Racal Instruments RA-117 User Manual page 20

and L63 in series with C152, C152A and C153. The final section consisting of L68
and L71 in series with C161 and C162, is damped by the series resistors R86, R87A
and R88 according to the bandwidth. In the L-C positions the output is taken from
a capacitive divider formed by C161 and C161A with C170, to equalize the gains in
the L-C and crystal bandwidth positions.
27. The L-C banwidths are obtained by varying the degree of coupling between each
section of the filter in addition to the damping resistors in the final stage. The capacitor
C175 is included to compensate for the effective reduction of the input capacitance of
V14, appearing across the tuned circuit, when switching from crystal to L-C positions.
28. To maintain the input capacitance of the L-C filter, in the crystal positions, a trimming
capacitor C148 is switched into circuits. This trimmer is adjusted to be equal to the
output capacitance of V26 and the screened cable. In the crystal bandwidth positions,
the L-C filter is operating in its narrow bandwidth positions, i.e. 1 2 kc/s.
NOTE:The 470-kilohm damping resistors R77 and R80 are disconnected except during
filter alignment.
FIRST 100 KC/S I.F. AMPLIFIER
29. The output from the L-C filter is passed through a coupling capacitor C164 to the
control grid of the pentode amplifier valve V14. This grid is returned via R96 to the
A.V.C. line which is filtered at this point by R102 and C173. The screen potential is
derived from a potential divider formed by R93, R97 and RV4. This stage is coupled
to the second I.F. amplifier and the I.F. output stage by a double tuned transformer
having an over-coupled characteristic.
SECOND 100 KC/S I.F. AMPLIFIER
30. The signal from the first I.F. tranformer is fed through the grid stopper R114 to the
control grid of the second I.F. amplifier. H.T. is supplied to the screen via the dropping
resistor R113 and is decoupled by C181. The anode load is tuned circuit consisting of
L77, C192 and C191. This circuit is heavily damped by R112. The secondary winding
L78 and L79 is tuned by C195 and C195B with R120A as a damping resistor. The
output is fed to the diode detector anode.
DIODE DETECTOR
31. The low potential end of L79 is connected through the R.F. filter (C209, R128, C210,
C219 and C211) to the diode load R130. With the meter switched to R.F. LEVEL, the
meter indicates the detector diode current. The resistor R131 is incluced to complete
the diode detector circuit when the meter is switched out of circuit.
NOISE LIMITER
32. The noise limiter diode (pins 2 and 5 of V21) is connected in a series circuit to operate
at approximately 30% modulation. its operation is explained with reference to fig.5.
33. The d.c. path from point A is through R134, R135, the diode and R137. The A.F. signal
path from detector diode load is through C216, the diode and C218 when S8 is open.
In the presence of a signal, a negative potential varying with the depth of modulation,
will be developed at point A thus causing the diode to conduct. The negative potential
Detailed Circuit Description 20