Samsung UN55MU8000 - Backlight But No Picture Problem

[KYBOSH]

Hello all

I pick this up free as a repair project for the holidays.
The original symptom was power cycling after 3-4 seconds with a single red light flash.

To remedy this I went through and unplugged each component 1x1 and discovered that when the FFC cables were disconnected the backlights stayed on and the TV did not reboot.

I tried remove one or the other to see which side of the display was causing the issue (expecting to see half a picture to show) but didn't see anything. But the boot loop only appears when the top FFC cable (right side of the display) was connected. I tracked down the offending/shorted tracks to the first 2 traces. I taped those off and reconnected everything. No reboots but no picture.

This unit comes with a One Connect box. With these models when the One Connect box is not connected we get a warning image upon telling you to connect the box. There was no warning image... When I connected the One Connect box I still didn't see anything but on a whim I started pressing some buttons on the remote control (specifically the menu button) and I started hearing input sounds as I scrolled thru the icons.

This told me that the one connect box was functional.
It also told me that the mainboard was at least semi-functional.

I read a few threads and watched a few videos about panel display issues, COF, ect. Unfortunately similar repairs to the MU series has not been document on these forums.

I went through every component on the LCD Panel PCB and found only one shorted capacitor (highlighted in RED in the pics). No other SMD was shorted. I also when through and tested for voltages across the panel board while the TV powered up and only saw voltages of 3.3v in a few places and ~1.8v in others.

I cannot find any place on these LCD Panel PCB marked VGH of VGL. I cannot find any place on the panel board OR the mainboard where 25v is present or supposed to be supplied.

This is my first go at LCD Panel board repair. I want to learn about it but this TV seems different than the ones I am seeing on YouTube. I speaking directly to the lack of a marked VGH test point.

I wanted to consult you good ppl of BadCaps before I did anything other than probing.

Obviously I'd want to removed the bad cap.
Should I replace it or leave it open?
Would that alone be the cause of a No Picture issue?

Below are some pics of what I am dealing with and seeing.

Your input is greatly appreciated!

[Biruslapio]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

remove the shorted capacitor, if the short is gone on the board, it should just need a replacement and then be back to working state.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

remove the shorted capacitor, if the short is gone on the board, it should just need a replacement and then be back to working state.

Thank you very much @Biruslapio.
Would you have any advice on what size cap this should replace it with?

[nomoresonys]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

yep, remove the cap, test it off the board, should be short, can try the tv with cap removed, just maybe dont let it run a long time, replace cap, should be able to grab one off a scrap board, put one in that is the same physical size, then see if its cured.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Okay I went back and checked that one cap that I said was shorted but it wasn't shorted.
I always like to measure twice and cut once! Glad I did.

I believe I was also inadvertently touching the top of the cap right below it with my test lead. When I checked it again this morning it read short intermittently. I adjusted my probe and there was no short.

Now that I have eliminated that cap as a suspect I dont have any bad SMDs on this board that I can tell.

What should my next step be?

[nomoresonys]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

seems like a short, maybe try the tape cut-off method on that right side.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

seems like a short, maybe try the tape cut-off method on that right side.

Thank you for the response.

I've been looking at the Tape Cut Method but TBH I have not found a resource which tells me exactly what Im supposed to be cutting and what Im not supposed to be cutting. It often seems like he ppl who make the videos dont know either. They just learned to cut a little here and here and there. I base this on their lack of explanation.

Seeing that cutting is pretty irreversible I'd like to learn as much as I can so I can avoid making a mistake.

Would you (or anyone here) have know of a good place to learn about this (links to webpages, videos, etc).

Thank you in advance!

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Would someone help me identify which testpoints on my LCD panel PCBs are the equivalent to VGH and VGL?

[nomoresonys]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

No, tape cut-off method is just taping off/blocking one or some of the pins of the connector to block the bad signals, no cutting with this method.

[mmaven806]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

On the main board, next to D401 is the VGH test point. Should be about +30vdc. Then check AVDD test point next to D402. ( dont short test probe to heatsink).
I dont see VGL (-10vdc) test point. It may be on back side of board.
Hope this helps.
Oh yea, VGL might be labeled VOFF.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

No, tape cut-off method is just taping off/blocking one or some of the pins of the connector to block the bad signals, no cutting with this method.

Thank you for clarifying @nomoresonys.
The cut-off tape method worked for me to get the TV booting properly but that's all. I think in order for the method to work you need to have at least one of the LCD panel PCBs producing a picture. This way you know you are going in the right or wrong direction. I get no image with either FFCs connected. There could technical be a short/problem with both sides but how to know for sure (without at least a flicker of an image) is hard to fathom.

On the main board, next to D401 is the VGH test point. Should be about +30vdc. Then check AVDD test point next to D402. ( dont short test probe to heatsink).
I dont see VGL (-10vdc) test point. It may be on back side of board.
Hope this helps.
Oh yea, VGL might be labeled VOFF.

Thank you for this @mmaven806! Where exactly is D401? I see D402, D403 but dont see D401. Could it be under the heat sink?

