Heh, I've got a Grundy Newbrain PSU here
that's missing its plug so I'll be
having similar fun trying to sort that one out :)
I have the Newbrain schematics, so I can find the pinout of the power
connector (IIRC 4 pins used) on the Newbrain. Then it's just a matter of
taking the PSU apart, finding the common ground wire, and identifying the
voltages on the other 3 wires. Not hard at all.
-tony