Blown Mosfets - Worth it to fix?

[skyking421]

Hopefully, I will get my photos in here. (attached)
I'd like to repair this Dell XPS 710 motherboard, that I got not working.
I don't see any swollen caps, but these two mosfets were nuked.
They are cheap enough, so I ordered some from Digi-key.
My question is, am I right in assuming that these burned up from some direct short the previous owner did? They may be used as voltage regulators, but I may be way off with that idea.
Any ideas why these burned up and what I might do to prevent the next ones doing the same?
They are labeled as 79n03s and there are a few more pairs of them around the board. The others seem ok.
Thanks gents.

[mockingbird]

Re: Blown Mosfets - Worth it to fix?

Those nearby UCC KZG capacitors may have had something to do with it. Either that or it could have been the PSU. You should have ordered replacement capacitors as well.

[skyking421]

Re: Blown Mosfets - Worth it to fix?

Thanks for the advice.
I'll check those caps!

[mockingbird]

Re: Blown Mosfets - Worth it to fix?

Those two 1000uF black caps near the UCC brown caps - are they TMx or KZJ series? They should be replaced. Not just the ones near the blown FET, all KZG and KZJ. I think it's also routine to replace the small sub-100uF valued caps on these motherboards, depending on what they used. Leave the Rubycon and Sanyo caps, as well as the polymer VRM low caps alone though. Remember, if you're going to test caps, capacitance measurement is useless. You need to check their ESR and see if that's within spec.

[c_hegge]

Re: Blown Mosfets - Worth it to fix?

Yup. It's probably bad motherboard caps. Chemi-con KZG and KZJ do often fail without bulging.

Your PSU is probably a Dell H750P-00. If so, I wouldn't worry about it. Those PSUs are built like tanks.

[mockingbird]

Re: Blown Mosfets - Worth it to fix?

Maybe we can get a peek inside one for your next review c_hegge...

[c_hegge]

Re: Blown Mosfets - Worth it to fix?

^
I published it ages ago (http://hardwareinsights.com/wp/dell-h750p-00-review/). My unit did have a bit of a coil hiss to it, but aside from that and the fact that it's non-standard, it's an excellent PSU. Entirely Jap caps, Ripple was <25mV on the 12V rails and <20mV on the 5 and 3.3 and 3% voltage regulation. The best part is, it survived over 900W on the load tester.

[skyking421]

Re: Blown Mosfets - Worth it to fix?

Mockingbird,
Yes the Black caps are KZJ 6.3v 1800uF and the Brown ones are KZG 16v 680uF.
They are not swollen or leaking. (yet).
I know it's not a proper test, but I used the ohms scale to test them for shorts or open. Also took note of the capacitive charge/discharge swing of the needle.
Of course they are still "in circuit", but they all read similar values, when comparing paired caps.
Maybe they are all the same amount of "bad", but isn't that unlikely? Or is it?

Do we have a topic on testing ESR?
Thanks

[selldoor]

Re: Blown Mosfets - Worth it to fix?

Do we have a topic on testing ESR?

Lots and lots of them -use the search above. However unless you have an ESR meter there is little point in reading them.?

[mockingbird]

Re: Blown Mosfets - Worth it to fix?

Mockingbird,
Yes the Black caps are KZJ 6.3v 1800uF and the Brown ones are KZG 16v 680uF.
They are not swollen or leaking. (yet).
I know it's not a proper test, but I used the ohms scale to test them for shorts or open. Also took note of the capacitive charge/discharge swing of the needle.
Of course they are still "in circuit", but they all read similar values, when comparing paired caps.
Maybe they are all the same amount of "bad", but isn't that unlikely? Or is it?

Do we have a topic on testing ESR?
Thanks

I'll save you a little time... You need a relatively expensive meter to accurately test capacitors. Save yourself time and money, just replace -ALL- KZG and KZJ on the motherboard. In my experience, KZG and KZJ will more often fail and remain fine in appearance externally than fail and show any signs.

