[weblacky]

Hello All,

I'm finally back with some news.
I received the chips and some others components.

This week, i replaced the G7G5 DS1780 chip which is in charge for measuring the PCI 5V.
And i have the regret to inform you that the issue remained.

So after an extensive reading of the LTC1629 datasheet and measuring some key tensions ( RUN/SS, EAIN, Ith, etc).
I decided to exchange this chip with the one in charge of the 3.3V.
And to my surprise the PCI 5V is now operational and so does the 3.3V next door.

Well at least the 3.3V was operational for a few minutes, and is now faulty with 0.98V.
So i guess that the LTC1629 is faulty and i'm gonna need to bite another bullet (16€/piece).

Thank you for reading me and I will keep you informed ASAP.

While this is a longshot before you go removing and replacing that chip again. SGI uses that chip on a lot of their late model boards. It's like a bit of a rubber-stamped design. The data sheet on that claims that the C-out capacitor for each voltage rail is designed to be low ESR and sometimes even ceramic (MLCC) on that controller.  For SGI this is SMD solid aluminum electrolytic or on some of them SMD polymer.

That was a very new thing for these converters back then. If you read troubleshooting documentation on these kind of converter controllers they claim that the converter process can experience odd issues on the C-out Cap that will cause a very odd wobble in error correction algorithm when the output capacitors ESR changes or is slightly too high that it basically makes the error correction nose dive after having an odd oscillation that goes wider and wider as the controller attempts to correct the voltage and this can result in very odd behavior where the voltage would drop especially if you saw it drop slowly but steadily over a period of let's say 30 seconds to several minutes. Because a lot of old electrolytic caps will change ESR suddenly as they heat up from usage this is why they work for the first couple seconds or minutes until their state basically changes from being heated up and now they've tried to reform from use but things obviously went wrong. So before you rip that new IC out I would urge you to look at your C-out capacitor for that 3.3 rail on that rail and check for what the ESR might be or just unusual capacitance or any other reading. Maybe even just replace it blind and see if suddenly you're back up and going. 

It's my belief that this phenomenon may be responsible for quite a few low-voltage VRM buses going bad and Tezro model machines as well as O350 1.8v rail.

It's easy to find out. Just try the replacement of the final output capacitor on that rail and let us know what happens. It can't make it any worse.

Also you could accidentally lose a diode in the simple configuration and have something similar happen. So I would also check that as well but the diodes may will measure really weird in-circuit because they're in reverse polarity for half the configuration and the other half they are designed to clean up the signal to the MOSFET gate so consider blind replacing every pair of SMB diode you encounter. Most of the old stuff is 600 mV forward bias you can't find anymore. I've replaced these with 700 mV forward bias with zero problem but make sure you replaced them in pairs because they have to fire in pairs so if they don't have the same forward bias they'd fire unevenly and do damage . But again a cheap part that's easy to check and easy to solder in and solder out of the layout

If either of these end up being your answer please do let us know as this is my working theory so I have a bit of a vested interest in the outcome.