2009 Samsumg UN55b8000xfza - backlight - no picture

[Cryptic78]

I have a samsung un55b8000 LEDtv (model #un55b8000xfza) that when I turn on I can see the backlight turn on but no picture. I hear the power on and off melody though speakers. The blue light flashes when turned on.

It used to take a few attempts, turning on and off a few times or let it warm up then off/on again to get the video to come through. Now it just never does. I suspected caps. Nothing is visually bad on the power board.

I've only probed around doing in-circuit testing on the power board.
I've got two 20N60C3 MOSFETs that read .480 from drain to source but read 1.5 from drain to gate. That's again in-circuit. The rest of them read around the .450ish range on both side. Not sure if I am onto anything or not.

Things tried:

1.) Disconnect the multi pin power harness that goes from the Main board. Result: TV will instant on when plugged in and I see backlight only.

2.) Disconnect the ribbon cable from the TCON board
Result: When turned on, Blue light flashes, backlight turns on, i can hear a single relay click, power on sound through speakers.


Tools available to me:
-Fluke 73 DMM
-MESR-100 v2 ESR meter
-soldering iron and vacuum.

I'd really love to fix this TV and learn how to troubleshoot this stuff.

Thanks,
Andy

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Pics of powerboard and main

[tvtimmy]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Check the 2 fuses on the tcon.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

F1 and F2 both have continuity. I checked voltage as well. 12.2v on both.

I checked all 3 fuses on the power board. All good.

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

If you look real close at the T0CON board you will see some test points for the Voltages I.E. 'VIN' 'AVDD;, etc.
This LCD panel has the tab bonding board on the bottom edge of the screen which it usually gets damaged by the cleaning liquid drips down on to the board and cause corrosion and shorts circuit.
You can also try having one ribbon cable connected between the LCD panel and the T-CON board connected one at a time to see what you get on the screen.
What is the history of this TV?

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

If you look real close at the T0CON board you will see some test points for the Voltages I.E. 'VIN' 'AVDD;, etc.
This LCD panel has the tab bonding board on the bottom edge of the screen which it usually gets damaged by the cleaning liquid drips down on to the board and cause corrosion and shorts circuit.
You can also try having one ribbon cable connected between the LCD panel and the T-CON board connected one at a time to see what you get on the screen.
What is the history of this TV?

History: Been mine since Black Friday of 2009 when it was first released. we have a spritz bottle we use to clean TV screens. I doubt I ever let cleaner run down the screen.

Not finding VIN or AVDD..... I'll try again tomorrow. I gotta turn in for the night

Thanks,
Andy

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

VIN = 34mv
AVDD = 13mv


Please advise and thank you.

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

VIN = 34mv
AVDD = 13mv


Please advise and thank you.

Looks like you need to start probing in the area of ICD1 (TPS65167A), you need to trace out VIN to see where it goes. I expect 12V feeding it.
Also check the resistance between GND and VIN test point to see what you get.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Found continuity to two locations, they all read the same voltage
Resistance between GND and VIN = 131k ohms (took awhile for it to settle)

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

That VIN pad has bunch of VIAS that have to go to the power source some where.
That VIN should have direct connection to pin 1 VINB per APP Circuit on page 7 of the IC spec sheet.
Right now it is a matter of tracing out the board or try new T-CON board.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

This is the backside of the board. Highlighted in yellow is where the VIN pad should be... so I am a little confused on where to proceed from here.

I guess I am a little ignorant here. What am I looking for, for a power source?

I'll try to probe around some more

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Found a few more spots with continuity to VIN, but still no 6-12v source.

How else do I approach this?

[budwich]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

you didn't follow up on budm ask of disconnecting one tcon to panel cable and see the response. reverse the test doing the other tcon to panel cable. what happens?

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

you didn't follow up on budm ask of disconnecting one tcon to panel cable and see the response. reverse the test doing the other tcon to panel cable. what happens?

I disconnected each ribbon cable from the TCON to the LCD (one at a time) and I do not see any difference. Just a backlight.

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Found a few more spots with continuity to VIN, but still no 6-12v source.

How else do I approach this?

You are on the right track, that ICD4 (MP2380 Stepdown converter) pin 8 is the VCC input that also gets the same Vin power source from some where.
This is reverse engineering so it will be time consuming, I have done it many times so it is up to you to keep probing to see where else VIN is connected to beside ICD4.

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

See if you get continuity (about 1 Ohm or less) between point 'A' (VIN) and point 'B'.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

See if you get continuity (about 1 Ohm or less) between point 'A' (VIN) and point 'B'.

Nothing. OL

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Keep on probing then to see where else VIN goes to.
I am expecting to see the low (<1 Ohm) resistance reading between VIN and the FUSE for the 12V feeding the T-CON board, that would be typical T-CON board setup.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Keep on probing then to see where else VIN goes to.
I am expecting to see the low (<1 Ohm) resistance reading between VIN and the FUSE for the 12V feeding the T-CON board, that would be typical T-CON board setup.

OK getting some different results now.

VIN to BD103 (like you highlighted before) now reads 12.5v. I also notice a fast pulse sound coming from somewhere when I test these two points.

VIN to Panel VCC and the top of C11 has continuity and reads .3 ohms and stray voltage around 0-13mv.

VIN to the bottom of C11 reads OL

Hopefully we are getting somewhere

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

"VIN to BD103 (like you highlighted before) now reads 12.5v. I also notice a fast pulse sound coming from somewhere when I test these two points."
Can you explain that a little bit? Are you putting black probe of the meter on the chassis and Red probe to BD103 nad getting steady 12V? You should have steady 12V at both ends of that Ferrite bead inductor BD103 when TV is turned on.
BTW, what is the P/N of that IC101? It looks to be a switched MOSFET.
We are getting close!