Samsung 4K led model UE50NU7445U no picture

[PowerAmpFreak]

Hi all,
I've got an Samsung UE50NU7445U 4k led screen which was stuck in some kind of "boot-loop".
I replaced the mainboard as a first try which did not help.
PSU seems fully OK, both backlight voltages and the mainboard supply is OK.
Now it boils down to the panel itself.
So I started up the set without the two flatcables disconnected between MB and panel. Then no boot-loop anymore.
So I guess the diagnostics detect an abnormality on the panel.
I removed the cover which hide all the panelconnection to the two "buffer" boards (could call it)
There are not very much on these board except for some decoupling caps and what it seems a flash memory.

Any advice?

[Storyteller12]

These “NU” series tv’s suffer from bad panels and backlights. This was just a bad year for Samsung TVs unfortunately. Sounds like yours maybe a panel issue. Have to check all those caps on the buffet boards or try the tape method to see if you get any results.

Good luck and keep us posted

[PowerAmpFreak]

Thanks, what does the tape method mean ?

[Storyteller12]

YouTube tv tape method.

It’s covering the traces from the flat cables going to the lcd panel to try and bypass the bad signals.

[brad2157]

Originally Posted by PowerAmpFreak

Thanks, what does the tape method mean ?

There are better videos out there, but this is just one I found.

https://youtu.be/pfrRhw7-ZII

[dskall]

I would be checking for shorts first.

[PowerAmpFreak]

Independtly what I find in terms of shorts (if not caused by a shorted ceramic decoupling cap) there will not be possible to fix it since the fault is inside the panel?

[Davi.p]

Check shorts on the bigger caps and eventually on diodes/diode packs + big resistors. Often are fixable

[PowerAmpFreak]

Please see the pic in the link below:
https://www.bildtagg.se/bild/b6npwso9b7epr2wky96iqlmq

There is 35ohms across a set of caps on both left and right buffer board.
MB disconnected so they have no connection through.

All the "big" caps measures no shorts (just a diode drop) but as mentioned the "small" in the same repeating pattern across both boards show only 35 ohms which seems faulty.

[Davi.p]

that's a normal value i guess. Sorry, i meant short on diodes / open on resistors..

[PowerAmpFreak]

35ohms cannot be normal, right ?

[PowerAmpFreak]

See picture:
https://www.bildtagg.se/bild/i7z3e4oghexkhx06790k05z7

There is not even half picture if I disconnect one of the ribbons.
Does not matter left or right.
The only difference is:
If one ribbon is disconnected (does not matter which) the set will stay on, backlight on.
If both connected - as soon as BL turns on, two seconds later the set will restart, flashing two times on the standby led.
Anyone having access to service manual for this chassis and could tell what 2 blinks mean ?

[Davi.p]

35 ohm is normal, that cap is for the 3,3v prob., 3,3v/35ohm=95mA that is normal for the cof supply... what about the rest??

[PowerAmpFreak]

The rest are hi-Z so no problem.
But there are not any 3V3 across the "35ohm" line. I suspect the MB shuts it off due to abnormal detection.

[Davi.p]

usually tcon disables supply output with a short, hi-z what? i told also resistors.. are there diode packs? can you photograph outers of the panel boards?

[PowerAmpFreak]

Hi-z means hi impedance so not a problem.
Still I suspect the 35ohm reading across one bank of caps is not normal.
It indicates a faulty COF in my opinion.
There should not be a resistive load of 35ohm across the supply lines.
Diodes packs measures fine.
Post pictures tomorrow when I see the patient again

[Davi.p]

the faults are common on the boosted lines.. Avdd,Vgh

[PowerAmpFreak]

Pics on buffer boards:
https://www.bildtagg.se/bild/3msuyww1tbu5eqhvqlvvaie
https://www.bildtagg.se/bild/o147wdppblsc0u7xiiaom0e

Diode packs visible on board edges

Is it one COF on each ribbon between buffer and panel ?
Do they all share the same supply lines ?

[Davi.p]

Source cofs have probably 3,3v supply, but gate cof/cog can have 1,8v as supply, each source cof receive signals from the pcb.
You must check every big resistor, one of them can be open or higher ohms, that is connected to one diode pack, i suggest you to search to the forum to previous months, this summer some useful info on how to test and source those diodes.. it's a very common fault..

[PowerAmpFreak]

Thanks.
The "bigger" resistors are 100R each measures OK.
BUT - when both bufferboards are connected to MB there is only 74R across one of them.
Indicate some short somewhere.
I have to figure out how to see the pinout of the diode packs.

Is it safe to remove the diode packs and run without them for a short while ?