pn51d550c1fxzc no 5V stb

[cheech3629]

this tv had no standby light on in front--took off the back and there was no stb voltage , but there is pson voltage.
i went online to see what i could find and an hour later when i returned the tv was on and working perfectly. i could turn it off and back on with no issues.
today i`m back to no stb voltage.
there is 5 volts at the transistors on the back of the board but no voltage to the other two legs.there is also 5 volts in at the input pin of the IC at the back of the board----now i`m stuck.

[attainteddragon]

Re: pn51d550c1fxzc no 5V stb

when you have no front led, do you have stby on pin2 of cn801 (pics are blury, so i hope that is right), and what does pin 1 of cn801 give you as you try to power the set up?

[budm]

Re: pn51d550c1fxzc no 5V stb

"there was no stb voltage , but there is pson voltage."
No way you can have PS-ON unless the main board is getting the STANDBY Voltage to run. Recheck your measurement.
Pictures are too poor to read the pin labels.
Also where are the Make and Model of this TV we are dealing with.

[cheech3629]

Re: pn51d550c1fxzc no 5V stb

samsung model is pn51d550c1fxzc
cn801 at
pin 1 = 2.944
pin 2 =.004
no change in anything when trying to power up.
not sure if it matters but the only other place there is voltage is at
J821 pin 5 = 2.944

[dskall]

Re: pn51d550c1fxzc no 5V stb

Well is seeing believing?

[budm]

Re: pn51d550c1fxzc no 5V stb

Unplug the TV from the outlet, put black probe on the chassis and the red probe on STBY pin, plug the TV in and see if Voltage on STBY (A5V) pin jumps up for seconds or not, this is the main key point to check.
BTW, if I remember correctly on this board, PS-ON has to go low to turn on the power supply.
Looking at the bottom side of the board, this what I am seeing: There is a switched POWER MOFSET (QB810?) for the STBY (A5V), it will go into shutdown if there is over load on the output. The PS-ON is held high by ICQ802 (P/N???) which gets the 5V VCC from the 5V that is fed to QB810, so if QB 810 is shutdown you will still have Voltage on PS-ON, the PS-ON is pulled LOW by the main board if the main board is running.
BN44-00444A, right?
Pictures provided by shopjimmy

[dskall]

Re: pn51d550c1fxzc no 5V stb

So you are getting 5v just not to standby pin. Check under board at your 5v standby and see if you have bad connection at your connector. With your cable disconnected you probably still have 2.9v on ps_on

[cheech3629]

Re: pn51d550c1fxzc no 5V stb

When i plug the tv in there is no change in the STBY pin at all.
QB810 5.3 V at the top but nothing at either of the bottom legs
ICQ802 has 5.3 V at pin 1 but nowhere else

I have another board--BN44-00330A that uses the same mosfet--in position QB810 as well. this one has 5.24 at the top , 5.25 on one leg and 9.86 at the other. The IC on this board is different.

any ideas on what to test next ? i checked all the solder points for cracks or poor connections and did not see anything .

[cheech3629]

Re: pn51d550c1fxzc no 5V stb

the problem board is BN44-00444A like you said.

[dskall]

Re: pn51d550c1fxzc no 5V stb

Try running your trace back from the stby pin to where you pick up 5v

[cheech3629]

Re: pn51d550c1fxzc no 5V stb

QB810 supplies the 5V to the STBY pin.

I am not very good with the terms but if i understand this --
so the QB810 isnt receiving the voltage signal at the --gate ?
to allow the 5V to go to the drain ? which feeds the standby pin.

so i need to find out where the QB810 is supposed to get it`s signal from?

[budm]

Re: pn51d550c1fxzc no 5V stb

When i plug the tv in there is no change in the STBY pin at all.
QB810 5.3 V at the top but nothing at either of the bottom legs
ICQ802 has 5.3 V at pin 1 but nowhere else

I have another board--BN44-00330A that uses the same mosfet--in position QB810 as well. this one has 5.24 at the top , 5.25 on one leg and 9.86 at the other. The IC on this board is different.

any ideas on what to test next ? i checked all the solder points for cracks or poor connections and did not see anything .

So you did leave the probes on the terminals as suggested? then next thing is to try it again but without the connectors on the power supply board except the power cord. I am trying to find out if the MOSFET was turned on for seconds or not, if not then the Gate drive is not working.

[cheech3629]

Re: pn51d550c1fxzc no 5V stb

Yes i tried it connected to the tv and then separately with just the powerboard on the bench. there is never a change on the stby pin.

[cheech3629]

Re: pn51d550c1fxzc no 5V stb

what is the gate drive ?

[budm]

Re: pn51d550c1fxzc no 5V stb

For the MOSFET to be on, the typical Gate Voltage will usually has to be about 3V or higher than the Voltage at Source pin to be ON, that the GATE drive Voltage. There is small SMD Transistor connected to GATE pin of that 5VSTBY MOSFET, may be the MOSFET itself is bad or that Gate drive transistor is bad, or that GATE drive transistor is not getting the ON command from that IC to turn it on.
The MOSFET can be bypass so you will have 5VSTBY all the time but you will no longer have the protection circuit (this is the second protection circuit beside the over current protection in the Primary drive circuit in the HOT side, you can try that and see if the TV will function properly when 5VSTBY is present or you may have more than one problem.

GATE drive:
https://cdn.badcaps-static.com/pdfs/...2d4372a282.pdf