2009 Samsumg UN55b8000xfza - backlight - no picture

[Cryptic78]

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!

TV on:
BD103 (top or bottom) to VIN = 12.40v
BD103 (top or bottom) to chassis ground = 12.55v
VIN to chassis ground = 18.5mv and OL testing continuity.

IC101 = 4435B MOSFET

I hope that helps.

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

OK, I am confused about how you measure this one:
"BD103 (top or bottom) to VIN = 12.40v" you put the probe between VIN and BD103 top, VIN and BD103 Bottom?
Right now we know that BD103 is OK.
VIN looks to be wired to get the 12V through that IC101 Switched MOSFET.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

OK, I am confused about how you measure this one:
"BD103 (top or bottom) to VIN = 12.40v" you put the probe between VIN and BD103 top, VIN and BD103 Bottom?
Right now we know that BD103 is OK.
VIN looks to be wired to get the 12V through that IC101 Switched MOSFET.

Yes on how I measured BD103. Just noting the slight difference in voltage. I don't know if that is relevant or not.

What's our next steps? Is that MOSFET not passing the voltage as it should?

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Need to get the P/N as printed on top of that MOSFET to get the pin out and find out which pin/s is the Gate so we can see if there is Voltage at the Gate to turn on the MOSFET or not.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

need to get the p/n as printed on top of that mosfet to get the pin out and find out which pin/s is the gate so we can see if there is voltage at the gate to turn on the mosfet or not.

ic101 = 4435b mosfet

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

AO4435/FDS4435/SI4435 P-CH MOSFET.
Check DC voltage on pin 4 with ref to GND (leave the probes in place) when TV is OFF, then turn the TV on to see if the Voltage will go low and stay low or not.
The Voltage at Gate pin 4 has to be about 3V less than the Source pin (pin 1~3 which all tied together) for the MOSFET to be ON.
As you can see, the 12V feeding the T-CON is fed into this MOSFET's Source Pins 1~3 which is being used as the electronics switch, the output is on the Drain pins 5~8 which feeds the VIN pad.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

AO4435/FDS4435/SI4435 P-CH MOSFET.
Check DC voltage on pin 4 with ref to GND (leave the probes in place) when TV is OFF, then turn the TV on to see if the Voltage will go low and stay low or not.
The Voltage at Gate pin 4 has to be about 3V less than the Source pin (pin 1~3 which all tied together) for the MOSFET to be ON.
As you can see, the 12V feeding the T-CON is fed into this MOSFET's Source Pins 1~3 which is being used as the electronics switch, the output is on the Drain pins 5~8 which feeds the VIN pad.

Pin 4 basically says at 12.38v while pin 1 reads 12.55v

I repeated the test a few times... On pin 4 I can see a few blips where voltage goes down, but not long enough for it to stabilize on a DMM, but it ultimately stabilizes at 12.38v

The drains are reading just millivolts

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

That pin 4 needs to go low, so if you see it goes low for a blip then what ever control that line is not working, it should go low and stay.
So the possibilities are:
1) Which ever IC that control that line is not working.
2) May be the protection circuit kicks in due to over current draw so the MOSFET is turned off right away.
3) We know that VIN has high resistance but the switching power supplies generated by ICD1 may be the problem, another IC the buck converter circuit may be the problem. You will have to check the resistance of each one of those power supply output.
4) You can force on the MOSFET by connecting a 470 Ohms resistor between GND and the pin 4 of the MOSFET and see if anything will get really hot or even smoke..
At this point, you may have to get the new T-CON board.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

At this point, you may have to get the new T-CON board.

I'm OK with that. $63 +$10 shipping on ebay for a NEW board. I appreciate the help and learning experience.

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Try having the two cables from the T-CON to LCD panel disconnected and see if VIN will be present or not when TV is turned on.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Try having the two cables from the T-CON to LCD panel disconnected and see if VIN will be present or not when TV is turned on.

Still no VIN with TCON to LCD ribbon cables disconnected. Reads ~15mv still.

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

OK, at least that may have eliminate the possibility of shorts circuit on the panel that may have caused over current draw and shut down the MOSFET, hopefully you will get good working T-CON board.

[Cryptic78]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

New T-CON board came in... TV is working perfectly again.

Thanks for all the help Budm!

[budm]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

Thanks for the follow-up.

[triplefour]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

I have acquired this same TV and am experiencing an almost identical situation.
startup and shutdown chimes are heard.
while on, i get blank backlit screen and the blue status light blinks on and off infinitely.
tv takes about 8 seconds to hear the power on chime from power button press.
with tcon disconnected from mainboard, tv only takes 2 seconds before chime is heard. (not sure what this means but i noticed it)

disconnecting one side at a time, or both ribbon cables from the panel doesnt change anything.

i also am getting a very low voltage on the VIN test point (45mv)
and i can see it is related also to IC101 not outputting any voltage after
it receives 12.5v on pins1-3
on pin 4 i read 11.4v
which according to what budm said, is a couple volts too high to turn on the
mosfet.

he mentioned wiring a 470ohm resistor between pin4 and ground to pull that voltage lower and trick the mosfet to turning on, which im itching to try
but i just wanted to be sure since im not too clear on the resistor math involved, if i should use a different value since i am getting a different voltage there than the original poster?

also what is the danger here?
if it works, could i just leave it in place?

i would love it if someone would explain to me the simple process of figuring out what resistance value to use to take a 11.4v down to the 9.5v value that should turn on the mosfet.

thanks for any help!

[triplefour]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

i couldnt wait so i figured err on the side of caution and use a larger resistor (1k) and connect pin4 to ground.
it lowered the voltage to around half a volt, and did succeed in turning on the mosfet, sending the 12v down to the VIN pin,
but the voltage doesnt get much farther than that as far as i can tell.
it goes into pin1 of a really tiny IC (ICD2) and comes out of pin4, but the other pins and everything else in that area reads 0v...
the blue light is still flashing infinitely, and i still dont see any voltage down by the ribbon connectors

markings on ICD2: 58AG
having trouble finding a datahseet from this list
http://www.alldatasheet.com/view.jsp...=58AG&sField=3

[triplefour]

Re: 2009 Samsumg UN55b8000xfza - backlight - no picture

little help? anyone?