So, I came into a couple of inoperable amplifiers/receivers recently as I was putting together a budget audio solution, thought they might provide a good opportunity to learn how to actually test circuits and diagnose issues with these types of devices. I'm struggling to find substantive resources on line that can help me start this journey, though. The Internet's now flooded with IT Help Desk style "turn it off and on again" type articles that just end with "if that doesn't work, send it in to be repaired".

I know there are, like, college courses for this kind of stuff, and that's probably going a step too far. I'm not actually looking to do repairs myself (not yet at least), just learn how to use the tools and where to poke them so that I can go "ah, so that's what's wrong!"

Are there reasonable resources for dipping my toes into this? Good intro textbooks? MOOC lectures? Video tutorial series? Or even just specific terms I should be using while searching for such things that will help clear up my queries' signal:noise ratio?