Please list diameters for the caps and I will point you to the correct replacements that the owner of this site sells. You should also list the series of the small sub-100uF caps. If they're KY or KZE class, leave them alone. If they're SMx or Vx class, replace them too.

[skyking421]

Re: Blown Mosfets - Worth it to fix?

Mockingbird,
Thanks for the advice. Here are the diameters.

Black caps are KZJ 6.3v 1800uF - about 7 or 8 mm diam.
Brown caps are KZG 16v 680uF - about 10 mm diam.

I have already ordered the Opti-mosfets, so I'll install them and try again.
Do these caps degrade to the point of causing that damage to the mosfets?
The mosfets are rated at pretty high amperage.
Am I risking blowing out the new mosfets?

[mockingbird]

Re: Blown Mosfets - Worth it to fix?

Mockingbird,
Thanks for the advice. Here are the diameters.

Black caps are KZJ 6.3v 1800uF - about 7 or 8 mm diam.
Brown caps are KZG 16v 680uF - about 10 mm diam.

1) This or this.
2) This

I have already ordered the Opti-mosfets, so I'll install them and try again.
Do these caps degrade to the point of causing that damage to the mosfets?
The mosfets are rated at pretty high amperage.
Am I risking blowing out the new mosfets?

Yes. Don't turn it on before replacing the caps... Again, I'm reminding you of the small 6.3mm or less caps on the motherboard... One of them that is shorted can wreak havoc.

[skyking421]

Re: Blown Mosfets - Worth it to fix?

Great advice. Thanks.
This motherboard's symptoms are high cpu and case fans on power up, but no other indications of life. Power led on board does light.
No Dell diagnostic lights are lit.
No heat from the cpu (with unplugged fan).
No heat from the chipset.
No video response at all (tried two cards) no onboard video on this board.

Are caps the typical cure for these symptoms?
Richard

[johnnyblazing]

Re: Blown Mosfets - Worth it to fix?

Update, I replaced the bad caps with the ones I ordered and its working like a new pc. Thanks for all of the help guys, Y'all are the best. Thanks a million.

[mockingbird]

Re: Blown Mosfets - Worth it to fix?

How did you replace the MOSFETs?

[johnnyblazing]

Re: Blown Mosfets - Worth it to fix?

I didn't have to replace anything other than the capacitors. I brought all of them, but only replaced the 2 that were visibly bad. I then powered it up to success I'll tear it back down this weekend and change everyone of them. Thanks to the info on this site I was able to bring it back to life.

[ben7]

Re: Blown Mosfets - Worth it to fix?

I didn't have to replace anything other than the capacitors. I brought all of them, but only replaced the 2 that were visibly bad. I then powered it up to success I'll tear it back down this weekend and change everyone of them. Thanks to the info on this site I was able to bring it back to life.

Uh.... you should replace the MOSFETs.
Otherwise, the other MOSFETs might be getting stressed out from the extra power they have to handle.

[johnnyblazing]

Re: Blown Mosfets - Worth it to fix?

hate to ask but I'm a little green as to the mosfets, can you id them for me, then I'll order them from here. Thanks in advance.

[johnnyblazing]

Re: Blown Mosfets - Worth it to fix?

skyking421, where did you order your MOSFETs from and what are the specs

[ben7]

Re: Blown Mosfets - Worth it to fix?

hate to ask but I'm a little green as to the mosfets, can you id them for me, then I'll order them from here. Thanks in advance.

They both appear to be BSC079N03S from Infineon


I am not sure if you will be able to replace them though, because you might need special equipment. The reason is because the MOSFETs have a big metal pad underneath that is soldered to the PCB for better heat dissipation and lower resistance. Scroll down to page 8 of that datasheet to see the mechanical drawings ... the one on the right shows the underneath of the MOSFET, and you can see the pad I am talking about.