Samsung LA26R71BA cannot power up

[dick_barton]

Re: Samsung LA26R71BA cannot power up

Resistance
Pin1 to 2 = 8.3M ohm
Pin2 to 3 = 5.6M ohm
Pin3 to 4 = 12.25k ohm
Pin4 to 5 = 22.4k ohm
Pin5 to 6 = 22.1k ohm
Pin6 to 7 = 99.5k ohm
Pin7 to 8 = 106k ohm

The thing on pin4 is solder.

So there's no solder bridging between pins and you say that the copper looking colour on pin 4 is just a solder joint. I would use a toothbrush and some alcohol to clean up the solder joints on the IC and remove the spots of flux and solder.

[dick_barton]

Re: Samsung LA26R71BA cannot power up

The NT811S blowing shows that there was a massive inrush current however the testing the mosfet (QP801S) shows that it is not short circuit so could not be responsible for blowing the NT811S.

I think it's time to start testing other components

Can you check DP801 and DP802 using your meter in diode mode and also the bridge rectifier with the set disconnected from the mains.

https://www.youtube.com/watch?v=dqkT6hF0O3E

[john_doe]

Re: Samsung LA26R71BA cannot power up

DP801 and DP802, both 0.413vdc one way, the other way is OL.

BD801S:
Pin1(pos) to pin2(neg) = OL
Pin1(neg) to pin2(pos) = 0.453vdc

Pin2(pos) to pin3(neg) = 0.453vdc
Pin2(neg) to pin3(pos) = OL

Pin3(pos) to pin4(neg) = OL
Pin3(neg) to pin4(pos) = 0.453vdc

BD801S looked faulty. Am i right?

[dick_barton]

Re: Samsung LA26R71BA cannot power up

DP801 and DP802, both 0.413vdc one way, the other way is OL.

They look fine


Edit
The bridge rectifier test does look like there's a problem with it.
The two middle pins (marked ~ ) should be OL in both directions

Measure in diode mode in both directions between 1 - 3 and 4 - 2




Can you also test the mosfet which should be high or OL except in one direction across the diode.

I'm still trying to work out why it went bang?

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

so what new parts have you fitted so far just the one ic?

[dick_barton]

Re: Samsung LA26R71BA cannot power up

From what I can make out the only parts that have been changed were the QP801S mosfet and ICP801S (TDA4863G).

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

mosfet correct type not shorting on the heatsink?

[dick_barton]

Re: Samsung LA26R71BA cannot power up

mosfet correct type not shorting on the heatsink?

Interesting possibility.

John_Doe
Was the original mosfet a TO-220 as shown in the picture or a TO-220F and did you replace like for like.

[vinceroger69]

Re: Samsung LA26R71BA cannot power up

its odd all the tests you have asked him to carry out are passing ok im not sure why that part blew maybe budm when hes next online on has a idea what may cause it to blow.

[dick_barton]

Re: Samsung LA26R71BA cannot power up

its odd all the tests you have asked him to carry out are passing ok im not sure why that part blew maybe budm when hes next online on has a idea what may cause it to blow.

The inrush varistor blew and it looks like the bridge rectifier isn't looking too good but I've yet to find out what took them down.
The possibility of the mosfet being a metal tab looks like it may be the problem. It would cause the damage if that was the case but I'm awaiting an answer.

[john_doe]

Re: Samsung LA26R71BA cannot power up

The original mosfet is the TO-220F type. The mosfet I replaced is the FCPF11N60NT and it is the TO-220F with the plastic tab. It is not shorted to the heatsink.

I attach the datasheet.

[john_doe]

Re: Samsung LA26R71BA cannot power up

Just to be clear, only NT811S blew up. The fuse FP801S is ok.

[budm]

Re: Samsung LA26R71BA cannot power up

Are you 100% sure that QP801s Source and Drain pin is not showing very low resistance (<10 Ohms)?
RP819 is still showing <1 Ohm?
I also expect the QP801S to be rated at 800V or more.

[john_doe]

Re: Samsung LA26R71BA cannot power up

QP801S: Resistance
Pin3 (pos) to pin2 (neg) = 192.3k ohm
Pin3 (neg) to pin2 (pos) = 280k ohm

Pin3 is shorted to heatsink. Strange?l

Resistance of RP819 = 0.4 ohm

[dick_barton]

Re: Samsung LA26R71BA cannot power up

John-doe
Can you confirm that when the NT811S burnt up that the fuse did not blow?

What was the result of testing the bridge rectifier between pins 1-3, 2-4 (post #64)

There's also an internal diode across D-S of QP801S (see pdf) so can you check with your meter in diode mode across these two pins that the diode is OK

The mosfets Source pin is connected to hot ground via RP819 and the heat sink is probably also connected to hot ground which would explain your last comment

[john_doe]

Re: Samsung LA26R71BA cannot power up

I confirmed only NT811S burnt up and fuse FB801S is still ok.

Bridge rectifier pin1 to 3 shorted, pin 2 to 4 also shorted.

QP801S:
Pin2 (pos) to pin3 (neg) = OL
Pin2 (neg) to pin3 (pos) = 0.454vdc

[dick_barton]

Re: Samsung LA26R71BA cannot power up

The mosfet certainly appears OK
The bridge rectifier is damaged and is why the NT811S burnt up.
Both need to be replaced,

Why this happened is really unclear since the mosfet did not cause it or any fault with DP801 or DP802 so I can only assume that when the mosfet blew the first time it must have damaged the bridge rectifier but blew the fuse before it damaged the NT811S.

Can you check RP816, RP817, RP818, DP804, DZP801 and also if you can measure the ohms value across CP803 but ensure there is no voltage across it.

[john_doe]

Re: Samsung LA26R71BA cannot power up

RP816 = 99.5k ohm
RP817 = 7 ohm
RP818 = 15 ohm

DP804 (in circuit) = shorted
DZP801 (in circuit) = 0.9v 1way, 0.6v the other way. I know it is not accurate to test in circuit.

CP803 = 2 M ohm and the value is dropping.

[dick_barton]

Re: Samsung LA26R71BA cannot power up

RP816, RP817, RP818 all measure correctly.

Best to unsolder both diodes and then test because there are low value resistors in parallel with them that would make them look like a short circuit.

There is no short circuit across the supply (CP803) which is good.

[budm]

Re: Samsung LA26R71BA cannot power up

I would remove QP801S, lift one leg of RM801 off the board.
That will leave the STBY power supply section and the bridge only in place, then use the lamp in place of the NTC to see if you will get the STBY power working or the lamp will still stay on bright, if it does then remove fuse FB802S and try again to see if the lamp will stay on bright or not.
I still do not understand what was done to the board to cause the QP801S to shorted out since in post 1 you were getting the stby Voltage OK.