SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

[triplefour]

TV flashes backlight then reboots over and over. no image. Client said it used to have image for a bit before rebooting. Got worse over time.

tcon is built into mainboard.
Only one cable goes to panel boards.
there is a cable from one panel board to the other that links them.
cost cutting measure that results in us having harder time to diagnose.
removing the link to the second panel board allows TV to stay on with stable backlight but no image.
used cut off tape method to mask 1 maybe 2 pins on the side of the cable that goes to that second panel board. maybe pin 11 or 12. that causes TV to come on and remain stable but still no image. TV has some voice feature that talks and says something about setting up the tv.
checking panel boards I see no 12v coming in.
checking on the mainboard I see no 12v on the caps close to the lvds cable or on the coils around there, or on the voltage regulator.
seeming like it must be a main board problem but do I also have a panel problem since I was able to use cut off tape method to gain stability? Why does having the second panel board plugged in make it reboot? if there's no 12v going to the panel boards anyway, why should masking a pin do anything at all to change operation?
there is no 12v but the tiny voltages that carry the picture information are there and I assume the clock lines too. some error there maybe tells the main board to shut down?

This is the newest tv I've been called to work on. Made in 2020. Shame on samsung. I see evidence of the challeges to come for us as they keep cutting costs and making things easier to break and harder to fix. The back of this TV had no screws which might be nice if it wasn't such a pain to separate all the clips. Ends up being more work in the long run. The tcon built into the main, I have never liked, but only one cable to the panel and daisy chaining the panel boards makes it so u can't easily test left and right of the panel by disconnecting a cable. You can only easily test the first board. Oh and the power supply being directly connected to the back lights with no wires is probably another issue that could make diagnosis harder in some cases. Whew!

[dskall]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

Not sure the t-con is on the panel. Probably a short on board or panel causing reboot.

[nomoresonys]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

do you have a steady backlight with cable from powerboard to mainboard unhooked? what is the discoloration on the pins on picture 6?

[Topcat]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

This is the newest tv I've been called to work on. Made in 2020. Shame on samsung. I see evidence of the challeges to come for us as they keep cutting costs and making things easier to break and harder to fix. The back of this TV had no screws which might be nice if it wasn't such a pain to separate all the clips. Ends up being more work in the long run. The tcon built into the main, I have never liked, but only one cable to the panel and daisy chaining the panel boards makes it so u can't easily test left and right of the panel by disconnecting a cable. You can only easily test the first board. Oh and the power supply being directly connected to the back lights with no wires is probably another issue that could make diagnosis harder in some cases. Whew!

I've got a 65" and a 70" samsung with the screwless backs I just haven't been brave enough to try breaking open....and a 43" that I never could figure out what was wrong with....ended up scrapping it. Samsung has taken a big dump the last few years with TV build quality.

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

do you have a steady backlight with cable from powerboard to mainboard unhooked? what is the discoloration on the pins on picture 6?

Yes back light tests good. And that's not discoloration it's the piece of tape and plastic that I used to mask one of the pins on the main board side of the cable. I colored it black to make it easier for me to see as I inched it along, looking for the pin that when blocked, produced stability. I did this thinking I had a panel problem but I hadn't tested voltages going to the panel yet. Should have done that first before chasing that..masking pins is pretty tedious. But it did yield an interesting result that I am puzzled by. (It stays on but with no image) Maybe it's the error pin that I blocked!
Clearly tho, the mainboard not producing the 12v to run the panel is the major problem I need to solve first. The client probably prefers to spend the 45$ for a new mainboard and skip the time it will take to chase this one down. He would give me the old dead mainboard but it would probably be hard to repair without the matching power supply...

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

I did some more poking around and discovered that 12v goes into the IC marked G4435SS but doesn't come out the other side. I see 12.9v on pin 1,2,3 and 12.8 on pin 4 which I can't quite figure out how it's getting that. That pin 4 has high resistance between it and 1,2,3 and looks separate like it's the gate or something. Datasheet time.

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

Hmm according to the datasheet if I'm reading it right, a typical threshold voltage at the gate would be -1.7v so a much higher voltage which +12.8v definitely is would keep the transistor from switching on, thus no voltage seen at the drain. Am I correct in my thinking?

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

I get 150k resistance between the source and gate so it's not like the gate is getting the 12.8v from an internal short...but I'm having trouble seeing where it comes from. Trouble tracing these tiny capacitors and resistors. Is there any chance I'd be able to find a schematic ?

[dskall]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

Can we get some good picture of panel boards??

