Constant fuse blowing

[Techboxcomputer]

Hi All


Hi I have a Samsung UN40HU6950 TV that blew it's fuse on the power board. The board is a SE9B505102. I bought a matched ceramic fuse for it, reinstalled the board and it immediately blew the new one. I am at a loss of what to do next

Thanks in advance = )

[diif]

Re: Constant fuse blowing

The fuse is blowing for a reason, usually shorted a shorted component(s) on the power supply.
You will need a multimeter to help find the faulty part(s) and to help us good photos of the board. I think it's a BN44-00752A ?

[Techboxcomputer]

Re: Constant fuse blowing

I have a multimeter and I am not totally unfamiliar with it's use. The board is exactly the one you have mentioned. Tell me what you want photos off and I will post them

[budm]

Re: Constant fuse blowing

Good clear high res straight shot of the whole top and bottom side of this power supply board.

[Techboxcomputer]

Re: Constant fuse blowing

OK - here they are. Let me know if you need anything better.

[CapLeaker]

Re: Constant fuse blowing

check out the bridge rectifier BD8015 to see if it is shorted and the PFC fets next to it.

[steves7839]

Re: Constant fuse blowing

One way to detect a shorted board is to measure resistance on the "load" or protected side of the fuse to circuit ground. Usually it will read just a few ohms if a downstream short exists.
To isolate whether the short is located on the power board or another board, disconnect cables from the power board and remeasure.

[Techboxcomputer]

Re: Constant fuse blowing

OK, lets address this one a a time, here are what my readings on my bridge rectifier is.
With positive lead on leg one ( + ) and negative on leg 2 - .502
With positive lead on leg one ( + ) and negative on leg 3 - .502
With positive lead on leg one ( + ) and negative on leg 2 - tests through


As far as the PFc's go, I am assuming you are telling me DP803 & DP804 These I am unsure how to test I now have noticed a slight discoloration around DP801c on the back of the board This sits between QP802cs & QP801cs and they read differently

[budm]

Re: Constant fuse blowing

Check the resistance between the two legs of the main filter cap, and also between the two outer legs of the bridge rectifier.

[Techboxcomputer]

Re: Constant fuse blowing

The 2 outer legs ring continuity on BD801s

If you are referring to the 2 500v68uf capacitors as the main filter cap(s) they both ring continuity also

The mosfet I am suspicious of is posted below. I may be wayyyy off as I am new to this but it looks like there was a heat issue with the middle one.

[budm]

Re: Constant fuse blowing

We need the reading in Ohms, continuity test does not mean much when looking for resistance value, continuity is telling you that you have connection between the two points, the resistance will tell me if it is normal or not.
Make sure to report if the reading is in Ohm, K Ohm, or M Ohm.
Yes, those are the two power MOSFET's for the PFC circuit and one PFC diode on that heatsink.

[Techboxcomputer]

Re: Constant fuse blowing

Ok, as I said, I am still learning, so be patient please
Here is what I went by
" Connect the 'Source' of the MosFet to the meter's negative (-) lead.

1) Hold the MosFet by the case or the tab but don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed.

2) First, touch the meter positive lead onto the MosFet's 'Gate'.

3) Now move the positive probe to the 'Drain'. You should get a 'low' reading. The MosFet's internal capacitance on the gate has now been charged up by the meter and the device is 'turned-on'.

4) With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you like, it does not matter at this stage). The gate will be discharged through your finger and the meter reading should go high, indicating a non-conductive device."

Now that you read that (if that is correct) here are my results testing the 3 mosfets I pictured. On the backside of the board I put my negative on the left and my positive on the right. My meter is set in "diode mode" Here is where I am confused. When set on that on my expensive multimeter it just beeped continuity, when I pulled out my cheap radio shack multimeter I got a reading. If I am on the right path, please continue to point me towards what to test. If not redirect me please
Mosfet 1 - 0.0
Mosfet 2 - 0.0
Mosfet 3 -.356

*note - pulled out my more expensive meter again, and it beeped on the first two as before, but on the third mosfet it measured 1.16 odd that the other meter measured .356?

Thank you

[budm]

Re: Constant fuse blowing

No power applied to the board, then just put meter in Ohm mode and read the resistance between Source pin and the Drain pin and then tell use how many Ohms you get on the meter, make sure to report exactly what you see on the meter to whether it is in Ohm, K Ohm, or M Ohm.

[Techboxcomputer]

Re: Constant fuse blowing

Okay from the mosfets pictured in my last attachment left to right (This is taken on the back of the board with one lead on the right pin and one lead on the middle pin)

1 .5 OHMS
2 1.1 OHMS
3 .7 OHMS

Hope this helps

Thanks

[Dumah Brazorf]

Re: Constant fuse blowing

OK, lets address this one a a time, here are what my readings on my bridge rectifier is.
With positive lead on leg one ( + ) and negative on leg 2 - .502
With positive lead on leg one ( + ) and negative on leg 3 - .502
With positive lead on leg one ( + ) and negative on leg 2 - tests through

Sorry i'm not english and i don't understand what do you mean by "tests through".
You did not test all possible connection, at least 6 measures, and ensure there's .5V or more.

[Moreno83]

Re: Constant fuse blowing

I think he means continuity with " tests through ".

[budm]

Re: Constant fuse blowing

Okay from the mosfets pictured in my last attachment left to right (This is taken on the back of the board with one lead on the right pin and one lead on the middle pin)

1 .5 OHMS
2 1.1 OHMS
3 .7 OHMS

Hope this helps

Thanks

Remove one MOSFET one at a time and then test it off the board again, so far you you do have shorted MOSFET.

[CapLeaker]

Re: Constant fuse blowing

something is shorted all right... but like budm already suggested: take one FET out at a time and check it, then put it back in its place. Not that you get mixed up later on where did what go.

[Techboxcomputer]

Re: Constant fuse blowing

So, rather then checking in circuit, you want me to desolder and resolder each mosfet? What am I looking for on the read for each mosfet?

Thanks

[budm]

Re: Constant fuse blowing

So, rather then checking in circuit, you want me to desolder and resolder each mosfet? What am I looking for on the read for each mosfet?

Thanks

Yes, remove it and test it of the board the same way as you tested before, the one we looking for the most is the resistance reading between Source and Drain pin.