What are lines on my CRT?

[Console User]

I had to start using a new converter box on my Analog RCA Set. The box I have is a LasonicTA260 Converter Box. I have a really nice picture but have one issue. If you are close enough to the CRT you can sometimes see red& green vertical lines in dark backgrounds. I remember using this same box on another RCA Set of about the same time era & it had those same lines on it. I didn't have these lines when I used my previous box that quit working.

Anyone that would know what is causing this or what I can do to remove these lines would be appreciated.