(from TM 11-866)

19.  BEAT OSCILLATOR. --  The beat oscillator tube (V10)
and associated oscillator circuit (T5) provide an IF voltage of
approximately 465 kc.  This voltage, when introduced into the input
circuit of the 2nd detector (V8) by means of the small coupling
capacitor (C41), mixes with the IF signal being delivered to the
detector by the 3rd IF amplifier tube (V7).  The mixture of these
two similar frequencies results in a "beat" or difference frequency in
the output of the 2nd detector.  By adjusting the beat oscillator
frequency to the proper value, the pitch of the difference frequency
can be controlled at will.  Fine adjustment of the frequency is
accomplished by means of the BEAT OSCILLATOR control (E17)
on the front panel which turns a small variable capacitor (C47)
in transformer T5.  The oscillator is turned on by throwing the
SIGNAL-MOD-CW switch (SW3) to CW.  In addition to being
necessary for proper reception of CW code signals, the beat oscillator
is useful for locating weak signals of any kind.