GA-8S655FX with very hot FET

[anonbeat]

Hello,
I have a Gigabyte GA-8S655FX motherboard that do weird things. Start when have been powered off for some time and after a while running it locks.
I have been trying to find what the problem is and found a FET that gets too hot. its near the Ram slots. Its the one in the Picture 1.
This FET gets about 8.6 volts in the gate pin and I think this is wrong. I tracked the pin and it comes from another chip with reference ST M104I that is the one in the picture 2.
The picture 3 is the motherboard.
All the caps in the MB looks good.
Anyone know what I can check now ? What could the problem ?

Thanks in advance

[anonbeat]

Re: GA-8S655FX with very hot FET

I can now add that the M104I is a quad differential amplifier and the inputs of the amplifier that handles this FET has 2.89v.

Any ideas ? could this be a badcaps problem ?

Thanks in advance

[anonbeat]

Re: GA-8S655FX with very hot FET

Nobody has any idea? do you think I should recap the mobo ? What could I test ?

Thanks in advance

[davmax]

Re: GA-8S655FX with very hot FET

You have not said how hot the MOSFET gets. The photos do not indicate excess heat. How do you know that this component is your problem?

What tests have you run on your computer? For example:
1. Has the hard drive been checked for errors? Error checking in the properties of the hard drive.
2. Has memory been checked for errors? Using memtest86

Errors in the above can cause your problem.

[anonbeat]

Re: GA-8S655FX with very hot FET

I start the system for testing only with memory and cdrom. The memory is working fine. The problem is that it starts and after a few minutes it locks. The fet gets so hot I cant touch it with my finger for more than a second after it have been running for a little while. This is the component that gets hotter. This is what made me think it could be the problem. If you look the area around the fet in picture 1 you can see its darker couse the temp the fet gets.

Im not sure if its the fet or not. I tested the FET with multimeter and its not crossed so I think it could be something related to it.

Thank you for your help.

[Brian C]

Re: GA-8S655FX with very hot FET

Is the capacitor next to the FET nichicon HM series ? Be careful of nichicon HM cap. Anyway I believe that is the AGP voltage regulator circuit. Could you please check the voltage level ?

[davmax]

Re: GA-8S655FX with very hot FET

I start the system for testing only with memory and cdrom. The memory is working fine. The problem is that it starts and after a few minutes it locks. The fet gets so hot I cant touch it with my finger for more than a second after it have been running for a little while. This is the component that gets hotter. This is what made me think it could be the problem. If you look the area around the fet in picture 1 you can see its darker couse the temp the fet gets.

Im not sure if its the fet or not. I tested the FET with multimeter and its not crossed so I think it could be something related to it.

Thank you for your help.

You say memory is fine. Was it tested thoroughly with Memtest?
If you are only using CDROM and no hard drive I presume the system is not booting. Is that right?
What is the system doing when it locks? Are you in the system Bios?
What are the voltages on the MOSFET drain and source, if it is a MOSFET. Is there a part number?
Some devices do run that hot normally. ie you cannot keep finger on it.

[PCBONEZ]

Re: GA-8S655FX with very hot FET

You have KZG near CPU socket.
KZG don't like heat and go bad there.
They don't always bloat.


HN and HM made between 2002 and 2004 (or 2005 depending on who you ask) were bad.
-> Manufacturing error.
Date code is on cap and looks something like: D0326
D[03]26 - This spot is year, year 2003
D03[26] - This spot is week of year, week 26
I dunno what the letter means.

.

[anonbeat]

Re: GA-8S655FX with very hot FET

The capacitors near the CPU socket in the input VRMs ar
HM 3300uF 6.3v H0411
HM 1500uF 16v H0409
KZG 1500uf 6.3v 39 W(3)

Near the Hotter FET there is a nichicon 1000uF 6.3v A0411

The fet part number is On Semiconductor NTD3055-150 . Its a Power MOSFET 9.0 Amps, 60 Volts N–Channel DPAK

When I power on the system I get this voltages in the FET that gets hot.
D - 3.32v
S - 1.90v
G - 6.30v

Ive tested memory with memtest86 on other system and is fine. Now Im going to read AGP and Memory slots volts.

