Really old stuff, with vacuum tubes, can have a "hot chassis" with mains voltage on it, and/or 200-300 volts on the plates of the tubes, be careful. Tube testers can be had, and so can tubes, for the most part.
Newer stuff, with transistors, usually has dead electrolytic capacitors that need to be replaced, and sometimes there will be other issues, most of the time it's a matter of tracing signals through a system to see which stage is at fault.
When you get to the newer stuff, with surface mount components, you'll need to have schematics and block diagrams in order to even start to figure out what's going on. The really new stuff has transistors and other passive parts so small you'll have to have a microscope to work on them with. You'll need hot air reworking gear to repair that type of stuff.
But first.... as I said earlier, start with the basics... programmers don't have good intuition about parallel and series circuits, etc. You can get multimeters and oscilloscopes very cheap these days.
If you want to get into radio, investing and playing with an RTL-SDR dongle and GNU Radio is a great way to get a feel for how things work, and to learn about negative frequencies, and all things signal processing related.