The S-Meter sensitivity control R-40 varies the sensitivity of the meter
for calibration purposes by varying the resistance in series with it.
I. Noise Blanker
The noise blanker in the R-4A attenuates a received noise pulse by
shunting to ground the output of V-4 during the time interval of the
pulse. In other words, the receiver is turned off during the pulse.
The 50 KC I. F. output of the second converter V3 is fed into the grid of the
12BA6 Noise Blanker IF amplifier V9.
Here it is amplified to a level sufficient to operate the pulse detector and
separation circuit composed of Dl and D2 and their associated circuitry.
This circuit operates in such a way that when a noise pulse is received of
a level higher than the received signal, a negative pulse is presented to
the grid of the noise shaper Vl0A.
This stage is normally conducting and
a negative pulse causes a large positive pulse to appear at the plate.
This pulse is applied to the grid of the pulse amplifier Vl0B.
The 18 K resistor R66 and the 100 K resistor R65 form a voltage divider
which applies positive cathode bias to Vl0B and diode D3 preventing
both from conducting.
When a positive pulse is applied to the grid of Vl0B, the bias is overcome
causing both Vl0B and D3 to conduct. When D3 conducts, it shunts to
ground the output of V4 through Cl04 and C105.
J. Crystal Calibrator
The cathode, grid #l, and grid #2, of the 12BA6 crystal calibrator (V12)
are connected in a Pierce oscillator circuit which operates at 100 KC.
The operating point of the plate of this tube is such that it is quite rich
in harmonics. This harmonic output is coupled to the antenna jack through
capacitor C118.
The cathode is disconnected from ground in all but the CAL position of
the FUNCTION switch, thus disabling the stage.
K.
Power Supply
The full wave power supply uses two IN3194 diodes. Condenser input
is used with +140 and +160 volts output.
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