I found a 7" Bosch angle grinder at the dump. Didn't work, but the good news was that it was totally dead - meaning that a broken switch or wire could be the problem. Unlike: it runs, but poorly - which means bad winding or armature.
I took it apart & quickly isolated it to a bad switch. Good! Worse case is buy a new switch, or jury rig a generic switch into it.
It got better: the switch comes apart! They're usually "welded" together.
Inside there's a bunch of parts. Both sides of the circuit are switched & each has a spring, contacts, levers & a little shield that swings up between the contacts to interrupt any arc. Given that it's rated for
13A & DC (universal motor), an arc-interrupter is nice.
Well, the only problem was that a couple of the parts had gotten rearranged. E.g., one end of a spring had come out of its seat. That's good, what was bad it that it was not at all obvious what the correct arrangement is. What with a snap action required via a flat spring going from convex to concave.
After a few iterations I got it together properly & it works great! I'm going to pull it all apart for cleaning & a lube. Then I'll have a tool that I'll probably hardly ever use, but I will have the satisfaction of resurrecting it.
Bob