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

[triplefour]

TCL 75S421
PSU BOARD: 40-L402H8-PWD1CG
it shows not even a flash of backlight when powering up.
but the tv appeas to turn on and i can hear the sound of the menus moving and clicking. i can power down the tv with the button.
testing the backlights with my tester all 4 strings lit up and showed similar voltage drops. around 110 or so.
unplug mb and leave only psu plugged into backlights (PSU + BL SELF TEST MODE?) does not light the backlights.
i do see 24v but 0v on PSON and BLON (i didnt write this down tho so cant be sure)
with mb connected, PSON is
2.4v both before and after pressing power button. this seems odd. i expect 0v when in standby. also not sure if 2.4v is correct. older tvs were always around 4 or 5 volts but newer tvs use lower voltages for their turning on signals?
BLON is 0v in standby and 3.2v after power on
ADIM goes from 0.3 to 2.3
3DEN stays at 0 (3d not enabled i guess)
PDIM goes from 0.8 to 3.22v
on the backlight connector there are 4 strips.
4 pairs of black and red.
the top 2 pairs dont read any voltage
the bottom 2 pairs show 0.6v after the tv powers up.

all signs point towards bad backlight circuit in the PSU
but ive been burned here before just recently on a samsung tv where i replaced the power supply but still no backlights even tho i tested them ok with my tester. so i have to point out any areas of doubt:

1. my LED tester doesnt output enough amps to fully test the backlights. maybe they only fail under the heavy current that the actual working PSU puts out.
but i would expect that if my tester can make them light, that i would see them at least light up a little before failing from the PSU's higher current. i should see some kind of backlight flash. maybe im wrong about this?
it sure would be nice if someone could recommend a stronger backlight tester to save my pennies for.

2. maybe the PSON voltage being 2.4v whether power on or off is some indication that the mainboard is not giving the correct voltage to fully turn on the PSU or the Backlight? does this model tcl have the power on self test that tests PSU and backlights without MB connected, or is that a samsung only thing? it kinda looks like a vizio tv to me with the way the back comes off but who knows.


3. need service manual, schematic, other working tv, or someone with experience to tell what the expected voltages would be there

4. i forgot to check/record voltages with mb not connected
mainly i wonder if that 2.4v is there all the time even with no MB

next steps are:

look for service manual/schematic
make badcaps forum post (CHECK!)
lookup IC's and map out circuit / test components related to backlight circuit

board looks to be in the 80 to 120$ range on ebay so replacing the board if it isnt the problem would be hurtful. shopjimmy is out of stock for just the power supply board by itself but they have 8 in stock of complete parts repair kits that include all boards for 175$. PSU board by itself is 80$ ... i wonder if they would break one of the sets up for me to just buy the PSU. couldnt hurt to ask i guess.


any guidance here would be much appreciated. happy repairing, everyone!

[EazyBone]

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

I know I've repairs some 65s on this model and bl not working did not give a flash either. Give a picture of the PSU. On the 70inch model there was a cap that always failed right by the BLs. I would test the BLs on this model and they all would light up. Maybe there is something similar to this model.

Picture please. Also, does TV show dim picture with flashlight? I didn't see you mention that

Edit, I just googled a pic of it, check those 4 caps by the BL connector for any bulge.

Maybe Sony can chime in, he suggests heating caps with a hairdryer and turning TV on while they still warm.

[EazyBone]

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

I also started to reread, no disc MB from PSU doesn't do self LED tests like Samsung with these tvs, that I've found so far anyway.

Also the voltages as you say for PSU to LEDs seem like a problem. I think you are on the right track to having a bad PSU circuit with BL. I had a Samsung literally yesterday that had this problem (disc main board for self test, no flash nothing) and it was 3 caps for LED line were bulging. Replaced and started right up. Those psus look expensive for this TV so it'd be better if you could replace caps of that was problem.

Pic shows what the caps looked like that were bad. They can also go bad without bulge I hear. Not sure if you can see bulge in pic

[nomoresonys]

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

Could be backlights, there's the joy of changing them out: https://www.youtube.com/watch?v=MlJu1udNp1U

[nomoresonys]

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

I would try to check everything else out that can be checked first, solder joints, diodes, resistors, transistors, maybe the backlight driver IC, and check the backlight voltages when you turn on and off the tv, should be an indication in the voltages if one or more are bad.

[EazyBone]

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

That's the perfect video lol I hope it's not the backlights. Changing 75 inches ain't the easiest.

You think if he hit those 4 caps sony it would possibly do something?

I may have my very first BL failure that the LED tester lights up myself. I bought the PSU and lights just so I can know for sure since it was so cheap.

[nomoresonys]

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

Well those caps look like chang or sum shit, I wouldn't think it a bad move to change them out with good ones but might not be the problem.

[nomoresonys]

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

I think that backlight testing, where you turn tv on and off and check those voltages is prolly the easiest and pretty damn accurate way of telling if there's a backlight led problem, Budm was always explaining how to do it, that man sure had a lot of patience. The backlight driver IC should not be overlooked when testing.

[nicholas1110]

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.

[EazyBone]

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.

That's what I was thinking! Let's see what he says

[R_J]

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

That C614 is the transformer return capacitor, some Samsung power supplies had that same issue. It goes open and the transformer primary has no AC path to ground.

[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?

[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.

[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.

[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]

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.

[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

[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]

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]

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