this might help.
http://groups.yahoo.com/neo/groups/heathkit/conversations/topics/12369QUOTE:
" The 3-pin connector Heath used is "nearly" the same as the old HP test
equipment power connector but the spacing between pins and the offset of the
ground (center) pin is different. The closest actual match I've found was a
cord used on office equipment (adding machines and electric typewriters) from
the 60s. Those cords fit 100% but are damn hard to find in decent condition..
What a friend of mine did was to use one of the 3-pin "Mickey Mouse Ears"
connectors from an old Compaq laptop with a built-in power supply (I think it
was an Armada 7700). The chassis mounted end mounting spacing is nearly
identical to the Heathkit connector - elongating the holes in the mounting
ears of the chassis mount connector enabled the holes in the rear panel of
the Heath unit to line up WITHOUT modifing the rear panel.
The cords for that connector are easily available (including on the web
site one of the earlier posters posted). "
http://www.ebay.co.uk/itm/AC-6ft-Power-cord-Mickey-Mouse-Used-for-Compaq-ARMADA-Series-7400-7700-800-/310635354091?pt=US_Power_Cables_Connectors&hash=item48534f57eb