PSU dead - no 5VSB

got an OCZ Mod Xtreme Pro 500W PSU from Craigslist free stuff, received as-is/not working and found to be completely unresponsive when I got it back to the workbench. Nothing on the 5v standby rail even. Checked the board visually and I can't find anything obviously burnt or otherwise physically damaged. Changing the high-side DC filter capacitor (single 400V / 330uF) seemed like a good first step since bad output/low-side filter caps shouldn't cause a complete and total failure like this. I removed it and found the capacitance to be in the nanofarad(!) range, did an autopsy and found it rather dry inside but the vent was not popped so it looks to have dried out slowly and not in a single violent event. I replaced it with a 250V/560uF cap (I won't be running this off of 240 and I won't be giving it away to anyone who will so I'm not too worried about the lower voltage rating) but there's no change in behavior.

There is power making it through the initial mains filtering/rectification section as the replacement DC filter cap is getting a charge (I intentionally shorted the cap's pads on the bottom of the board with the unit freshly unplugged & got a big honkin' spark) which leads me to believe that it's an issue in the standby linear regulation section itself since any failure downstream of that should(?) allow the unit to at least try to start turning on. The weird thing is, I can't find a 5v linear regular on the high/input side. I checked the datasheet on the controller IC that's on the high side and it's just a PWM controller with no built-in passive regulation. Usually when I pull a PSU apart there's an LM7805 or similar on the high side in the vicinity of the platform controller or the bootstrapping SMPS for the FET gates but I'm not seeing that here and thusly I'm stuck for the moment. My working hypothesis is that the massive ripple caused by the failed filter cap resulted in something in high side control/bootstrapping section pulling too much current between waves and gradually burning itself out, but I'm not sure which component that would be. I'm going to open it back up later and poke around a bit more but I figured I'd ask [H] for thoughts in the meantime (thanks for reading thru!)

KATEKATEKATE said:

I replaced it with a 250V/560uF cap (I won't be running this off of 240 and I won't be giving it away to anyone who will so I'm not too worried about the lower voltage rating) but there's no change in behavior.

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Just because you don't have ~220V mains doesn't mean you can lower the voltage of the main filtering capacitor. Some switching power supply topologies use voltage doublers on the primary side to boost the voltage up into the 350v range. You need to verify this with a multimeter before you start substituting capacitors. Using a drastically different capacitance is also a very bad idea, you should be using the exact same rating on the capacitor you replace unless you know exactly how the circuit works.

KATEKATEKATE said:

My working hypothesis is that the massive ripple caused by the failed filter cap resulted in something in high side control/bootstrapping section pulling too much current between waves and gradually burning itself out, but I'm not sure which component that would be. I'm going to open it back up later and poke around a bit more but I figured I'd ask [H] for thoughts in the meantime (thanks for reading thru!)

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Check for failed resistors, especially some of the larger higher power rated ones. They can short out, go open or drift way out of tolerance.