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Old 11-11-2012, 09:54 PM   #7
momaka
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Default Re: Need help with Shuttle SB83G5A - fans spin but that's all

It's really been a while since I last updated the info in this thread. I removed the lower 4404 MOSFET a long time ago and tested it out of circuit - there was a short circuit (6-8 Ohms actually) not only between Source and Drain of that MOSFET but also between Gate and Drain! Checking the resistance between the Gate pad on the motherboard for that MOSFET and ground revealed a short circuit too. So looks like the MOSFET took out the controller with it after it shorted. GREAT! The controller that's responsible for driving this buck circuit is a RichTek RT9203.

And now for some more bad news...
Before I even found the gate drive on the RT9203 controller for that MOSFET was shorted, I tried wiring up a 60N03 MOSFET in place of the dead 4404 to see if the NB voltage would come up. And in this process, I managed to rip the pad on the board for the Gate of that MOSFET. So now even if I do get a replacement RT9203 controller and 4404 MOSFET, I would still have to do some ghetto hacks to wire the Gate drive.

I've been eager to test that computer so I've also been working on a 1.5V DC-DC power supply. Finally made a design not too long ago that actually works well enough with more than 0.5A of load, but I think it's oscillating at high loads (2A and above). It's a linear setup with a single MOSFET and XRA10358 (equivalent to LM358) op-amp. I'll leave that for another thread, though. Maybe if I get that working, I'll use it instead of buying replacement MOSFET and controller. Not that the controller is that expensive or anything. Just eager to build my own working DC-DC low-voltage power supply. Right now, the circuit I have works fine with 0.5A of load or less. When I go above 1A, the voltage goes up to 1.6V - I'm not sure if the Intel 915G NB will like that. Finally, when I increase the load to 5A, the output drops to 1.42 to 1.48V (it's fluctuating all the time).

Last edited by momaka; 11-11-2012 at 10:00 PM..
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