Samsung UN55MU8000 - Backlight But No Picture Problem

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

How can you check a pmic? maybe pmic has internal registers and those are cleared... need mainboard replacement, but on the market there is a gadged of a tiny board reproducing all standard trimmable tensions of a lcd display.. this only for testing purposes... china market..

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

You don't need to supply any power, you just need to put a jumper between FB_TRDY_1 and FB_TRDY_3.

Okay I am back with voltage readings after jumping FB_TRDY_1 and FB_TRDY_3.

AVDD_17V = 0V
HAVDD_8.5V = 0V
VGH = ~0V (actually it was 0.065v
STVP1 = 0V
Panel_3.3V = 3.3V

There were a few other test points I hadn't noticed or paid attention to before so I took their measurements anyway.

SFC1 = 1.85V
VCCA_1.9V = 1.9V


If I recall correctly there is no change from before jumpered the FB_TRDY_1 and FB_TRDY_3.

Does this mean that the T-Con portion of the mainboard is non-operational?
Is that a firmware issue? I remember checking all the PMICs and they seemed okay.

[lotas]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

You don't need to supply any power, you just need to put a jumper between FB_TRDY_1 and FB_TRDY_3.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Who told you that the node should apply 3.3v to the FB_TRDY_1 point?

https://www.badcaps.net/forum/showpo...3&postcount=74

My apologies! The way i read the conversation between you and Davi.p was he mentioned injecting 3.3v to the T-CON test point but you came in and said it needed to be injected to FB-TRDY 1 & 3. So i take that there is no need to supply voltages anywhere for this particular test.

So when i get home i will connect FB-TRDY 1 & 3 together and check the test points again.

Thank you for the clarification!

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

sorry i have read so but never tested, anyway no problem, no damage, sure enough...

[lotas]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Who told you that the node should apply 3.3v to the FB_TRDY_1 point?

https://www.badcaps.net/forum/showpo...3&postcount=74

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

So I went in and added 3.3v to FB_TRDY_1.
I used the Panel Testpoint which has 3.3v and verified that it was getting to the correct pin.

After powering up the set and using the remote control to get to the main menu (via sound prompts) I tested AVDD_17V, HAVDD_8.5V, VGH and STVP1.

They all read 0V.

Just want to confirm that doing just FB_TRDY_1 is correct. I think this would simulate having just 1 LVDS cable connected which should still work as we often test these TVs with either panel cables connected.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Fantastic tech support @davi.p amd @lotas!
I will go in and jumper these 2 points tonight and report the results!

Invaluable knoweldge and insight. I really appreciate it.

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

good

[lotas]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

search if a test point named tcon ok is present and tie it to 3v

No, you need to connect the points FB_TRDY_1 and FB_TRDY_3.

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

search if a test point named tcon ok is present and tie it to 3v

[lotas]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

To run T-Con without a panel, you need to put a jumper on the motherboard (imitation of the panel).

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

this could be one of two causes , the mosfet switching the 12v is not functional, but rather difficult, the pmic is turning down dc dc converters due to short on panel, i vote for this

But wouldn't I see voltage if LVDS cables to the panel were disconnected? Since the PMIC no longer detects a short (bcos nothing is connected).

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

this could be one of two causes , the mosfet switching the 12v is not functional, but rather difficult, the pmic is turning down dc dc converters due to short on panel, i vote for this

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

maybe those voltages are not important, but there possibly is a mosfet on the mb that switches the voltage for the panel at a certain point of the initialize, must be check.. it probably switches 12v

I believe you.
Ive gone through and tested all the voltages at the common test points. I still haven't discovered why AVDD, HAVDD and VGH are all 0 volts. Must be a mosfet but I haven't found any that are shorted. I haven't identified which mosfet(s) are responsible for sending these voltages either.

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

maybe those voltages are not important, but there possibly is a mosfet on the mb that switches the voltage for the panel at a certain point of the initialize, must be check.. it probably switches 12v

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Which tension do you have on these points?
for the spi eprom you have to measure each one of the 2 spi eproms on the vcc pin in standby, the one related to tcon is the one not fed.

If by 'tension' you mean voltage then I have only seen voltages of 3.3v in a few places and ~1.8v in others on the LCD Panel board. I have not measured the voltage at that connector (on the mainboard) only at certain points on the panel board. I can test them if you think the LVDS cable maybe an issue.

Thank you for pointing out how to ID the t-con BIOS chip. I will power the TV see if I can get it to go into standby mode. The bios chip maybe under the heatsink as I dont immediately see them in the picture.

[Davi.p]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Which tension do you have on these points?
for the spi eprom you have to measure each one of the 2 spi eproms on the vcc pin in standby, the one related to tcon is the one not fed.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

Well i hesrd from @lotas and he tells me that the chip that i pulled off the LCD panel board woudlnt have anything on it that would prevent the panel from displaying a picture. If i understand it correctly thst chip is used to store calibration/corrections wheb its manufacturered.

He did point me to a proably suspect which would be any other 25-bios chip on tbe mainboard. It would be the one that stores the t-con firmware. And THAT would certsinly give us a no picture problem on the display. Im learning a lot with this particular repair. When inget home tonight i will try to identify this T-Con chip and see if i can get a dump off of it.

[KYBOSH]

Re: Samsung UN55MU8000 - Backlight But No Picture Problem

I was able to pull the firmware off of the panel board's chip and posted it to the eeprom dump thread. I had to pull it off and read it via an WSON-8 adapter and a CH314A programmer. Im hoping to hear back soon on what's what. Pretty excited to get here. I've never done this aspect of TV repair before. Also excited to see what new adventures this CH314A Programmer will lead to.