This testing instrument provide two alternate fuction: signal injector and signal tracer.  This audio signal tracer/injector is very helpful in trouble shooting audio circuits, when you need to test a circuit by injecting a signal and observe the output (by watching the oscilloscope or by hearing the loudspeaker for example), or by tracing some points inside the circuit when an audio signal is applied to the input. Here is the schematic diagram of the circuit:

audio injector tracer circuit schematic

This signal tracer/injector uses 9 volts supply from battery. An alligator clip is recommneded for the ground probe, so you can works with one hand to hold the board, and the other hand to target the test probe. The SPDT switch connected to the transitor and the earpiece is used to select the function, whether as a signal injector or a signal tracer.