Samsung UN50NU6900FXZA

[Eddie3]

I have a no power, no standby with BN96-49483A Main Board Power Supply for the UN50NU6900FXZA Samsung TV. It appears this is an All-In-One Main Board.

Upon a visual inspection, the fuse is blown, there is a burnt blue cap that is close to a heavily coated (silicone or conformal coated) power transistor. In-circuit testing with meter revealed shorts between all three terminals of that transistor (No reverse bias voltage). The burnt blue cap (marked R 221k 1kv) measures 137.9uf with 0.33 ohms (esr).

Is it possible to repair board by replacing just these 3 components, and how can I figure out what the value of this transistor is if I cannot see the marking on it?

Any help would be greatly appreciated.

Thanks!

[dskall]

Re: Samsung UN50NU6900FXZA

You will need to remove the heatsink to read the mosfet.

[Eddie3]

Re: Samsung UN50NU6900FXZA

You will need to remove the heatsink to read the mosfet.

Thank you for your reply; I saw what looked like a threaded screw (bottom end) on that large heat sink.

I had such a heck of a time getting the cover off (no screws on back cover) and was so disappointed with that! I worry that I might damage the board by removing it due to the fact it is also attached without screws.

I will try to remove it and check it out, and will be inspecting the bottom for arcing marks or burnt traces / components.

Will try to post some pictures of the underside and that MOSFET.

I hope these TV engineers bring the screws back…

[nomoresonys]

Re: Samsung UN50NU6900FXZA

Had one similar recently, didn't record it tho, this guy did, it's a little tricky: https://www.youtube.com/watch?v=g0ojfT2rJtQ

[Unspun01]

Re: Samsung UN50NU6900FXZA

This is a Samsung? Those dirty fucks! A combined power supply and mainboard and TCON? May as well buy an RCA. What a garbage TV.

[Unspun01]

Re: Samsung UN50NU6900FXZA

If the fuse is open, be sure to test the 4 pin bridge rectifier near the main fuse for shorts. Test all pin combinations (pins 1-2, 1-3, 1-4, 2-3, 2-4, 3-4, etc). It's mounted on one of the heat sinks.

If this rectifier blows, it can take out the fuse and mosfet.

Also, check the surge arrester directly beside the fuse (black heat shrink over round coin sized arrester (also called a transient voltage surge suppressor TVSS). Should measure high/infinite resistance if ok. If it experienced an over-voltage and conducted, it will take our the fuse to protect and prevent further damage to the rest of the power supply section. If shorted, it has failed and must be removed/replaced when you replace fuse.

[Eddie3]

Re: Samsung UN50NU6900FXZA

Had one similar recently, didn't record it tho, this guy did, it's a little tricky: https://www.youtube.com/watch?v=g0ojfT2rJtQ

Thanks for that link; I'll try that when I get the chance.

[Eddie3]

Re: Samsung UN50NU6900FXZA

If the fuse is open, be sure to test the 4 pin bridge rectifier near the main fuse for shorts. Test all pin combinations (pins 1-2, 1-3, 1-4, 2-3, 2-4, 3-4, etc). It's mounted on one of the heat sinks.

If this rectifier blows, it can take out the fuse and mosfet.

Also, check the surge arrester directly beside the fuse (black heat shrink over round coin sized arrester (also called a transient voltage surge suppressor TVSS). Should measure high/infinite resistance if ok. If it experienced an over-voltage and conducted, it will take our the fuse to protect and prevent further damage to the rest of the power supply section. If shorted, it has failed and must be removed/replaced when you replace fuse.

Not sure if this is an MOV, but it read 7.8 Megohms. Don't think it's blown.

Not sure how the rectifier is in the circuit because I haven't seen the bottom, nor any schematic diagram (wish I had one). In-circuit testing shows short between 1 and 4, the outer ends, both ways. All others show reverse bias voltage measured both ways. Seems like a bad, situation?

I still haven't had a chance to remove the board but was starting to think maybe I should consider shotgunning the thing, picking up a salvaged board.

Is this what most people end up doing?

Thanks

[Eddie3]

Re: Samsung UN50NU6900FXZA

Sorry but I could not add this picture via edit. It shows the varistor and BRIDGE RECTIFER device.

[dskall]

Re: Samsung UN50NU6900FXZA

If your mosfet is short you will find a short at + and - of bridge rectifier. Most likly your short is at mosfet.

[Unspun01]

Re: Samsung UN50NU6900FXZA

Not sure if this is an MOV, but it read 7.8 Megohms. Don't think it's blown.

