My advice: get into it gently.

Most problems can be solved by simply knowing what the various modules do, and replacing them when necessary. The next level is to understand the basics of electronics, so that you can identify problems visually or using a multimeter. That will help you target most problems. After that, the more you know the more you can do. But you are also in the realm of diminishing returns (i.e. you need to know a lot more to do a little bit more).