Though there is a huge variety of electric power tools, the basic electrical components -- cords, switches and brushes -- are similar regardless of the tool's purpose or manufacturer.
From the article:
"(The) procedure for tracing problems begins with testing and examining wires, then switches and finally brushes. The only special tool you will need is a multi-tester, available from an electronics shop for about $25. With it, a host of problems can be traced right to the source. Many electrical components on power tools can be replaced for $20 or less if you do the repair work yourself."
Here's a link to the article, Basic Repairs for Portable Power Tools