is quite expensive so it warrants taking the time for circuit chasing!

i do miss budm....havent seen his post in forever...did he leave us? or earth?

also there was talk of checking the voltage at the LED connector as the power is turned on? i reported on that in my original post. 2 strings stay at 0 and the other 2 jump up to 0.6v...

i didnt do the picture with flashlight test because i saw right away that we had a backlight problem that needs fixing anyway. but in hindsight i definitely should have done that before i took the tv off the wall. so many details! i get excited and forget!


edit: sure enough visual inspection on c614a reveals it is bulged on the sides. never would have noticed if nicolas had not told me to look!
now i have another visual check to make when i see these box shaped capacitors. the sides can bulge! wow

so i guess ill be pulling it off, posting a picture and a reading, and then looking for a new one... any chance i can salvage one from a junk board lying around?