Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

[DXseekerMO]

I'm working on the stand-by portion of a HiSense/Sharp power supply #192020. I found a transistor blown apart to the point I can only read a couple numbers. Can anyone help me ID this?

[linksys1127]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

I happen to have one of these on hand. GB1 is the marking. Here's the picture.

[DXseekerMO]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

I happen to have one of these on hand. GB1 is the marking. Here's the picture.

Shout out to linksys1127 and Hernando MS! Thank you for posting that, super stoked up you had that board!

The placement of this and the other transistor in the circuit makes me wonder if the over voltage event originated on the main board?

Thank you, I appreciate it!

[linksys1127]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Glad I could give back! Good luck with your repair!

[budm]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

https://www.shopjimmy.com/sharp-1920...led-board.htm#

It is on the cold side, that section looks to be something to do with driving those two Opto N881, 891.
Is it tested bad?
Are you not getting the 12V and 18V standby Voltages?

[DXseekerMO]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

https://www.shopjimmy.com/sharp-1920...led-board.htm#

It is on the cold side.
Is it tested bad?

Yes it's on the cold side and it does test bad.

[budm]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Are you not getting the 12V and 18V standby Voltages?

[DXseekerMO]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Correct, no stand-by. There is voltage on the hot side tho. Only issue I saw on the hot side was up and down voltage on the rectifier diode, DC side.

[budm]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Correct, no stand-by. There is voltage on the hot side tho. Only issue I saw on the hot side was up and down voltage on the rectifier diode, DC side.

Please clarify, are you saying the DC Voltage on the main filter cap is going up and down?

[DXseekerMO]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Main cap voltage across is solid at 166V, but when I check directly across the DC side of the rectifier diode the voltage goes from 166v down to about 142V repeatedly.

Red probe on + side and black probe on - checked directly on the DC side of the rectifier diode pins

[DXseekerMO]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Glad I could give back! Good luck with your repair!

Thank you!

[budm]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Main cap voltage across is solid at 166V, but when I check directly across the DC side of the rectifier diode the voltage goes from 166v down to about 142V repeatedly.

Red probe on + side and black probe on - checked directly on the DC side of the rectifier diode pins

The DC Voltage at the +/- leg of the rectifer bridge is pulsing dc since there is no filter cap, the + of the rectifier is fed to the anode of diode VD804, the cathode is the connected to the + leg of the filter cap.
So 166V steady if is fine for running the standby power supply section.
Is R818 OK? It should be less than 1 Ohm.
P/N of the SMPS IC N834?
Did you remove that transistor off board and tested it? Is it shorted between E and C pins?

[DXseekerMO]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

The DC Voltage at the +/- leg of the rectifer bridge is pulsing dc since there is no filter cap, the + of the rectifier is fed to the anode of diode VD804, the cathode is the connected to the + leg of the filter cap.
So 166V steady if is fine for running the standby power supply section.
Is R818 OK? It should be less than 1 Ohm.
P/N of the SMPS IC N834?
Did you remove that transistor off board and tested it? Is it shorted between E and C pins?

R818 checks less than 1ohm

N834 is 1271A, believed to be NCP1271, datasheet attached

I was unable to conclusively find a datasheet for either of those transistors. I'm having a hard time turning a the numbers on them in to a part number so I an get the right sheet.

The 1P transistor is directly short on the top center and lower left pins

EDIT/ADD: I checked the components near N834 and all appear to check OK in-circuit. The IC itself is not shorted to ground and ground is not shorted to any other pins

[R_J]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

That GB1 is a TL431 reference ic
Using hot ground, check if you have VCC on N834 pin 6, Also check for aprox +160v on pin 8. Then check the base of V812, it should have +16 volts (VZ810 is a 16v zener)

[budm]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

RJ, great info.

[DXseekerMO]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

That GB1 is a TL431 reference ic

Thank you R_J, how did you translate the number on the IC to the number you gave me?

RJ, great info.

Agreed, for sure!

[DXseekerMO]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

That GB1 is a TL431 reference ic
Using hot ground, check if you have VCC on N834 pin 6, Also check for aprox +160v on pin 8. Then check the base of V812, it should have +16 volts (VZ810 is a 16v zener)

N834:

PIN6 is 6.6v to 11.1v up and down
PIN8 checks just like across the rectifier diode, about 144v to 161v

Base of 812:

6.7 to 6.8v, up and down rhythmically

[R_J]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Check R801,R887,R803, Are you sure that was the base of V812 you checked? AC3 comes from the + of the bridge rectifier.
maybe check all 3 pins of V812
See if this helps

[DXseekerMO]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Check R801,R887,R803, Are you sure that was the base of V812 you checked? AC3 comes from the + of the bridge rectifier.
maybe check all 3 pins of V812
See if this helps

I am sure I checked the base of V812, verified with the datasheet. The other two pins of V812 check at 6.2v

R801 = 628K in one direction and about 852K in the other
R887 = 632K in one direction and about 852K in the other
R803 = 628K in one direction and about 750K in the other


EDIT/ADD: The schematic above shows R803 as 270K resistor. The one installed on my board has the numbers 684 on it, same as R801 & 807.

After that series of resistors the test point has 6.8v and is mostly stable. At the first resistor that comes directly from the + of the bridge rectifier the voltage is up and down from 141V to 165V.

[R_J]

Re: Sharp LC-55N620CU PSU 192020 help ID'ing a transistor

Have you replaced N851 ? If it is still in circuit and shorted, it could be shutting down the power supply by turning on the optocpupler. If you don't have sot23-3 find a to-92 style and use it for now, most smps use them.

"PIN6 is 6.6v to 11.1v up and down" That tells me the ic's startup cell is working, the ic should pulse the fet's gate and the fet should drive the transformer, this will give you the +20 volts on C819, so check that R821 & R823 are ok.
Also check that you have +160 on the V807 Fet