Would just recapping solve my problem - desoldered MOSFETs

[Smithrll46]

I have a gigabyte GA-7VAXP motherboard which has bulging and leaking caps.

It has been running OK. When I examined the motherboard, I found the bad caps, and also saw that several of the MOSFETs in the VR circuit had overheated and melted the solder onto the motherboard on their flat side - none are scorched or melted. I presume (but don't know for sure) that this was caused by the bad caps.


I'm worried about a fire-hazard here. If I recap the motherboard, would the MOSFETs be OK and not further overheat?

Should I replace the MOSFETs as well ? - I'm not sure how I would desolder the flat side of them from the motherboard if I had to replace them.

If I should, and can replace them, can anyone recommend a source for the new MOSFETs ?

Thanks

[willawake]

Re: Would just recapping solve my problem - desoldered MOSFETs

pic?

[Smithrll46]

Re: Would just recapping solve my problem - desoldered MOSFETs

Here are the pictures:

Bad MOSFET



Blown Caps

[Rainbow]

Re: Would just recapping solve my problem - desoldered MOSFETs

Ouch, you were VERY close to a disaster. The caps need to be replaced, of course. The MOSFET is probably not shorted if the board works so it only needs desoldering and soldering back to its place.

[kc8adu]

Re: Would just recapping solve my problem - desoldered MOSFETs

i would replace the sliders and recap it.
that board was really close to letting the smoke out.

[larrymoencurly]

Re: Would just recapping solve my problem - desoldered MOSFETs

Do MOSFETs degrade partially, as bipolar transistors are known to do?

[davmax]

Re: Would just recapping solve my problem - desoldered MOSFETs

I would suspect the continued reliability of the MOSFET in centre picture. The temp melted solder, then rose higher due to a poor connection to the copper plane heat sink. The full part number is 2SK3405. It seems hard to obtain this device but STmicroelectronics offer a replacement (superior spec) to be found at Digikey part number 497-3515-1-ND at $1.95 single unit.

here: http://www.digikey.com/scripts/dksea...t=STB75NH02LT4

[yanz]

Re: Would just recapping solve my problem - desoldered MOSFETs

at what temperature the soder can melt? >100C?

btw, "nice" pics there...

[Rainbow]

Re: Would just recapping solve my problem - desoldered MOSFETs

It's above 200 degC. I'm surprised what can these MOSFETs survive.

[davmax]

Re: Would just recapping solve my problem - desoldered MOSFETs

Check this post: https://www.badcaps.net/forum/showth...0687#post10687

[subgr]

Re: Would just recapping solve my problem - desoldered MOSFETs

Hi all.
I have a GA-7VAXP rev 1.1 mobo which recently stopped working. When I opened the case to investigate the reason I found that there was a "NEC K3405 24M" wondering around. The original location of this MOSFET was as the one showed at picture posted by Smithrll46. The MOSFET has been destroyed as there are two cracks at both sides.

If I replace it will the motherboard work?
I can't find a MOSFET with this part number, is the “2SK3405” as mentioned by davmax compatible with "NEC K3405 24M" too (because on the picture is 23M)?
I found from a UK supplier a MOSFET 24v rated but 150A instead of 75A
http://uk.farnell.com/jsp/endecaSear...=9512560&N=401
do you think is suitable?

Thanks

[kc8adu]

Re: Would just recapping solve my problem - desoldered MOSFETs

ignore the 24m
its a datecode.

[WeStSiDePLaYa]

Re: Would just recapping solve my problem - desoldered MOSFETs

i dont think recapping will fix your problem,

i dont think the failed caps caused the fets to overheat.

i think the over heated fets caused the caps to fail, if its hot enough to desolder itself, its way too hot for caps.

so it appears something else is the problem, and recapping and resoldering the fets is just setting yourself to fail again soon.

[gonzo0815]

Re: Would just recapping solve my problem - desoldered MOSFETs

It is well known that bad caps will increase the duty cycle of an smps and could lead to severe dammage to the mosfets due to overheating. This could be easily seen e.g with ecs k7s5a and several of the older epox boards. So i don`t think that there is anything else the cause than bad caps. Any other short circuit condition would probably had exploded the mosfet and not let him slowly melting its solder.
It is clealry that the heat could dmaged the caps futher to an obvious degree, but i am shure that they had already bad esr values.

[burswood]

Re: Would just recapping solve my problem - desoldered MOSFETs

Had exactly the same problem as shown in Smithrll46's photograph, except the Mosfet on the left of the photograph had fallen off the motherboard. Difficult to remember now, but I am convinced that all the Memory cards had lifted themselves out of their sockets. Heat ???

I replaced all the pyramid topped caps and the Mosfet.

Excellent results, up and running again.

4 days later the PC failed again. A hole had blown in the Mosfet between the centre pin and the right hand pin, (looking towards the pins).

Replaced Mosfet again, but PC will not Boot Up. Tried most things, including booting from floppy, with no success.

ANY SUGGESTIONS PLEASE.

I have another board where the Mosfet in the photograph had come away from the heatsink. I replaced in and the board is now okay, but I had to Boot Up from a Windows Start Up disc.

Roger.

[gastorgrab]

Re: Would just recapping solve my problem - desoldered MOSFETs

Is it OK to replace just the one MOSFET or should 'the set' be replaced so everything is balanced? The new part could have different specs just because it's new couldn't it?


.

[Rainbow]

Re: Would just recapping solve my problem - desoldered MOSFETs

Check other MOSFETs too - some of them might be bad.

[Penumbrian]

Re: Would just recapping solve my problem - desoldered MOSFETs

The body of these MOSFETs is soldered to the board, not just the two pins, correct? I have one (same failed mosfet and caps) I want to try to repair.

[Per Hansson]

Re: Would just recapping solve my problem - desoldered MOSFETs

Yes, that is correct

Quite allot of heat is necessary to desolder/solder them in place so it's good if you have something to practice on first...

[skafiskafnjak]

Re: Would just recapping solve my problem - desoldered MOSFETs

I would say before that MOSFET on the picture was like that when it was new.

Transistor usually can not handle over 90C and solder melt around 260C BUT in this case temperature had to go over 330C. There is no way to resolder that FET with temperature under 300C because board is draining temperature.

(unless you have 150W soldering station with fast tepmerature recovery)