Samsung TV UN75NU7100/Power Board BN44-00874C

[Larrob19]

Hello,

I have a Samsung power board BN 44–00 874C that is dead. I have looked for a schematic but no such luck. I opened it up and found two Mosfets burnt QP801CS and QP802CS, also the main fuse. I replaced them and checked the two blue capacitors in front of them. They both tested good. I checked the Diode on the same heat sink and the two capacitors in front and they checked out OK. I plugged it in again and that blew the fuse and the Mosfets the second time. I plugging it in again hooked to a 100 W watt light bulb. The light bulb starting started flashing off and on. I checked the three big capacitors and they seem to be OK and also the two bridge rectifiers. I did find 4 small caps CM851-854 that were bulging and out of spec so I changed them also.
I plugged it in again and still have the bulb flashing 1 to 2 times per second.
Would some somebody know why the light bulb would be pulsing like that?
Have included some photos.
This has got me stumped. Any help would be appreciated.

[Diah]

usually when replace Mosfets we change the ceramic caps too against new one,
you need to add load just plug the BL socket at it and see the behavior of yours DIY lamp and the output at sockets which goes to MB

[Larrob19]

I changed the ceramic caps that you mentioned and plugged it in with 100w bulb. The lighted flashed for a few seconds and went out. I tested the 3 big capacitors and they are 156 volts. the standby voltage is 12.7 volts and I am getting 281 volts at some channel outputs. This is from testing on the bench. Is there any other test that need to be done?

[Diah]

from yours reports yours psu work as it should.. plug it back in set

[Larrob19]

I will try it and report back.

[Larrob19]

I installed the board back in the TV and plugged it in with the 100w bulb. The relay clicked and the LEDs lite up and the 100w bulb started to glow. Then the relay clicked off and back on again, over and over. Could there be another problem or is the 100w bulb causing the problem. I don't know if I should try it without the bulb as it might burn the Mosfets again. A little guidance would be helpful.

[EazyBone]

What's with this bulb? Did you replace the blue caps and the mosfets? And check the additional mosfet next to it? If yes then plug it in. You don't need to put it into the tv, just plug it in on the bench and test the outputs.

It blew up because you didn't replace those caps likely..

[lotas]

You have a PFC on your power supply and a 100w lamp won't handle it, you need a 300-500w one.

[Diah]

no need to make it complex... his post #3 psu ok... just start tv with out any lamp.... but first disconnect the lvds betenn MB and Panel.. because this NU samsung 99% panel defect... your sign will be Tv start the optic remain lit red

[Larrob19]

Diah, There is only 1 line to disconnect from the PB to the MB. CNM802. Disconnecting this makes the LEDs flash on and off.

[Diah]

Diah, There is only 1 line to disconnect from the PB to the MB. CNM802. Disconnecting this makes the LEDs flash on and off.

yours test must be with out lamp.. if you do it with out lamp and BL flashing then need inspect the PSU again.

[Larrob19]

OK, I tried it without the lamp, the relay clicked and the LEDs came on dim, then went bright then the relay clicked and they went off. Then started all over again. Next step?

[EazyBone]

So the tv keeps rebooting? LEDs on and shut off and repeat cycle?

[lotas]

Disconnect the cable from the power supply to the motherboard and turn it on. How will the power supply behave with the backlight?

[Larrob19]

Yes that's it. As I said in post #1 I replaced the 4 small caps because they were bulging and out of spec. Could there be other Caps not doing there job? But where?

[Larrob19]

Iotas, I did that see post #10. On and Off

[nomoresonys]

Do the simple test in post # 14, see what you get.

[Larrob19]

What I am getting when the power supply is disconnected from the motherboard is the LEDs flashing on and off at what I am guessing is full brightness at the same time the relay is clicking.

[nomoresonys]

Ok, we have progress, you still have a problem with the powerboard or backlights. You could also have more issues but the usual process is verify powerboard and backlights and move to next if necessary.

[nomoresonys]

Do you have a backlight tester?