Alc Circuit - Icom IC-701 Instruction Manual

In the receiving mode using the transverter, +9 Volts DC
is applied through a 470 ohm resistor to J4, superimposed
on the incomming signal from the transverter. Then D18,
D19, and D40 are turned ON, and D28 to D33 of the RF
amplifier circuit for each band are turned OFF by +9 Volts
DC from J4, and D41 is turned OFF by +8 Volts DC from
pin 11 of J1. Thus the incomming signal from the tran-
sverter is fed to the mixer circuit composed of D11 to D14,
through J4, D18, D19, and D40.
In the transmitting mode, J4 is grounded for DC, then D16,
Di7 and one of D28 to D33 are turned ON and D40 is
turned ON and D41 and Q2 are turned OFF by +8 Volts
from Pin 11 of J1. Thus T/R relay RL1 turns to the
transmit side so the mixed SSB signal from the transmit
mixer circuit is fed to the RF amplifier circuit, and the
amplified signal is sent to J4 through D16 and D17.
The output level is about 200mV across a 50 ohm load.
6-2-7 EXCITER CIRCUIT
The output from the RF unit J6 is applied to O1 on the
"B' unit and amplified class A to about 500mV. L3 and
R6 are negative feedback circuits for obtaining the wide
band characteristics from 1.8 to 30MHz. This signal is
amplified class B push pull to about 7 Watts by Q2 and Q3.
The junction voltage of D2 and D3 is amplified by Q9 and
applied to the bases of O2 and Q3, from the emitter of Q9,
for the bias voltage.
D2 and D3 are connected to the cases of Q2 and O3
respectively, and compensate the bias voltage by the
temperature of O2 and Q3. L8, L9, R10, and R11 are
negative feedback components for obtaining a wide fre-
quency range.
6-2-8 FINAL POWER AMPLIFIER CIRCUIT
The 7 Watt signal from the exciter stage is amplified class
B push-pull in the final amplifier stage transistors, Q1 and
Q2 of the PA unit to a level of about 100 Watts. Bias
voltage is treated by D1, D2 and Q3, the same as D2, D3
and O9 of the "'B" unit, and fed to the bases of Q1 and
Q2. L4, R7, to R10, C3, C4 and C8 are negative feedback
circuits.
6-2-9 ANTENNA SWITCHING CIRCUIT
The Antenna switching circuit is composed of D24 to D26,
Q4 and Q5 located on the "B" unit. O4 is ON when the
voltage for transmit (+9V) is applied to it's base through
R14 during transmit. Accordingly, O5 is turned ON, which
turns on D24 which passes the RF. Q17 is OFF since it's
emitter voltage is 0, and D25 and D26 are cut OFF. The
signal from the PA unit is conducted to the filter section
through D24. In this case, the high frequency voltage is
rectified by D25 and D26 and this voltage, about 80V, is
fed to the cathode of D25. This reverse biases D25 and
D26 and turns them OFF, so output signals do not feed
into the receive section.
In the receive mode, the base of O4 goes to OV, so 04 and
Q5 are turned OFF, and —9V is applied to D24 through
R19 and R108, thus D24 is turned OFF completely. Also
+9V is applied to D25 and D26, through Q17 and R70, so
D25 and D26 are turned ON, thus incoming signals are
fed to the receiving section.
6-2-10 LOW PASS FILTER CIRCUIT
The PA output, which has passed through the antenna
switching circuit, is fed through the low pass filter of the
filter unit, which serves to attenuate harmonics by about
40dB in order to get a clean transmit output. The low
pass filter is set for each band, and is switched by the
rotary relay switch. The relay switch is driven by Q1 to
Q8. The voltage set in the band switching circuit on the
front panel is applied to one of O1's bases through R8,
while the voltage set according to the contact of the rotary
relay is applied to the other base through R12. If there isa
potential difference between both bases of Q1, Q2 and O3
amplify the DC current, and 04 applies the bias voltage to
Q5's base. Asa result, the multivibrator composed of Q5
and Q6- operate and the rotary relay is driven by Q7 and O8.
The relay continues to operate until the voltage of both
bases of Q1 are the same. At that time, Q1's collectors are
equal, O2, O3, and O4 are all OFF. Therefore Q7 and
O8 are OFF and the relay stops.
6-2-11 ALC CIRCUIT
L3 of the ALC circuit is an SWR pick up coil, and D3 and
D4 are the detection diodes. The voltage of a traveling
wave is detected by D3 as a negative DC voltage, and a
reflected wave is detected as a negative DC voltage by D4.
They pass through the SWR switch (forward-reverse switch-
ing) and operate the meter on the front panel. D1 is the
ALC detection diode.
The bias voltage determined by R24 in the ''B" unit is
applied to the cathode of D1 through R2 in the ALC
circuit. If RF voltage applied to the D1 anode is above
this bias voltage, negative ALC voltage is detected by D1
and fed to the base of Q10 in the "'B" unit, through L2
and R3.
The ALC output voltage is DC amplified By Q10, Q11,
Q12, and Q13 in the "B" unit and then applied to the gate
of O17 in the "A" unit and controls the driving power to
the PA stage. If the antenna connected to the transmitter
is inferior in SWR, the detected voltage of the reflected
wave is applied to the ALC output circuit Q10 in the 'B'"'
unit through D2 and R1. Therefore, the ALC output
negative voltage becomes extremely high and so the driving
power to the final PA stage becomes low and protects the
PA transistors.
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