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Old 10-24-2011, 11:01 PM   #6
momaka
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Default Re: Need help with Shuttle SB83G5A - fans spin but that's all

Quote:
Originally Posted by kc8adu
look at that vrm again.iirc 915 uses 1.5v core.could also be shorted.does nb or vrm get hot?
Correct, 915 uses 1.5v core. Nothing gets hot, though, and that's what worries me a bit. I measured resistance between the NB/SB VRM low and ground and the lowest I got was 40 Ohms. Switching the probes around gives higher readings. In any case, I can't get a short circuit across either of the MOSFETs.

Quote:
Originally Posted by retiredcaps View Post
I'm not sure if my understanding of how this works is correct or not, but the AO4404 datasheet says that the gate threshold voltage minimum is 0.7V, typical is 1.0V, and max is 1.4V.
Vgs(th) indicates just what voltage the MOSFET needs in order to turn on.

Good thing you brought it up, though. I measured the Gate voltage on both MOSFETs and here's what I got:
- the lower (supposedly bad) MOSFET that's connected between 3.3v rail and NB/SB VRM low has 1.4v on its Gate.
- the upper MOSFET that's connected between ground and NB/SB VRM low has 0.0v on its Gate.

So if I'm not mistaken, it looks like the PWM controller controlling these MOSFETs is trying to make the lower MOSFET conduct, but perhaps the lower MOSFET is open?
As for the 0.6v present on the NB/SB VRM, my only guess is that this is caused from leakage current from other components. Any thoughts on this?

If not, I guess I'll remove the lower MOSFET, test it out of circuit, and then proceed from there.

Last edited by momaka; 10-24-2011 at 11:05 PM..
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