Here's a signal injector for those of us like me who don't have a bench full of nice test equipment. It can be used for tracing audio circuits for troubleshooting, or for working on audio circuits that require some kind of signal for testing.
The injector is a single-transistor implementation of the classic twin-T oscillator circuit. I actually found this in the ARRL (Amateur Radio Radio League) Handbook, and adapted it for my purposes. The transistor type is absolutely non-critical: any small-signal NPN transistor will do. Likewise, component values aren't crucial, and you can change them for different frequency output if you like. (As built, the injector gives about a 1 kHz tone at about 1v p/p.)
The whole thing can be built on a tiny scrap of perfboard and mounted in a small box (I used an Altoids-type metal lozenge box).