maple classic™ 
you know what?
http://www.tomshardware.com/reviews/psu-repair,4147-14.html
this whole article is very similar to what i'm having
the old phenom I built with this psu back in 2011 did not use 5vsb constantly
This new AM4 motherboard however, keeps 5vsb powering USB devices constantly, and I in fact used one of them to power an USB night light while my computer was off
That might've worn down the 5vsb output, which would explain a single output cap failing...
I'll trace back which capacitor that one I suspected is, if it is in fact the 5vsb output, I'm pretty sure that would fix it
however, the replacement cap I have, while similarly rated, is not high-ESR, so it's probably not a great fix
even then, at least it would keep this psu on its last legs while I wait for its replacement??