Read the FAQ on CRTs in this forum. If you're using a black and white classic it may or may not be interchangeable depending on if you have the Revision A or Revision B analog board/CRT in your Classic. (Revision A will work in a 512K).
However, your problem sounds more like an issue with the focus on your machine. You'll need a TV adjustment tool (you can find these at Radio Shack) to adjust the knob on the side of the analog board that controls the focus. CRTs tend to need this adjustment once they hit ten years of age--I had it happen with a TV and have seen it on over 50% of the compact Macs I've worked on lately.
However, your problem sounds more like an issue with the focus on your machine. You'll need a TV adjustment tool (you can find these at Radio Shack) to adjust the knob on the side of the analog board that controls the focus. CRTs tend to need this adjustment once they hit ten years of age--I had it happen with a TV and have seen it on over 50% of the compact Macs I've worked on lately.