Samsung UN65NU6900 hot cof's vertical lines

[Kilroy2k1]

Samsung un65nu6900f

I've had no luck scrounging a schematic for this unit so time to ask for community help.

I bought this tv used and shortly after it started doing these vertical coloured lines over the entire screen.
Another symptom is the COF tabs are super, super hot to the point of burning skin (dont ask how i know.. lol)
Research has shown this was a common fault but I could not even locate the service bulletin that was mentioned.
I'm hopeful someone has experience with this problem.

voltages are as follows:
VCC : 3.2v
VCOM1 : 5.55v
VCOM2 : 5.67v
VCOM_REF : 5.5v
VGL1_-5V : -7v
VGL2_-10V : -14.9v
VGH1 : 27.9v
VGHD : 0.0v
VDD_16v : 16.7v
VCORE_1.2V : 1.12v
Pannel_13V : 12.75v

The only issue I can see are the VGL 1 and 2 are a little on the high side.
Also the RT6950 chip that generates these voltages must be made for samsung as Richtek will not supply the datasheet so I cant verify any of that circuit.
I hope I covered all the details, anything I missed let me know.
Thanks
Tom.

[Nick's Tvs]

Re: Samsung UN65NU6900 hot cof's vertical lines

Based on symptoms I would put money on the panel being bad. This is however something that can be fixed if the failure is located on the address boards.

It would be nice if you could provide us a picture of the lines you are describing. The way the lines display on screen does matter and can point us in one direction or the other, so it would be nice to get a visual.

Now on the address boards, you will either have a shorted SMD capacitor, burnt SMD resistors, or an IC malfunction.
The first two are easily repairable.

Disconnect the panel from the main board, and check the address boards for the above mentioned. I would start with resistors as those are going to be easy to spot visually. Just do a careful visual check on all the resistors and see if any look burned. They will be obvious and will fail open meaning they will have much higher resistance than they should.

If no burned resistors are found, then check the address boards for shorted capacitors. If you find a shorted SMD ceramic cap, you will find most likely at least half a dozen others in parallel also showing a short. That however doesn't necessarily mean all of them are shorted. We usually carefully measure each cap very carefull and keep our leads on the cap for at least 10 seconds and notate exactly to the hundredth of an ohm the resistance of the cap. Once you do this for all of them, find the one with the lowest of the readings and remove it. That will be the faulty one. If multiple capacitors have identical readings, do a thermal test. Aka put your finger on all of them and see if any of them get hot. The one that gets hot is the bad one.

If no burned resistors or shorted capacitors are found, then you may either have a bad IC, or I could be completely wrong and you may have a bad main board.

[Kilroy2k1]

Re: Samsung UN65NU6900 hot cof's vertical lines

I forgot to add the detail in to what extent I have checked things out.

I'll try to get and post a picture of the lines in the morning.

I have gone over the address boards with a fine toothed comb, no shorted caps or open resistors. There is very little logic on the boards at all, just an eeprom and opamp i believe.
It is all the cof's that get seriously hot and once they get hot enough the lines will appear.
Thanks

[Davi.p]

Re: Samsung UN65NU6900 hot cof's vertical lines

You forget VGH2.. isn't there? is VGHD connected to GND?

[Kilroy2k1]

Re: Samsung UN65NU6900 hot cof's vertical lines

Sorry for the delay,
I dont find a VGH2 marked anywhere and I dont have schematics available to me so no idea if it even exists.
VGHD is tied to ground yes.

[Davi.p]

Re: Samsung UN65NU6900 hot cof's vertical lines

Check D401/D404, i suppose D401 is connected to L401, if you can't figure out a problem, unsolder D401 and place on its cathode a tension of -10v with an insulated psu or regulated battery output and check the image. Check the resistance of VT404, i don't know what it is maybe a varistor, check resistance of R401-R402. If VT404 is not infinite unsolder it hand test the negative tensions.

