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
ranges of the equipment being worked on.
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
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.
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.
are the protection against DC voltages and AC coming
back from the device up the probes leads , it also can absorb surges
nothing special and the whole thing could be made from
junk box parts.