Well wait. I get the idea but you left it with no power into it? That's not in what you described (unless that's the leaving overnight without), I don't think.  And also when doing that remove the power board.  Unplug everything.  Remove power board.  Wait a bit, replace power board.  Replace AC (not battery), try soft boot.

I'm not an electrician but I suspect that one is trying to drain all the capacitors.  (That's just based on observation from my various 3400c exploits)