Racal Instruments RA-117 User Manual page 21

at B, will be lower than at A and will be maintained at a constant level due to the
long time constant of R134 and C217. R135 allows the cathode potential to vary in
sympathy with the modulation provided the modulation depth does not exceed 30%.
The potential appearing at the cathode of the noise limiter diode therefore consists of
a steady negative potential with the modulation superimposed. When noise impulses
corresponding to high modulation peaks appear at point A and via C216 at point C,
the voltage across the diode changes sign thereby causing the diode to stop conducting
and open-circuit the A.F. signal path. With S8 in the OFF position the limiter is
inoperative.
A.V.C. AND T.C. DIODE
34. The signal appearing at the anode of V16 is passed through the capacitor C139 to the
anode of the A.V.C. diode. The diode load is formed by R116. A positive potential
derived from R120, R121 and R122, supplies the required A.V.C. delay voltage to the
cathode of this diode.When A.V.C. switch is in the SHORT position and the SYSTEM
switch set to a position in which the A.V.C. is operative, i.e. A.V.C., CAL. or CHECK
B.F.O., the anode of the A.V.C. diode is connected to the A.V.C. line via L81 and
R127. The choke L81 is tuned by C203 to a frequency slightly below 100 kc/s so that
is presents a small capacitance at 100 kc/s, thus R127 is prevented from shunting
the diode load. When the signal level falls, the capacitors C182 and C173 discharge
through R118, R127 and L81 into the diode load resistor R116. The A.V.C. potential
is brought out via R123 to the tag strip at the rear of the receiver for external use if
required. With the SYSTEM switch set to the MANUAL position, the A.V.C. line is
connected to the R.F./I.F. GAIN control RV1, thus the gain of the 100 kc/s amplifiers
may be varied by adjusting the negative potential applied to the A.V.C. line.
AUDIO OUTPUT
35. Audio frequencies are applied to the control grid of V23B via RV2 the A.F. GAIN
control. The output transformer (T2) provides four separate outputs as follows: 1W
into 3-ohms, and three windings supplying 3mW into 600- ohms.
36. The internal loudspeaker (which may be switched out of circuit by operating S11) is
connected across the 3-ohm winding. The headphone jacks JK1 and JK2 are connected
across one of the 600-ohms windings.
A.F. LINE OUTPUT
37. The audio frequencies are also applied to the grid of V23A via RV3, the A.F. GAIN
LEVEL control; this control presets the level from output transformer T3. The trans-
former provides a 10mW output at 600-ohms which is suitable for direct connection to
landlines. A bridge rectifier MR1 is connected across the output via R142 and R143.
Th meter may be switched across the rectifier circuit so that the operator can monitor
the A.F. output.
BEAT FREQUENCY OSCILLATOR
38. The beat frequency oscillator (V19) employs an electron-coupled Harley circuit. The
oscillation frequency is determined by a fixed inductor L82 and a variable capacitor
C200 in parallel with C202 and C201. the trimming capacitor C201 is adjusted to
Detailed Circuit Description 21