Transmitting Circuits - Icom IC-730 Maintenance Manual

3 – 2 TRANSMITTING CIRCUITS
3 – 2 – 1 AF CIRCUITS
The audio signal from the microphone is fed to pin 3 of IC4,
operational amplifier on the MAIN unit, through the MIC GAIN
control R14-2 on the front panel. The amplified AF signal is
put out from pin 1, then fed to pin 5 of the balanced
modulator, IC2 in the DET unit, which is the same one for the
receiver product detector. To pin 7, the BFO signal is fed and
mixed with the audio signal, and a 9.013M H z or 9.010MH z
modulated DSB (carrier suppressed double side band) signal
is put out from pin 3.
In the receive mode and SSB transmit mode, bias voltages
adjusted by R44 and R45 are applied to pins 5 and 7
respectively to place IC2 in a carrier null condition. In the AM
and CW transmit modes, Q6 is turned OFF and an offset
voltage is applied to pin 5 through D4. Thus IC2 is in an
unbalanced condition and a 9.0115MHz AM signal or carrier
is put out from pin 3. In the other modes, Q6 is turned ON and
the offset voltage is shunted to the ground and IC2 is in a
balanced condition.
In the receive mode and CW transmit mode, Q5 is turned ON
and shunts the AF signal from the microphone to ground to
prevent the AF signal is applied to IC2.
3 – 2 – 2 IF CIRCUITS
The signal output from IC2 in the DET unit is fed to the 1st
gate of Q3, mixer, the same one for receive 3rd mixer,
through D3 in the MAIN unit to be converted into a 455KHz
signal. The local oscillator for this mixer is also the same one
for the receive. However, the oscillation frequency is fixed at
9.4665MHz.
In the receive mode, Q2 is turned ON and shunts the signal
fed from the DET unit to ground to prevent the signal from
leaking into the receiver IF circuits.
In the SSB and CW modes, the 455KHz signal is fed to Fl1,
MF-455-11GZ (or -11AZ), mechanical filter which has a
2.4KHz bandwidth, to remove unwanted sideband signal.
In the AM mode, the 455KHz AM signal is fed to Fl2,
CWF455HT, ceramic filter which has a 6KHz bandwidth, and
the signal passes through the filter unchanged.
The output from the selected filter is then fed to the balanced
mixer consisting of Q4 and Q5 which is the same one used in
the receiver 's 4th mixer. The mixer's purpose is to convert
the incoming signal to 9.0115MHz. The 9.0115MHz signal is
then fed to the IF amplifier Q9.
In the CW mode and key up condition, a positive voltage is
applied to the source of Q9 through D19, and Q9 is turned off
and the signal is not fed to the next stages. In key down
condition, Q10 is turned ON and the positive voltage applied
to the source is shunted to ground through Q10. Thus the
positive voltage is not applied, Q9 functions in norma l
condition and the signal is amplified then fed to the next
stages.
The amplified IF signal is then fed to FI1, 9M10A 9.0115
MHz crystal filter (or installed optional crystal filter) in the 2ND
IF unit.
Then the signal is fed to the doubly balanced mixer consisting
of D1 through D4, which is the same one used in the
receiver's 2nd mixer. In this case, the mixer's purpose is to
convert the incoming signal to 39.7315MHz.
The 39.7315MHz signal is fed to the amplifier Q7, then the
mixer consisting of D4 through D7, which is the same one for
receiver 1st mixer, for conversion to the desired operating
frequency.
When 10MHz, 18MHz or 24.5MHz band is selected, a
positive voltage is applied to the source of Q7 through Q27,
D26 or D25 respectively. This turns Q7 OFF and mutes
transmission on these bands. To transmit on these bands, cut
lead of the diode for the desired band.
The desired operating frequency signal is fed to the band-
pass filter to produce a clean output. The appropriate filter is
selected with the BAND switch. The signal is then amplified
by Q4 and fed to the PA unit.
3 – 2 – 3 RF POWER AMPLIFIER CIRCUITS
The transmit signal fed to the PA unit is amplified by Q1 up to
about 1 watt. Q1 is a class A amplifier and maintains high
linearity.
L2 gives the correct phase signals (180 degrees apart) for the
push-pull amplifier Q2 and Q3. Q2 and Q3 are class AB
amplifiers and amplify the signals up to about 6 watts. The
bases of Q2 and Q3 are biased by means of the barrier
voltage set by D1. D1 functions as temperature compensator
for Q2 and Q3, and is attached to the case of Q2. Negative
feed back by R and C applied across each collector and base
of Q2 and Q3, provides stability and broadband
characteristics.
L4, a broad band transformer provides balanced DC feed to
the collectors of Q2 and Q3, and for matching the collector-to-
collector impedance.
Then the signal is fed to each base of Q4 and Q5 through L4
secondary for impedance matching and correct phasing. Q4
and Q5 are class AB push-pull amplifiers which produce 100
watts output.
The bases of Q4 and Q5 are biased for class AB operation by
the emitter voltage of Q6 which is controlled by the barrier
voltage of D2. D2 voltage is adjusted by R21 to give proper
idling current.
D2 also functions as a temperature compensator to prevent
runaway caused by heating, and is attached to the case of
Q4.
R17 and R18 are resistors which provide negative feedback
from L7. L7 samples the output and provides stability and
broadband characteristics.
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