Edit to add: I just found it! Its towards the bottom (of the picture) next to the big IC401. I will be testing it shortly.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

@mmaven806

I have found the two diodes you mentioned but dont see the nearby test points you indicated. I tested all points in the vicinity of D401 and didn't get any voltages. I extended my probing to farther components and test point but didn't see any voltages greater than 1v-3v. This was both with and without the panel's FFCs connected. Can you show me exactly where Im supposed to be testing for voltages?

[diif]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

VDD _17V is just to the left of the diode you have highlighted.

[mmaven806]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Yes, avdd to on left side of d402, little solder ball sitting on top of pad or via.
Vgh is above d401 (top of picture).

[mmaven806]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Opps, I meant d410, not d401.
Stupid fingers.

[Storyteller12]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

If tape cut off method go the tv booting correctly you just need to keep moving the tape until you get a picture. It’s trial and error here.

Unplug 1 side of the lvds cable at a time and see if the tv boots normal (still no pic).
If it boots normal then the cable that’s unplugged is the one the tape needs to be on.

Good luck

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Thank you one and thank you all for coming to my aid.

I found the test points you all mentioned and measured them for voltage.
I didn't get a single volt on either of them.
I haven't found the VGL test point yet but I will flip the board over and have a look.

I tested the voltages under different conditions:

1) No FFCs or One Connect box connected.
2) Only the bottom FFC connected without the box.
3) Only the top FFC (the one where I taped off the offending traces) connected without the box.
4) Both FFCs connected without the box.
5) Both FFCs connect WITH the box connected (ensuring I hear a OSD sounds).

I unplugged the TV every time I changed a configuration allowing the system to do a full reboot.

I did not remove the 2 traces on the top FFC for this test as that would undoubtedly send the TV into a boot loop.

I used the foot/mount at the bottom of the board to ground them.

No voltages were seen on either VGH or VDD test points.

I assume these voltages are to be always present on the mainboard regardless of what's connected to it. Is this proof the mainboard is bad or did I miss something here?


Edit To Add:
I added pics of the back of the mainboard.
One of the whole board and another of the small chain of test point islands in the middle.
On first glance I didn't see VGL.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Just wanted repose the question just in-case it got lost in the details and explanation in my previous post.

Does NO VOLTAGE on VGH and/or VDD at the mainboard indicate a fault in the mainboard or are those voltages waiting for some signal (perhaps from the panel) to initiate?

Thanks in advance!

[mmaven806]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

I'll offer an option, if i'm too far off base, someone please set me straight.
Samsung tends to use the same design (ckt), from board to board, that said.
On your main board, ic400 appears to be the pmic, I 'believe' it gets two voltage inputs (main).
One is VIN_12V, it supplies 1.8v, 1.9v and 3.3v (panel).
Second is HVIN_12V, it originates from the VIN_12V, and supplies the havdd, avdd, and vgh.
The two "4435" fets, it believe it was q211 and q2002 on your board supply these voltages to the pmic.
Check the voltage at the drains of the fets you should have 12vdc on both, however I'm betting you don't on one of them.
I'm not sure yet where the enable signals come from, I was thinking maybe fb_trdy_x, but those traces seem to lead to the processor, (on my boards at least).
I also just read that a corrupted rom could cause the processor not to enable the pmic outputs, this is what I am looking into currently on my boards.
Again, just my thoughts.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

I'll offer an option, if i'm too far off base, someone please set me straight.
Samsung tends to use the same design (ckt), from board to board, that said.
On your main board, ic400 appears to be the pmic, I 'believe' it gets two voltage inputs (main).
One is VIN_12V, it supplies 1.8v, 1.9v and 3.3v (panel).
Second is HVIN_12V, it originates from the VIN_12V, and supplies the havdd, avdd, and vgh.
The two "4435" fets, it believe it was q211 and q2002 on your board supply these voltages to the pmic.
Check the voltage at the drains of the fets you should have 12vdc on both, however I'm betting you don't on one of them.
I'm not sure yet where the enable signals come from, I was thinking maybe fb_trdy_x, but those traces seem to lead to the processor, (on my boards at least).
I also just read that a corrupted rom could cause the processor not to enable the pmic outputs, this is what I am looking into currently on my boards.
Again, just my thoughts.

Thank you so much for your indepth response and suggestion @mmaven806.
Please forgive me that it took so long to reply but I got back from visiting family.

I took time to study your response and look for each component/test point you mentioned.
I was able to find most of them but the two that eluded me were VIN_12V and HVIN_12V.
I even took the heatsink off but could not find them.
They were not marked on either of the LCD Panel PCBs either but I went ahead and tested each point.

With the TV on and all cables/ribbons connected I got the following voltage readings.

Panel_3.3V = 3.3v
AVDD_17V = 0v
HAVDD_3.5V = 0.v
VGH = 0v

Q211 measured 12.72v on all leads (both sources and drains) except for Pin 4 (which is supposed to be Ground) and that measured 6.35v.

Exact same behavior for Q2002.

This means something, somewhere is shorted right?
Where would I begin looking?

Please see attached diagrams and thank you again.