How to get to the Soul Sphere ("Supercharge") in Doom 64
level 5, Tech Center:

You'll first appear on this map in a small room surrounded
by doors and passages that lead off in 8 directions.
Once you get the yellow key, go back to that starting room
and then go through the yellow-keyed door.
Take the short elevator up and step into the small, lit-up
square of floorspace next to the elevator.
When standing on that lit square, turn so that the elevator
is to your immediate left.  You'll see some narrow vertical
bars just in front of you, and beyond that, a square, metal
vent in the wall ahead.  (The yellow-keyed door should now
be ahead of you and off to the right a bit.)
Use your pistol (or shotgun/chaingun), and shoot the vent.
The small square of floorspace you're standing on will rise
up temporarily.  While it's raised, turn to your right and
shoot an identical vent in the wall ahead of you.
The bars blocking access to the Soul Sphere will drop down
for a brief moment; Run to reach the Soul Sphere before the
bars rise up again.