You don't specify what make or model. On the older crts the gross adjustments are inside the monitor the outside controls are for fine tuning.
DANGER! DANGER!
To adjust the gross setting it involves removing the back cover and working near the high voltage flyback with the power on.
ONLY USE PLASTIC OR INSULATED TOOLS
(I ALSO WEAR RUBBER GLOVES)
CRTs can KILL....
There should be three plastic potentiometers with slots cut into them for a small flatblade.
On really old CRTs they also used small magnets attached to the sides of the picture tube as well.
Adjust the monitor slowly. If possible have someone else watch the picture while you adjust so you can insure not touching a live board.
You life is not worth an inch of black screen.