2011 i5 27" imac wired keyboard does not power up I have tried 3 keyboards and every one does not get power until after OS X starts it's boot sequence, it seem the USB bus is not getting power until about a minute after startup. I am therefore unable to launch diagnostics, reset NVRAM, PRAM and all the other good stuff I need at present because my SSD is dying and I'd like to upgrade it's firmware

Any ideas?
Plug out the iMac (from power) for 15+ seconds to reset SMC.