Start with something like The Art of Electronics by Horowitz and Winfield.
An example of the kinds of EE stuff you don't heave to know to be an effective troubleshooter of basic issues, might be, oh, say, what it means for the poles of a transfer function lie in the left half of the s-plane.
A stereo that loses the right channel unless you push on the board? Broken solder joint.
Visibly exploded caps? Bad caps.
Some basic understanding of electricity and current flow and the high-level concepts will help, but a detailed education is more for building your own than just repairing existing mass-produced setups.
(As an aside the electrical knowledge in a good ham radio study guide would be a great place to start - https://home.arrl.org/action/Store/Product-Details/productId... )