(from TM 11-866)

14.  CRYSTAL FILTER

    a.  The Quartz Crystal Filter (T1) couples the 1st detector
(V3) to the 1st IF amplifier (V5).  Its selectivity can be varied in
definite steps by the CRYSTAL SELECTIVITY switch (SW7) controlled
from the front panel by knob and pointer E7.  In addition,
its selectivity characteristics can be greatly sharpened on one side or
the other by adjusting the PHASING condenser (C32), which is
controlled by knob E8.

    b.  When the CRYSTAL SELECTIVITY switch is set at OFF the quartz
crystal is short-circuited and signal voltages present in the
secondary of the 1st detector plate coil (L26) are impressed
directly on the control grid of the 1st IF amplifier tube (V5).
At any other setting (1 to 5) the quartz crystal is in use and acts
as an extremely high "Q", high impedance, series tuned circuit
interposed between the secondary of plate coil (L26) and the 1st IF
grid circuit (L27, C33), which constitutes the load into which the
crystal works.  Selectivity is varied by altering the impedance of
this parallel tuned circuit (L27, C33), which is accomplished by adding
resistance (R42, R43, R44, R45) in series with coil L27 and capacitor
C33.  As this series resistance is increased (reducing the parallel
impedance of circuit L27, C33) the overall selectivity of the filter
is also increased.