UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt fix!

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Your in HI, so your going to be hit with shipping charges from almost everywhere, china is the only place that has secured themselves those shipping advantages they were allowed all those years ago when they were deemed a "developing economy" Well poor little china, I guess they're still a "developing economy": https://hbr.org/2020/02/how-much-mon...orld-owe-china

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

That one is 32 bucks: https://www.ebay.com/itm/20244289449...pid=8059995264

[Davi.p]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

You could make a simple cheap tester about 30$ with This
for input diodes you could use 4 1n4007 or an integrated one.. you could power until about 870mA Backlight.. if could be a problem you could limit inrush current with an NTC in series..

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Looking at amazon and ebay, it seems that they are only going to 150 mA and 320v so any I guess one of those, they seem to be about 40-50 bucks or so.

[triplefour]

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!

[triplefour]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Thx davi, yes I fixed that led anyway lol I wasn't gonna mess around. Taking apart a 75 is definitely a challenge. Plus they taped down the screen on 3 sides. Is that really friggin necessary lol.

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.

[triplefour]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

I don't understand if I should laugh...

yes sir. it was a joke. please laugh!

[EazyBone]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Thx davi, yes I fixed that led anyway lol I wasn't gonna mess around. Taking apart a 75 is definitely a challenge. Plus they taped down the screen on 3 sides. Is that really friggin necessary lol.

[Davi.p]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

I don't understand if I should laugh...

[triplefour]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

IF you haven't a led bar you can simulate a led with 3 1n4007 diodes in series..

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

[Davi.p]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

IF you haven't a led bar you can simulate a led with 3 1n4007 diodes in series..

[Davi.p]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

Answer to post #50:
yes, i've already thought about a very cheap repair by adding some extra hidden leds or you can add simply a normal diode with similar Vdrop, who knows? I'm no more in tv repair hobby business so i have repaired few bad leds tv.. i have still one of them of the type "1 out of 10 times B.L. does not lit" to repair.. maybe it's time to fix it..

Answer to post #51:
Eazybone one must know how much is rated your tv PSB output, surely is higher than your tester's 100mA, so the fewer the leds you test, the higher the inrush current them takes, you have in some way activated a sleeping led, but it is still a damaged led and must be replaced..

[triplefour]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

I'm prolly completely wrong with that guess, it's probably that your tester isn't putting out enough current to trigger the bad led so they light up, I don't really know what else would make it test at 280 instead of 192.

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 amps the thing is designed to provide...im not sure how it tells what the forward voltage drop should be so it doesnt burn out the LEDS...does this just happen automatically due to math of the constant current?

so this makes it look like my LED strips have open circuit according to the PSU
but according to my LED tester they have closed circuit. can the circuit open up that fast under "hot" condition (full power) that an LED wont even have time to flash? this is my gnawing question.

[triplefour]

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?
https://www.ebay.com/itm/174949839584

[triplefour]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

I'm prolly completely wrong with that guess, it's probably that your tester isn't putting out enough current to trigger the bad led so they light up, I don't really know what else would make it test at 280 instead of 192.

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!
https://www.badcaps.net/forum/showpo...0&postcount=18

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

I'm prolly completely wrong with that guess, it's probably that your tester isn't putting out enough current to trigger the bad led so they light up, I don't really know what else would make it test at 280 instead of 192.

[triplefour]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

You prolly should have checked for high resistance in the backlight connections while you had it stripped down.

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 tv now so i can take my time with it.

but then again if there was high resistance in that connection, wouldnt my weak tester not even be able to light things up at all?

i reported before that i was getting 100v between pin1 and 2, and 64v between pin 3 and 4 with my LED tester but when i tested it at home i am reading 96.8v with a slight wavering to 96.7v and 63.9v with a slight wavering to 63.8v. not sure if they should be rock solid but they dont waver much. anyway the math checks out: 96.8v/36 LEDS = 2.68v per LED which when i was testing them individually most of them read at 2.6 ... just a few read at 2.7
and 63.8/24=2.65 which again seems about right.

im on the quest to power them with more amps and see if something craps out.
or to find comparable strips and rig them up.
or insert a few extra LEDS in series as per davi.p suggestion.
and/or test resistance in the connection between backlight strips and the socket that connects to the PSU.
im not trying to buy new strips or another PSU before i figure out for sure whats wrong.

[nomoresonys]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

You prolly should have checked for high resistance in the backlight connections while you had it stripped down.

[nomoresonys]

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 help i still appreciate it!

i only come here with the hard problems. the easy ones have been fixed already!

Ok, so you have the bl-on and the ps-on, that shows that YES this tv should light up the backlights with cable from powerboard to mainboard unhooked, there's a possibility that the problem is in a connection of the backlight circuit and not necessarily an led.

[triplefour]

Re: UN65TU700DF picture and sound but no backlight. BL tests ok but repl. PSU didnt f

https://www.badcaps.net/forum/showpo...0&postcount=15
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 is though this LCD panel tester came with all kinds of wires and even some nice probes that would be handy to use for the LED backlight driver but for its output there is a 4 pin connector for which they have given me no adapter to plug the probes into. always something...gotta craft something to make it work!