build a signal injector

In Radio and Audio circuits a handy tool to have for servicing and fault finding them is a signal injector.
It is a wide band un-tuned oscillator that operates within the frequency ranges of the equipment being worked on.

Connecting the injector to the circuit you should be able to hear its tone in the speaker of the audio device.

Starting at one end of the device under investigation we can work our way around the circuit , following it's schematic to find where the tone is lost ...
This is the place where we should look closely for possible faulty parts.

Using a signal injector is quite straight forward and safe , but of course at any time a device is open and you could possibly be exposed  to high voltages or currents , extreme caution is very much recommended. (don't do it after drinking beer ! )

This one has some extra protection in case you do something dumb and come up against some high voltages with your probes tip.

This schematic was found somewhere on the net and although I built it in approximately 2003 it is still working fine today.
The circuit is nothing too special , I have altered it a little but it works fine from audio frequencies on up to about 10MHz.

It is useful for amplifier work and AM and Shortwave radio servicing , but above the AM broadcast band the injectors strength drops off.

Notable features are the protection against DC voltages and AC coming back from the device up the probes leads , it also can absorb surges from capacitors.
Components are nothing special and the whole thing could be made from junk box parts.