The simplest way I can think of is to go over to the shelf of test equipment
and grab the old Heath signal generator and attach a foot long wire close to
the S-38. Then tune the generator to 455 kc (whoops, that is kHz). You
will have a nice beat note.

Like the others have said, convert the if stage into an oscillator - then
reduce the feedback to get it to stop oscillating. That will then be sort
of a Q multiplier while in the feedback but not enough to oscillate stage.
You would need a control to put in and out of oscillation. A pot, a
variable capacitor or a switch.

Or, just a hum a few bars into the Ranger mike and tune for best audio.

73, Colin