The IF signal from T12 is also applied to V2 which
functions as a diode detector and an audio amplifier
in the AM receive mode. The output of this stage is
also fed to the Mode switch and is connected to
V17 through the Audio Gain control when the
Mode switch is in the X-AM position. The output
of V17 is applied through the audio output trans-
former to the phone jack, J5, and to pin 12 of the
power connector. Also, output from the plate of
V17 is applied to the anti
VOX
rectifier, D6, through
the ANTIVOX control.
A 100 kHz crystal calibrator, V5, is switched on
when the Mode switch is in the CAL position. Its
output is coupled to the grid of the RF amplifier
v7.
The S meter in the TR-4C operates in a bridge
circuit with the plates of a receiver IF amplifier,
Vl 1, and the transmitter IF amplifier, V15 in one
leg of the bridge and plate of the audio output tube
V17 in the other leg. Receiver AGC voltage applied
to Vl 1 on receive and transmit causes these tubes to
draw less current, thus unbalancing the bridge,
which causes the S meter to read up scale. The
bridge is balanced on receive by the ZERO control.
On transmit, the meter may rest up scale with no
modulation.
4-3.
TRANSMITTER CIRCUITRY.
Audio input from the microphone is applied to one
section of the microphone amplifier V18 where it
is amplified and applied to the remaining section of
this tube through one section of the XMTR GAIN
control. Output from the cathode of the second
triode of V18 is applied to the balanced modulator
through. the Mode switch for SSB operation. Out-
put from the plate is applied to the grid of the AM
screen modulator, V14, and to the grid of the first
VOX amplifier triode, V19A, through the VOX
control. The output from Vl9A is rectified by the
VOX rectifier, D5, and the resulting positive DC
voltage is applied to the grid of the relay control
triode, V19B, causing it to conduct and to close
the transmit/receive relay.
Audio voltage from V17 is rectified by the anti
VOX
rectifier, D6, which supplies negative voltage to the
grid of V19B, and preventing it from conducting
and closing the relay when
up audio from the speaker.
the microphone picks
Cut off bias for the relay tube is obtained from a
voltage divider. When the microphone push-to-talk
switch is activated the cut off bias is grounded,
causing VI 9B to conduct and close the relay.
RF from the 9.0 MHz crystal oscillator portion of
the product detector tube V16 is applied to the
balanced diode modulator through the carrier
balance control. On SSB the audio from V 18 is also
applied to the balanced modulator and the result is
a double sideband suppressed carrier signal which is
applied to the transmitter IF amplifier. The ampli-
fied output is coupled through T13 to the upper or
lower crystal filter where the undesired sideband is
filtered out. The resulting SSB signal is coupled
through T6 to the transmitter mixer, V4, where it
is combined with a premixer signal of the proper
frequency to give output on the desired amateur
band.
Output from V4 passes through the L/C circuit, T7,
T8 and C37, and is applied to the grid of the driver
tube, V6. Here it is amplified and applied to the
grids of V8, V9, and V l 0 through the L/C network,
T9, T10 and the other half of C37.
Three parellel power amplifier tubes boost the sig-
nal to a power level suitable for transmission. The
output impedance of the power amplifier tubes is
matched to a 52 ohm load by means of the pi net-
work circuit composed of L8, L9, C94 and C95.
At the first trace of flat topping in the final ampli-
fier tubes, a small amount of grid current will be
drawn. This produces a voltage drop across R47.
The small negative going voltage thus obtained is
applied through R46 to the cathode of V1 3B where
it is amplified. The amplified negative voltage is
applied to the grid of V15, thus reducing the level
of the driving signal.
When the Mode switch is placed in the X-CW
position, V2 becomes an audio phase shift oscilla-
tor which is grid-block keyed along with the trans-
mitter mixer, V4, and the driver, V6. The audio
output from V2 is applied to the grid of the
4 - 2
