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 OET unit, which is the
same one for the receiver product detector. To pin 7, the
BFa signal is fed and mixed with the audio signal, and a
9.013MHz or 9.010MHz modulated OSB (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 res-
pectively to place IC2 in a carrier null condition. In the
·AM and CW transmit modes, 06 is turned OFF and an off-
set voltage is applied to pin 5 through 04. 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, 06 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, 05 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 OET unit is fed to the
1 st gate of 03, mixer, the same one for receive 3rd mixer,
through 03 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 fre-
quency is fixed at 9.4665MHz.
In the receive mode, 02 is turned ON and shunts the signal
fed from the OET unit to ground to prevent the signal
from leaking into the receiver I F circuits.
In the SSB and CW modes, the 455kHz signal is fed to Fll,
MF-455-11GZ (or -llAZ), mechanical filter which has a
2.4kHz bandwidth, to remove unwanted sideband signal.
In the AM mode, the 455kHz AM signal is fed to FI2,
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 04 and 05 which is the same
one used in the receiver's 4th mixer. The mixer's porpuse is
to convert the incoming signal to 9.0115MHz. The 9.0115
MHz signal is then fed to the I F amplifier 09.
In the CW mode and key-up condition, a positive voltage is
applied to the source of 09 through 019, and 09 is turned
off and the signal is not fed to the next stages. In key-
down condition, 010 is turned ON and the positive voltage
applied to the source is shunted to ground through 010.
Thus the positive voltage is not applied, 09 functions in
normal condition and the signal is amplified then fed to the
next stages.
The amplified I F signal is then fed to F 11, 9M lOA 9.0115
3-4
MHz crystal filter (or installed optional crystal filter) in
the 2NO I F unit.
Then the signal is fed to the doubly balanced mixer con-
sisting of 01 through 04, 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 07, then the
mixer consisting of 04 through 07, which is the same one
for receiver 1 st mixer, for conversion to the desired ope-
rating frequency.
When 10MHz, 18MHz or 24.5MHz band is selected, a po-
sitive voltage is applied to the source of 07 through 027,
026 or 025 respectively. This turns 07 OF F 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 BANO switch. The signal is then
amplified by 04 and fed to the PA unit.
3 - 2 - 3
RF POWER AMPLIFIER CIRCUITS
The transmit signal fed to the PA unit is amplified by 01
up to about 1 watt. 01 is a class A amplifier and maintains
high linearity.
L2 gives the correct phase signals (180 degrees apart) for
the push-pull amplifier 02 and 03. 02 and 03 are class
AB amplifiers and amplify the signals up to about 6 watts.
The bases of 02 and 03 are biased by means of the barrier
voltage set by 01. 01 functions as temperature com-
pensator for 02 and 03, and is attached to the case of 02.
Negative feedback by Rand C applied across each collector
and base of 02 and 03, provides stablility and broadband
characteristics.
L4, a broadband transformer provides balanced OC feed to
the collectors of 02 and 03, and for matching the collector-
to-{;ollector impedance.
Then the signal is fed to each base of 04 and 05 through
L4 secondary for impedance matching and correct phasing_
04 and 05 are class AB push-pull amplifiers which produce
100 watts output.
The bases of 04 and 05 are biased for class AB operation
by the emitter voltage of 06 which is controlled by the
bamer voltage of 02. 02 voltage is adjusted by R21 to
give proper idling current.
02 also functions as a temperature compensator to prevent
runaway caused by heating, and is attached to the case of
04.
R17 and R18 are resistors which provide negative feedback
from L7. L7 samples the output and provides stablility and
broadband characteristics.
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