Ok, after thinking about your file for a second, I gigured maybe you had accidently selected all your objects and choosen Polygons>Combine because that's what it looks like you've done.

I created two different oscilloscopes in the file attached. The right-most oscilloscope is your version, the middle one is a version made up of several objects (the easiest one for you to create), and the left-most one is a version made from extruding faces.

For a simple object such as this, booleans are not really neccesary because you'll usually end up with more faces that you should. In addition to this, if the boolean were to work, you would have irregularly-shaped faces. Making the box-like portions and the knobs seperately without extruding or booleaning makes modeling go faster in some cases. This is one of them.

For the middle model's box-like parts, I simply took a cube, selected one face, extruded it inward by scaling the face, then extruded it inward with the move tool and placed it where ever I pleased on your main object.

For the bottom box-like part, you have a small extrusion that appears to be some kind of button I guess. To create this, I used the SplitPolyTool under the Editpolygons list to make an extra edge, movd the edge to the right, then extruded it. I'm fairly sure this is what you did as well in your version.

If you don't like having your oscilloscope being made up of several objects, you can always use the split poly tool to break the front face of the main object into many faces with which you can extrude from. I will explain exactly how to do this in the next post.