Thank you all for your help
Greets

[PCBONEZ]

Re: GA-8S655FX with very hot FET

Any or all of those caps could be a problem.
-
If the caps aren't dumping the ripple to ground the FET and MOSFET may be trying to compensate for ripple. They aren't made to switch that fast. It will burn them out.

[anonbeat]

Re: GA-8S655FX with very hot FET

The PCI and AGP volts are OK.

Just found this FET is the memory regulator and its not giving the 2.5v but 1.29

Any ideas of what could be the problem this is not giving 2.5 Volts ?

Thanks for all your help
Greets

[PCBONEZ]

Re: GA-8S655FX with very hot FET

Leaky cap.
As in: excessive leakage current.

[anonbeat]

Re: GA-8S655FX with very hot FET

Wich one do you suspect it could be the problem ?
How can I check wich ones are faulty ?

Thanks in advance
greets

[anonbeat]

Re: GA-8S655FX with very hot FET

Sorry I was wrong...
The mosfet that gets HOT powers the Southbridge SIS964 chip. Other mosfet gives power to the DDR slots and this last is giving 1.29V instead of 2.5volts and is not getting even warm.

This two fets gets power from the 3.3v line.

[PCBONEZ]

Re: GA-8S655FX with very hot FET

'Out' cap should be near FET and + side of cap in common with a FET lead.
There may not be an out cap.

'In' caps are usually many in parallel and can be all over the board.
As in, all the caps on 3.3v are in parallel.

.

[anonbeat]

Re: GA-8S655FX with very hot FET

Ok figured out that the 1.29v are fine!

As indicated here
the DDR slots needs 2.5v and 1.25v

The controller for the DDR memory voltages is the w83301. The w83301 datasheet have an application circuit which reflects exactly the gigabyte ddr voltage regulators used in this board and looks fine. The datasheet can be downloaded from here .

So I get back to the first FET which gets too hot as the problem.

Thank you all for your help
greets
anonbeat

[davmax]

Re: GA-8S655FX with very hot FET

You say the HOT MOSFET is powered from 3.3V and it has an output of 1.9V that you say powers the south bridge. I suspect this is operating correctly if the south bridge does indeed require 1.9 to 2.0 volts. The heat is high but most likely OK. The MOSFET can run at 80C or higher and that is hot. This heat is not good for any electrolytic nearby. The suggestion is that you change that electro and the KZG caps. The Nichicon HM caps should be OK. Really you need to borrow an infra red thermometer to check the MOSFET temp.

[davmax]

Re: GA-8S655FX with very hot FET

You have not made it clear what the computer is doing when it freezes. You have not mentioned a hard drive. It will give a better understanding if we know the complete system configuration and what process is going on. You may have a cap problem, often a cap problem will cause a boot failure.

[anonbeat]

Re: GA-8S655FX with very hot FET

Right now this board only boots with a 400/533 Mhz Pentium IV and not with 800Mhz one, but even when boots it locks after a few minutes. I removed all components but the strictly necesary like video card, memory and a cdrom drive to load Ubuntu or to run memtest86.
When boots you can get into BIOS. I have loaded BIOS safe defaults and rebooted to clear a posible BIOS corruption. I tried to load Ubuntu but got locked loading or just after it finished loading without the posibility to check any error or system log.
Im going to try to get a infrared termometer to check this FET temperature and post it here.
I would love to get the SIS 864 datasheet but looked in google with no sucess.

[PCBONEZ]

Re: GA-8S655FX with very hot FET

HM are date code week 11 of 2004.
- Probably NOT okay.
Thats the time bad HM were killing thousands of motherboards.
MAC G5 recall, Dell SX/GX-260/270/280 problems, many MANY others.
Just use google.
.