User Profile

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

in Troubleshooting TVs and Video Sources

01-11-2024, 05:06 PM
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...

See more | Go to post

Last edited by triplefour; 01-11-2024, 05:57 PM.
Leave a comment:
triplefour replied to TCL 75S421 has sound but no backlight not even a flash

in Troubleshooting TVs and Video Sources

01-11-2024, 03:27 PM
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.

See more | Go to post
Leave a comment:
triplefour replied to TCL 75S421 has sound but no backlight not even a flash

in Troubleshooting TVs and Video Sources

11-19-2023, 01:51 PM
Re: TCL 75S421 has sound but no backlight not even a flash

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...

See more | Go to post

Last edited by triplefour; 11-19-2023, 02:01 PM.
Leave a comment:
triplefour started a topic TCL 75S421 has sound but no backlight not even a flash

in Troubleshooting TVs and Video Sources

11-17-2023, 09:54 PM
TCL 75S421 has sound but no backlight not even a flash

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...

See more | Go to post
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-22-2023, 01:10 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

i will have to put this TV on hold for now... gonna order the stronger LED tester. wish someone would give me a recommendation which one to get though!

See more | Go to post
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-22-2023, 01:08 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

75 is huge! ive never opened the panel on one that big. this one did have tape holding down the bottom edge of the panel. SUPER annoying....

See more | Go to post
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-22-2023, 01:07 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

yes sir. it was a joke. please laugh!...

See more | Go to post
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-22-2023, 02:18 AM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

oh nice so just 180 of these diodes in series and i can trick the power supply into thinking the LEDS are there! ...

See more | Go to post
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 11:37 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

if the LED driver circuit on the PSU functions anything like my cheap LED tester then the open voltage of 280v would indicate open string... if the circuit were closed and current flowed, the voltage would drop based on what amperage the LED circuit wants to draw.

my LED tester will appear 320v when probes have no path to each other. when there is a path (through a single, or multiple LEDs) that voltage drops to whatever the forward voltage drop of the LED's is at whatever...

See more | Go to post

Last edited by triplefour; 09-21-2023, 11:43 PM.
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 11:33 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

can anyone recommend a more powerful LED tester? this one seems like a good deal at 50$ for claiming 150ma and it has adjustable outputs as well. do we believe it works as advertised?
[url]https://www.ebay.com/itm/174949839584[/url]Re: UN65TU700DF picture and sound bu... do we believe

See more | Go to post
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 11:26 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

yes im leaning more and more towards this as well. which is why im dreaming of getting a more powerful LED tester.

this one included in the panel tester says it outputs 120ma but it also seems to say it only outputs 30v...which would be pretty useless for any big tv...guess is its included for testing small monitors tvs or laptop screens.

i updated the thread with translated images of the panel tester. seems a useful tool if i ever find a use for it!...

See more | Go to post
Leave a comment:
triplefour replied to General question about LVDS voltages. Blank Backlit Screen.

in Troubleshooting TVs and Video Sources

09-21-2023, 11:22 PM
Re: General question about LVDS voltages. Blank Backlit Screen.

translated images for anyone curious as to what it says on this thing...

See more | Go to post
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 10:21 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

yes i also thought of this as i drove home with the tv in my car... dang it!
ill probably end up taking it apart again
the thing is because of its socketed design, and the way they put the backlights and the paper over the spot you need to get to, i would have had to take all the backlights out to even get to where the socket makes connection with the strips...or theres probably some other easy way that ill have to figure out when im back in there. i have the...

See more | Go to post

Last edited by triplefour; 09-21-2023, 10:36 PM.
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 10:11 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

[url]https://www.badcaps.net/forum/showpost.php?p=1240330&postcount=15[/url]
so i just realized that this tool i bought a while back (but still havent really figured out how to use) has a LED backlight driver in it that seems to claim to do 120ma ... and after looking at my cheapo tester and trying to figure out what ma it outputs at i really cant say. but it looks like similar looking and priced ones on ebay output about 35-50ma ... im not sure where i got the 10ma number from. the thing...

See more | Go to post
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 09:59 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

something wrong with that url i think... and i still cant really be sure whether i have bad backlights or if i was sent a bad PSU...
im thinking i need something that can test the backlights with more current,
or to do like davi.p is saying and try and add some LED's in series,
or wire up some salvage strips of comparable voltage and number...
maybe buy another PSU...(please...anything but this!)...

See more | Go to post
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 09:40 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

yes there are ways to rig this up for sure. and im sure ill try this if not on this tv then on another one. but now im thinking even though i get the 100ma tester, i still could have this problem occur...just if it occurs after 100ma...700ma is quite a lot more than 10ma...i dont think i was appreciating just how weak my little tester is! and my bench PSU can only do 30v5a so ....
for 200$ i could get this one... [url]https://www.amazon.com/NICE-POWER-Variable-Programmable-Adjustable-Regulated/dp/B0C7ZXYGHM?th=1[/url]...

See more | Go to post

Last edited by triplefour; 09-21-2023, 09:43 PM.
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 08:33 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

yes they are cutting costs wherever they can. and the tvs seem to always be panel or backlight failure with no reason why just one day they go to turn on the tv and its a black screen. easy board swaps to fix tv's are a blessing we see less and less of these days

is there any device like a load tester that can simulate an LED string of any length and voltage? this would be a nice tool to test the current output of the LED driver of the PSU without needing to use any...

See more | Go to post

Last edited by triplefour; 09-21-2023, 08:36 PM.
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 08:03 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

i did understand you eventually. thank you. i will try this when i can. in this case there is no pin that can be taken out. its the socket style that just clips in. no screws in this tv at all either! i would have to do something pretty destructive maybe to insert my LEDS here... ill think about it more.
well i guess it could be done if i kept the PSU disconnected from the BL socket and then soldered wires from PSU connector positive to positive, negative to one side of LED...

See more | Go to post

Last edited by triplefour; 09-21-2023, 08:10 PM.
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 07:45 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

i dont understand your question... yes these are the voltages i have stated and these are the voltages i still get with mb unplugged. BL connected. PSU plugged in to AC.
i get 280v across the LED output as well. which is basically open voltage no current flowing. i should expect to see it drop to around 190v if the BL were consuming current.
please tell me if i have an error in my thinking!
thank you for all your help. everyone who posts even if it doesnt...

See more | Go to post

Last edited by triplefour; 09-21-2023, 07:48 PM.
Leave a comment:
triplefour replied to UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

in Troubleshooting TVs and Video Sources

09-21-2023, 07:17 PM
Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

the seller has responded to my message.
my original message was before i took the panel apart. i simply said that i tested the backlights with my LED tester and they appeared ok but the PSU wont power them without the main connected.
he replied one sentence. saying i most likely have a mainboard issue. either he is unaware of the samsung power on self test, or hes bullshitting me, or he has intimate knowledge of this tv and knows that samsung has done something different here and the...

See more | Go to post

Last edited by triplefour; 09-21-2023, 07:52 PM.
Leave a comment: