TCL 75S421 New Backlight LED's Still No Backlight

[ChattFix]

Hello everyone, I got a TCL 75S421 with no backlight. I swapped out the LEDs and still getting nothing. I checked a few readings, see pictures below. Can anyone point me in the next testing area or what to look at next?

[nicholas1110]

Hello, you will want to check a capacitor. I have replaced many of them in this power supply. When you remove it you will feel a slight bulge on one of the sides. It is c614a . Hope this helps.

[ChattFix]

Hello, you will want to check a capacitor. I have replaced many of them in this power supply. When you remove it you will feel a slight bulge on one of the sides. It is c614a . Hope this helps.

Thank you for that info. I checked all the capacitors similar to C614A and none have bulged or no continuity. Should I just replace it just in case?

[nicholas1110]

I recommend removing c614a and feel it between your fingers. It’s usually very slightly bulged on one of the sides. Normally you cannot see it until after you have felt it. If you have an exact match for that capacitor definitely just replace it and see if that resolved your issue. Also one thing about this board it is very easy to short out the transistors when testing them. If you have tested them and got any kind of zap from your meter leads, that it’s likely something has shorted.

[ChattFix]

I recommend removing c614a and feel it between your fingers. It's usually very slightly bulged on one of the sides. Normally you cannot see it until after you have felt it. If you have an exact match for that capacitor definitely just replace it and see if that resolved your issue. Also one thing about this board it is very easy to short out the transistors when testing them. If you have tested them and got any kind of zap from your meter leads, that it's likely something has shorted.

I took this off before and just overlooked this tiny spot! Wild but it still doesn't show as open. Now I need to find a replacement!

[ChattFix]

I recommend removing c614a and feel it between your fingers. It’s usually very slightly bulged on one of the sides. Normally you cannot see it until after you have felt it. If you have an exact match for that capacitor definitely just replace it and see if that resolved your issue. Also one thing about this board it is very easy to short out the transistors when testing them. If you have tested them and got any kind of zap from your meter leads, that it’s likely something has shorted.

Is this a good candidate? Not sure what I should be looking for!

[ChattFix]

I recommend removing c614a and feel it between your fingers. It’s usually very slightly bulged on one of the sides. Normally you cannot see it until after you have felt it. If you have an exact match for that capacitor definitely just replace it and see if that resolved your issue. Also one thing about this board it is very easy to short out the transistors when testing them. If you have tested them and got any kind of zap from your meter leads, that it’s likely something has shorted.

This one seems to match more closure to what I have read so far. I'll wait to get confirmation.

[EazyBone]

Did you test the new leds with led tester from the connection that plugs into PSU? I have, to eat crow, done this a few times where I forgot to connect maybe one of the connectors and had to tear it apart again in order to see what I did, and a connector was off or loose. But led tester will show short on the line if so.

[ChattFix]

Did you test the new leds with led tester from the connection that plugs into PSU? I have, to eat crow, done this a few times where I forgot to connect maybe one of the connectors and had to tear it apart again in order to see what I did, and a connector was off or loose. But led tester will show short on the line if so.

to be hones I cant remember if I did or not. I want to say i did lol. Can I test from the LED power line? I know I can't test from the back lol.

[EazyBone]

Yea the power line. Disconnect it from PSU when testing.

[nicholas1110]

The 1000v 333j should be a good candidate to install.

[ChattFix]

Yea the power line. Disconnect it from PSU when testing.

With every line I'm getting 115v with 20amps. Does that line up with what you checked before?

[EazyBone]

With every line I'm getting 115v with 20amps. Does that line up with what you checked before?

Yup they look good then. It would read short otherwise. If people are saying a common failure is that component, then just go and change it, likely will be your fix.

[tech9pc]

Ya these TCL's are very temperamental lol. Done several of them all sizes. Right now working on a TCL 43 Inch 43S446-CA 4K Google Smart TV works great but no backlight and panel is ok. Model only has one led strip.Tested backlight and all leds individually and they are ok. Only has one LED strip 21 x (6V Leds) running at 5.238 Volts each. Measures total 110 Volts powering leds on tester. I powered the backlights seprately with my tester and everythings working when viewing tv on and watching .. but if I power the leds from the power supply board normally the screen just does not light up. So I suspect the power supply is culprit. They are made of really cheap short life components. Ordered one from aliexpress for 22.00 and hopefully solves issue. Will post back when it arrives.

[Diah]

LEDs if they are direct or edge one.. special if edge one with double LED as the case of @tech9pc ... these units build in protection zener diode.. with LED tester tjey will lighting however if the zener shorted or open.... with original PSU they will not--- so don't trust the test only by light... use yours DM in diode mode to check if there zener Diode build in

[tech9pc]

Hi Diah yes you are right thanks for your advice my friend. It's just going to cost more for diodes and stuff like 15.00 or more. I figured I might as well just order the whole power supply for only 22.00 and won't have to screw around removing capacitors and diodes etc. But I will try to repair the old one. Apparently these diodes on power board crap out frequently. Diodes Schottky SB5200, MBR3200 diode. I will see if I can find some off some old boards.

[Diah]

Hi Diah yes you are right thanks for your advice my friend. It's just going to cost more for diodes and stuff like 15.00 or more. I figured I might as well just order the whole power supply for only 22.00 and won't have to screw around removing capacitors and diodes etc. But I will try to repair the old one. Apparently these diodes on power board crap out frequently. Diodes Schottky SB5200, MBR3200 diode. I will see if I can find some off some old boards.

sorry friends.. you went at wrong direction, the protection zener i refer to it is build in at each LED unit.which ,mounted on the strips.... PSU had nothings to do

[tech9pc]

Oh ok thanks for that advice. I did not know about that zener diode protection. Thanks Diah! wonderful advice. I will have to check that out. I will wait for the new board anyways to see what happens, before ripping it all apart again lol.

[ChattFix]

Well thats a $200 lesson for my first TCL backlight replacement. Though I wonder why the screen is still blank? I was just going to use it in the shop but its still not showing a picture.

[EazyBone]

You got new PSU and lights show up now? Cracked screen only sometimes shows picture, not always.

Poor tv, been there done that. I personally take the TV down to the bones again and practice my disassemble when this happens. Take the new lights back off. save for next tv.

$200? What did you buy it for.