dynamically grounded plate is adjusted with R20 to control the balance of the modulator. This balance
is automatically upset in the AM mode by connection of R42 from one deflection plate to ground. Out-
put is taken across the two anodes of the 7360, and coupled to the four pole crystal f ilter by means of
T
l.
A mplif ication at 5. 2mc is provided by the common IF amplifier, V5, which follows the crystal
filter output. The bias on the grid of this stage is modified by the ALC control voltage, which acts to
decrease the stage gain when the peak input power to the final approaches the 200 watt level. Both the
output of the common IF amplifier and the output of the VFO, Ql and Q2, are applied
to
the transmitter
mixer tube, V12, whose output circuit is tuned to the frequency in use. A dual section tuning capacitor ,
C93, tunes this stage and the following driver plate circuit.
Two VFO ranges are employed. The 8. 7 to 9. 2Mc range provides for both 80 and 20 meter opera-
tion, and the 12. 2 to 12. 5Mc range yields 40 meter operation. On 80 meters, the transmitter mixer
selects the diff erence frequency between the 9Mc VFO and the 5. 2Mc modulated signal f rom the com-
mon IF amplifier. These same two signals are added for 20 meter transmission. The mixer subtracts
the 5. 2Mc IF output from the 12Mc VFO signal on 40 meters.
CW grid-block keying is accomplished in the transmitter mixer stage. In the CW mode this stage
is biased to cutoff with approximately 50 volts under key-up conditions. A relatively low plate voltage
is employed on the mixer stage, the diff erence between this voltage and the +250 volt supply being taken
up across R89. When the key is up and the stage is cut off , there is no current through R89, so that
at these times the full 250 volts appears at the plates of Vl2. R90 and R95 form a voltage divider
between the plate voltage of V12 and the -100 volt bias supply voltage. When the key is up and there
is 250 volts at the plates of V12, the voltage at the junction between R90 and R95 is some small posi-
tive value. As soon as the key is depressed and V l2 draws cur rent, the voltage at this junction becomes
negative. This change from positive to negative voltage when the key is depressed forms the trigger
signal which provides CW break-in operation through the means of the VOX circuit.
Af ter additional amplification by the driver stage V l3, the signal is applied to the power amplifier ,
V l4 and V15. The screen current in these tubes rises sharply as the control-grid voltage nears zero.
Thus, since the screen is supplied through dropping resistor R
l ll
from the +250 volt supply, the
screen voltage drops when the grid voltage rises. This variation is detected by diode CR5, and then
filtered before application to the control grid of the common IF amplif ier . The gain of the common
IF stage will vary depending on the output of this detector , and will be lowered when the final tends to
operate near its non-linear region above zero grid voltage. Zero signal grid voltage for the final
stage is set to approximately -55 volts by means of the BIAS adjust control, R108.
RECEIVING
The same PI-network that matches the output of the transmitter to the antenna is used to couple
the antenna to the receiver input amplifier , V16. Conversion of the amplified input to the IF frequency
of 5. 2Mc is accomplished in the receiver mixer , Vll, which is f ed by both the RF stage and the VFO.
The plate circuit of Vll is tuned to 5. 2Mc with the same transfor mer as is the balanced modulator.
Af ter passage through the crystal filter , three stages of IF amplification are provided by V5, V6, and
V7. At the output of V7, a diode detector, CR3 and CR4, develops the AGC control voltage which is
applied to the grids of V5, V6, Vl6 and Vll, as well as to the S-Meter amplifier , V4B.
One section of V8 functions as a product detector in the CW and SSB modes. Signal from the
third IF stage, V7, is applied to the grid of V8A while the output of the 5. 2Mc crystal oscillator, V4A,
is injected at the cathode. For AM reception, the cathode of V8A is grounded directly, causing this
stage to function as a grid-leak detector. The 5. 2Mc oscillator is disabled for AM reception. V8B
provides audio frequency voltage amplification, and V9 yields the AF output to the speaker or head-
phones.
A RECEIVER OFFSE T tuning control, R48, per mits the receiving frequency to be off set from the
transmit frequency by up to
±
lOkc.
It
operates by controlling the bias on a voltage variable capacitor
(varicap), CR6, across the VFO tuning capacitor in the receive condition. In the transmit condition,
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