I think it's fine. You would know if it was shorted - will read as a short, possibly even look damaged or have black soot around it.

If your mosfet is short you will find a short at + and - of bridge rectifier. Most likly your short is at mosfet.

Good point. The short across the bridge rectifier could be shorted bridge rectifier, or mosfet, or both. A short in either one would take out the fuse. This is different than an external high voltage surge that would take out the surge suppressor.

Either way, you have some shorted component to remove. 3 pin mosfet might be easier than 4 pin rectifier, but don't be surprised if both need to be replaced.

[Davi.p]

Re: Samsung UN50NU6900FXZA



To remove the board you must lift the pcb where indicated then slide the board to the left, you probably have only the bridge rectifier shorted, this due to if the mosfet would be blown it burned the sense resistors, so a good bridge would see an o.c., since that is the pfc mosfet that is from the B+ and B- and those are shorted by the bridge you see short on mosfet..
you must lift one leg of varistor to see if is infinite....

[Eddie3]

Re: Samsung UN50NU6900FXZA

I think it's fine. You would know if it was shorted - will read as a short, possibly even look damaged or have black soot around it.



Good point. The short across the bridge rectifier could be shorted bridge rectifier, or mosfet, or both. A short in either one would take out the fuse. This is different than an external high voltage surge that would take out the surge suppressor.

Either way, you have some shorted component to remove. 3 pin mosfet might be easier than 4 pin rectifier, but don't be surprised if both need to be replaced.

Thanks for the suggestions; I finally got the circuit board out and thought I'd get to the MOSFET “QP801CS” which has not been very easy with my 30 watt iron. It has 6 pins, if you count the 3 extra “heat sink posts” soldered into the board.

The Bridge rectifier “BD801S” has 4 pins, plus 4 more soldered points that hold the metal heat sink to it, for a total of eight. Was saving this for last.

Haven't lifted one leg of the varistor “VX801X” to test it out of circuit yet.

Fuse “FP801S” connects L (AC) at one end. The other end goes through Varistor, back to N (AC).

I'll post update later, if I get that MOSFET out. Might need to get a 40 or 60 watt iron?

[Eddie3]

Re: Samsung UN50NU6900FXZA

If your mosfet is short you will find a short at + and - of bridge rectifier. Most likly your short is at mosfet.

Thank you for your comment.

[Eddie3]

Re: Samsung UN50NU6900FXZA

To remove the board you must lift the pcb where indicated then slide the board to the left, you probably have only the bridge rectifier shorted, this due to if the mosfet would be blown it burned the sense resistors, so a good bridge would see an o.c., since that is the pfc mosfet that is from the B+ and B- and those are shorted by the bridge you see short on mosfet..
you must lift one leg of varistor to see if is infinite....

Thank you for your suggestions and I should have checked the varistor first because I needed to unsolder the one pad to lift that leg !!

but what are the sense resistors, please?

[Davi.p]

Re: Samsung UN50NU6900FXZA

I think you don't have took into real consideration what i have written or my English is so bad, the sense resistors are the two connected to the big mosfet..

[Eddie3]

Re: Samsung UN50NU6900FXZA

I think you don't have took into real consideration what i have written or my English is so bad, the sense resistors are the two connected to the big mosfet..

Your English is fine and I should have checked again when you mentioned it. I will have another look, because I thought I had seen a few black surface mounted components near that MOSFET.

I'm starting to wonder what might have caused that MOSFET and/or bridge rectifier to fail? Because I would be very disappointed if these components were replaced and then later, burn out again.

Thanks

[nomoresonys]

Re: Samsung UN50NU6900FXZA

could be it just wore out, mosfets work hard. Should use a good surge protecting strip too, to protect against surges, sometimes when the lights go out because of a storm or whatever, when the electricity is restored there is a surge that can take things out.

[Davi.p]

Re: Samsung UN50NU6900FXZA

The bridge rectifier is a common target of mains spikes, it's today's normality, many and many faults due to high power devices turning in operation, such as air conditioners, this is my bet, needed power surge suppressors in the "electric lever box"', sorry don't remember real name, have you done what i have said? so dismounting first the bridge? i don't read this..

[Eddie3]

Re: Samsung UN50NU6900FXZA

could be it just wore out, mosfets work hard. Should use a good surge protecting strip too, to protect against surges, sometimes when the lights go out because of a storm or whatever, when the electricity is restored there is a surge that can take things out.

Thanks for taking the time to explain that. I had a feeling it was associated with the power going out and coming back on. Will have to make sure it's protected with a good surge protector.