[Kilroy2k1]

Re: Samsung UN65NU6900 hot cof's vertical lines

Check D401/D404, i suppose D401 is connected to L401, if you can't figure out a problem, unsolder D401 and place on its cathode a tension of -10v with an insulated psu or regulated battery output and check the image. Check the resistance of VT404, i don't know what it is maybe a varistor, check resistance of R401-R402. If VT404 is not infinite unsolder it hand test the negative tensions.

I'll try and got to that tonight or tomorrow, i got some work in and of course the customer is in a panic. lol
Welcome to Monday!
Thanks for the direction and I'll get back with results soon.

[Kilroy2k1]

Re: Samsung UN65NU6900 hot cof's vertical lines

Sorry for the delay, real life keeps getting in the way. lol
Here is the photo as requested but i have not had a chance to do your other recommendation yet, I hope to get to it by tomorrow.

[Kilroy2k1]

Re: Samsung UN65NU6900 hot cof's vertical lines

Check D401/D404, i suppose D401 is connected to L401, if you can't figure out a problem, unsolder D401 and place on its cathode a tension of -10v with an insulated psu or regulated battery output and check the image. Check the resistance of VT404, i don't know what it is maybe a varistor, check resistance of R401-R402. If VT404 is not infinite unsolder it hand test the negative tensions.

I had some time to burn so i pulled out the meter, D401/404 test ok, 0.74 fw voltage, VT tests at 1.5K R402 also 1.5K. R401 Dead shorted! I removed and tested R401 and shes shorted. now for the fun part, finding its replacement value.

Anyone have a BN41-)2670A board around that can measure R401?

[Davi.p]

Re: Samsung UN65NU6900 hot cof's vertical lines

I think VT is a varistor and must show open, but if you test in circuit you are measuring R402 instead so VT404 is ok, it's hard to say but try to swap R401 with a 1,5k, don't throw it away, could be a zero ohm.

[Kilroy2k1]

Re: Samsung UN65NU6900 hot cof's vertical lines

I tried a 1.5K and then a 10K and it made no difference in the voltages.
I'll try and hook it up to a negative supply today and see how that goes.

This would be so much easier if Samsung would put out the schematics or even if richtek would release the datasheet for that chip.

[Davi.p]

Re: Samsung UN65NU6900 hot cof's vertical lines

I was searching for the resistor divider but it's not easy to find it. Otherwise the image don't tells me of a supply problem because the line colors are repeated in a pattern, so this brings me more to a logic problem, maybe ther's a flash eprom for the tcon part of mainboard, might be checked with a programmer and a dump from the web (russia)..

[bohaboha]

Re: Samsung UN65NU6900 hot cof's vertical lines

Hi Kilroy2k1 i had the same problem with a samsung 46 and i was advised by a member here to reflow the big chip and the 6 small ones around i have tried with it it worked for 10 mn because i did not let the mainboard cool down as it have to do.so now i still have the same problem and tv is waiting to be fixed.you can see in my posts

[Kilroy2k1]

Re: Samsung UN65NU6900 hot cof's vertical lines

I'll be looking for a board to swap in for testing and see if there is any difference.
I had one lined up on ebay, but it was the wrong sub model, Lucky the seller was sharp and caught the error before sending me the wrong board.

[uniram]

Re: Samsung UN65NU6900 hot cof's vertical lines

I know this thread is 6 months old, but Kilroy2k1, did you ever get any further on this one? I have the same issue and am about to begin trying to see if I can solve it. I started another thread before seeing this one (https://www.badcaps.net/forum/showthread.php?t=93917).

One difference I have is that the vertical lines on mine had a larger strip in between the repeating pattern. However, I no longer have anything but a black screen. I must have accidentally shorted something when testing. I have a new mainboard coming because I thought that was the issue until finding this thread.

I am going to go through this thread and the other one about the hot COFs and test all of the parts mentioned and see if I can get anywhere.

I am also attaching pictures of the 4 boards, labeled L1, L2, R1 and R2 from left to right. None of my COFs look like they have been damaged by heat.

[ReeceyBurger123]

Re: Samsung UN65NU6900 hot cof's vertical lines

I’d start by checking all ceramic caps on the PCB for S/C they seem to be very common these days on panel driver boards.