GA-945GCM-S2L - low voltage on VTT_GMCH

[caspian]

Dear members,

I have a GA-945GCM-S2L mainboard that I am fixing. All voltage rails are fine in this mainboard, except for VTT_GMCH.

When CPU is out of the board, VTT_GMCH is fine at 1.22v. In this case, we have:
Pin1 of Q287: 1.22v
Pin2 of Q287: 1.5v
Pin3 of Q287: 1v
Pin4 of U85: 12v
Pin5 of U85: 1.22v
Pin6 of U85: 1.22v
Pin7 of U85: 1v
VTT_PWRGD: 1v
CPU_VCORE: 1.06v
resistance on VTT_GMCH: 200 Ohms.

When CPU is inserted, VTT_GMCH is low at 0.2v. In this case, we have:
Pin1 of Q287: 0.4v
Pin2 of Q287: 1.5v
Pin3 of Q287: 0.2v
Pin4 of U85: 12v
Pin5 of U85: 1.22v
Pin6 of U85: 0.2v
Pin7 of U85: 3.6v
VTT_PWRGD: 0v
CPU_VCORE: 0.0v
resistance on VTT_GMCH: 9 Ohms.

I have tested it with 3 CPUs (two Celeron-D CPUs and one Pentium 4 CPU) and the mainboard has this same problem.
I have replaced Q287 and EC169 but the problem persists.
I did not replace U85 because I think it is working correctly.

What do you think is the problem?

Thanks

[momaka]

Re: GA-945GCM-S2L - low voltage on VTT_GMCH

When CPU is inserted, VTT_GMCH is low at 0.2v. In this case, we have:
Pin1 of Q287: 0.4v
Pin2 of Q287: 1.5v
Pin3 of Q287: 0.2v
Pin4 of U85: 12v
Pin5 of U85: 1.22v
Pin6 of U85: 0.2v
Pin7 of U85: 3.6v
VTT_PWRGD: 0v
CPU_VCORE: 0.0v
resistance on VTT_GMCH: 9 Ohms.

From what I can tell from this, there is only a 100-Ohm resistor (R1712) between the Gate of Q287 and the output of U85b. Therefore, start by checking R1712. It should be 100 Ohms. If not, that's probably the problem.

Also, I noticed the following that I think may be an error in the report here:

When CPU is out of the board, VTT_GMCH is fine at 1.22v. In this case, we have:
Pin1 of Q287: 1.22v
Pin2 of Q287: 1.5v
Pin3 of Q287: 1v
Pin4 of U85: 12v
Pin5 of U85: 1.22v
Pin6 of U85: 1.22v
Pin7 of U85: 1v
VTT_PWRGD: 1v
CPU_VCORE: 1.06v

Since pin 1 of Q287 is tied to the pin 7 (output) of U85b (via 100-Ohm resistor), then voltage on these two should be the same. I don't see how you can have a higher voltage on the MOSFET's Gate than what U85b provides. Likewise, if you measured only 1V on pin 3 of Q287, that would imply VTT_GMCH = 1V. So I can only -assume- you meant to say here that pin 1 of Q287 is 1V, and pin 3 of Q287 1.22V... yes?

Otherwise, I don't see why else VTT_GMCH would go low when there is a CPU... unless VTT_GMCH is used for deriving some other voltage rail on the motherboard that is shorted, but not turned On until valid CPU is inserted.

Since this is a socket 775 board, you could also try an unsupported Core 2 Duo CPU, then see if VTT_GMCH goes low or not. If VTT_GMCH is still present with the unsupported VTT_GMCH, then indeed it might be a case of the VTT_GMCH powering some other rail downstream that is shorted and only trying to activate when there is a valid CPU inserted.

[caspian]

Re: GA-945GCM-S2L - low voltage on VTT_GMCH

I am sorry. I had a typing mistake. I correct it as below:
When CPU is out of the board, VTT_GMCH is fine at 1.22v. In this case, we have:
Pin1 of Q287: 1v
Pin2 of Q287: 1.5v
Pin3 of Q287: 1v
Pin4 of U85: 12v
Pin5 of U85: 1.22v
Pin6 of U85: 1.22v
Pin7 of U85: 1v
VTT_PWRGD: 1v
CPU_VCORE: 1.06v
resistance on VTT_GMCH: 200 Ohms.

[caspian]

Re: GA-945GCM-S2L - low voltage on VTT_GMCH

Since the schematics of this mainboard includes an advertising link inside, I cannot attach it here. But it can be found by Google searching.
Currently, I have no more CPU to test on this mainboard.
I have checked all the rails connected to VTT_GMCH in the schematics. all of them has higher resistance than VTT_GMCH.
VTT_GMCH is connected to both PCH and CPU. The CPU connection of VTT_GMCH is 9 Ohms to GND whereas the PCH connection of VTT_GMCH is 200 Ohms to GND.
Maybe, this 9 Ohm resistance causes the problem. I do not know. I inserted this CPU in another mainboard, then VTT_GMCH become high on that mainboard.
All the 3 CPUs have about 9v to GND on VTT_GMCH.
R1712 is 100 Ohms.
I tested Q287. it does not seem to be shorted.
The current to VTT_GMCH is provided by VCC1_5.
U85 provides voltage on pin1 of Q287 to make it On.
When CPU is not inserted, U85 can make Q287 on easily by putting 1v on pin1 of Q287.
When CPU is inserted and VTT_GMCH is low, U85 feels this low voltage on its pin6. Then, U85 puts more voltage on its pin7 (about 3.6v) to make Q287 on. but this voltage drops at pin1 of Q287. I do not know why.
This voltage drop may be due to the low resistance on VTT_GMCH. But I am not sure. Maybe, the low resistance on VTT_GMCH results in a too high current from Pin2 to Pin3 of Q287 that causes Q287 to turn off.

So I am confused now. Any help is appreciated to discover the defective component.

[caspian]

Re: GA-945GCM-S2L - low voltage on VTT_GMCH

Once, when CPU is out of the board, I connected VTT_GMCH to GND using a 50 Ohm resistor. then, I observed VTT_GMCH was still stable at 1.22v.
That means, if the resistance of VTT_GMCH be 50 Ohms, then it will work in this mainboard.

[stj]

Re: GA-945GCM-S2L - low voltage on VTT_GMCH

maybe the fet is failing and cant supply the current.
if you can turn it on without the cpu, try loading it down with a resistor and see if the output changes.
it shouldnt because it has a feedback loop to the opamp

[caspian]

Re: GA-945GCM-S2L - low voltage on VTT_GMCH

When CPU is out of the board, I connected VTT_GMCH to GND using a 15 Ohm resistor and a 6 Ohm resistor. then, I observed VTT_GMCH was still stable at 1.22v with each one of these resistors.