[dskall]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

Hmm according to the datasheet if I'm reading it right, a typical threshold voltage at the gate would be -1.7v so a much higher voltage which +12.8v definitely is would keep the transistor from switching on, thus no voltage seen at the drain. Am I correct in my thinking?

Still learning, but I think on PNP that gate will need to be negative (-1.7v) in respect to source

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

Still learning, but I think on PNP that gate will need to be negative (-1.7v) in respect to source

If voltage is with respect to the source pins, the difference there is only .1 volt so I guess that would be -.1v at the gate?

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

Can we get some good picture of panel boards??

Here's some pics

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

well its officially a nightmare TV. Client ordered the replacement Mainboard and it is behaving exactly the same way. no 12v coming out of it to the panel. But since i was hasty and not paranoid enough, i installed the replacement mainboard before making any resistance checks, and installed the panel cable before making any voltage checks. so who knows. maybe it DID produce 12v but something on the panel fried the new Mainboard? is that even possible?
or is it possible the guy on ebay sent a part with the exact same problem even though he claimed it was fine?
both questions I would have a better answer to if i had made some checks before fully powering up and fully connecting.
I MUST STRESS MYSELF TO REMEMBER THIS IN THE FUTURE.
but anyway now i am back at square 1. i have 2 mainboards that seem to be either both fine, or both broken in the same way.
what could possibly be going on here? how could a problem on the panel keep the mainboard from even generating the 12v to go to the panel? doesnt it need that before it can even detect an error?

oh i guess i should note that the replacement part we ordered did not have a picture of the tv displaying any image. the seller is not the most thorough with regards to that but i have got 2 good parts from them in the past. there is a chance this third part is just broken in the exact same way....

seriously hoping someone has some insight here!

could it be that the 12.7v i saw on the gate of that IC which needs to be brought low, gets brought low by some connection thru the panel cable to a ground on the panel itself, and that connection somewhere on the panel has severed?
i tried tracing where that gate pin gets its signal from and i could only get as far as the 2 tiny mlccs next to it before it disapears. im in diode mode with one probe on that pin and just caressing every pin and solder blob i can think of on the board and dont hear any beeps except on those 2 caps, which are not shorted.

am i crazy?

[diif]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

Remove the cable between the power supply and mainboard, do the back lights come one and stay on ?
If yes, what are the two voltages on the test points next to the backlight connector ?

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

i dont believe i have a backlight problem. the backlights come on and stay on just fine if i remove the panel connector. also sound can be heard from the tv, and the mainboard functions fine for on/off
i am not getting 12v out of the IC1603B_UT which i am trying hard to trace at the moment. will upload a picture with measurements soon.
ill check those voltages u mentioned as well. thanks.

[diif]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

What's the 8 legged IC on the driver board ? a winbond eeprom ?

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

heres some voltages. does the 12.8 come from the 12.9 thats on pins 1-3 going thru that mlcc, and ending up at pin 4? the 12.8 disapears right there on the other side of what measures in circuit as a 30 ohm resistor just to the right of the tiny cap. i say disappear meaning it doesn't go to ground right there. if it went to ground then it would say 0v. its 12.8v there and i cant figure out if thats where the 12.8 comes from or thats where it goes, somewhere? via a via?
the mlcc in question (the tiny one on the left that has 12.9 on the bottom side and 12.8 on the top side)
does not measure short or low resistance. it measures quite high. well just shows a high resistance for a second before it goes infinite...which is what i expect a cap to look like.

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

Remove the cable between the power supply and mainboard, do the back lights come one and stay on ?
If yes, what are the two voltages on the test points next to the backlight connector ?

backlight stays on with mb disconnected.
12.9v at those test points

[diif]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

The other voltage? The lack of coils on the driver boards suggests there won't be 12v on there.

[triplefour]

Re: SAMSUNG UN50TU8000F rebooting, backlight ok. No 12v to panel boards.

The other voltage? The lack of coils on the driver boards suggests there won't be 12v on there.

im having trouble following you. what do you mean by driver boards? the panel boards? yes there are no coils i can see on them. those are on the mainboard and they are not producing 12v. most likely because they are not receiving it from the other side of that IC ive pointed out. which is at 0v and most likely the output of the IC is at 0v because the gate pin is at 12.8v when according to the datasheet it should be at 1.7v or something. im trying to figure out why im getting 12.8v at that gate pin.
aside from any of that though, back to basics:
shouldnt 12v be going to the "panel driver boards" on one or more of those pins on the panel connector?