What is a good circuit to use for teaching troubleshooting?

I'm talking about more complex circuits, so they can see something that might "scare" them at first but not be as scared after walking through it, personally I was never much scared of little things, I was talking about for example, rather than picking random spots on a PC board, how would troubleshoot for a specific component [I know you probably wouldn't, and would just replace the board, but say I wanted to find the diode that broke down, you wouldn't measure every diode on a pc board, but would look for ceratin things, what is a good circuit for teaching something like that? no offense but I'm going to throw up if I see another 555 in the next few weeks, HAHA It's the only dang IC they sell in this electrically foresaken city...