Icom IC-730 Maintenance Manual page 10

the MAIN unit. In the receive mode, 01 is turned ON and
010 is turned OF F by the RSV, and the signal is fed to the
1st gate of IF amplifier 01, dual gate MOS FET. To the
2nd gate, AGC voltage is applied. Its attack time is de-
termined by R4 and C4.
The amplified 2nd IF signal is fed to the 1st gate of the 3rd
mixer 03, dual gate MOS FET through 02, which is turned
ON in the receive mode. To the 2nd gate, 9.4665MHz local
oscillator signal for IF TUNE or PST is applied, and the
2nd I F signal is converted into 455kHz 3rd I F signal.
In SSS or CW mode, the 3rd IF signal is passed through FI1
mechanical filter through 04 and 06, which are turned ON.
M F -455-11 AZ or M F -455-11 GZ is employed for F Ilo
11 AZ has 1.5kohm and 240pF input/output impedance
and 11GZ has 1.5kohm and 20pF. Thus, C75 through
C7S are not used for the 11GZ.
In AM mode, the 3rd IF signal is passed through FI2
ceramic filter through 05 and 07.
The output from the 455kHz filters is fed to a balanced
mixer consisting of 04 and 05, and converted into a
9.0115MHz 4th IF signal again. The local oscillator signal
is the same one for the 3rd mixer, and fed to the center tap
of the input tuned circuit. The 4th I F signal is fed to IF
amplifier 06 through a switching diode OS, then amplifier
07. Soth amplifiers employ dual gate MOS FET res-
pectively. To the 1st gate, the IF signal is fed and to the
2nd gate, AGC voltage is applied respectively. The ampl ified
signal is fed to the OET UNIT through J6, Pin 5.
The source voltage of OS is varied according to AGC voltage
i.e., incoming signal level. This voltage is amplified by 07,
then fed to the S-meter. R42 is for meter zero-point ad-
justment and R41 is for full-scale adjustment.
3 - 1-
4 IF TUNE AND PBT CIRCUITS
The VXO circuit consisting of 011 and X1 oscillates at
9.4665MHz, and this frequency can be changed by ± 1.5kHz
by changing the voltage applied to 013 varactor diode.
This voltage is varied by R13, PASS BAND SHIFT
CONTROL on the front panel, and applied to 013 through
012 in the receive mode. In the transmit mode, a voltage
adjusted by R66 is applied to 013 through 011, and the
oscillation frequency is fixed at the center frequency,
9.4665MHz. This signal is fed to the 2nd gate of 03, 3rd
mixer, and 4th mixer 04 and 05 through a.buffer am-
plifier 012.
When the Pass-Band Shift Control is set at the center po-
sition, the VXO oscillates at 9.4665MHz, and 9.0115MHz,
the center frequency of the 2nd I F signal is converted into
455kHz, the center frequency of 3rd I F signal: This
455kHz signal is converted into 9.0115MHz signal again
with the 9.4665MHz VXO signal. This is the normal con-
dition of the Pass-Band Shift system and incoming signals
pass through both pass-bands of the filters, 9.0115MHz and
455kHz.
When the PB Shift Control is slid toward the right side, the
VXO frequency is increased, as an example; 9.4675MHz
3-3
(9.4665MHz+1kHz), 9.0115MHz, the center frequency of
the 2nd IF signal is converted into 456kHz, 3rd IF signal.
This 456kHz signal is converted into 9.0115MHz signal
again with the 9.4675MHz VXO signal. However, the
456kHz, 3rd IF signal is off from the center frequency of
the 455kHz filter. In other words, the pass-band of the
455kHz filter is shifted toward the lower side, and the total
pass-band is also shifted toward the lower side. At this
time, if the optional SSB filter FL-30 is installed, the total
band width is narrowed from the upper side. Thus the
center position of the control is the widest bandwidth and
is equivalent to the normal SSB bandwidth, and the band-
width is narrowed electrically from either the upper or
lower side continuously by up to SOOHz.
3 -1 - 5
DETECTOR CIRCUITS
In the SSB and CW modes, a 9.0115MHz, 4th IF signal is
fed to IC2, product detector in the OET unit. To the other
port of IC2, a BFO signal is applied and an AF signal is
put out from pin 3. The BFO is composed of OS, X1,
C39, C40, L2 through L4, etc. C39, C40 and L2 through
L4 are connected in series with X1, and L2 through L4 are
shunted to ground respectively according to the selected
mode to get proper BFO frequency.
The detected AF signal is fed to pin 5 of IC1 operational
amplifier. The amplified AF signal is put out from pin 7
and fed to 07, low-pass filter, then to the VOLUME control
on the front panel.
IntheAM mode, the 4th IF signal is fed to 01, IF amplifier.
A part of the amplified signal is then fed to
AM
detector
05.
Toe detected AF signal is fed to pin 3 of IC1, operational
amplifier. The amplified AF signal is put out from pin 1
and then fed to low-pass filter 07.
As an AGC voltage, a part of the 4th IF signal at 01 col-
lector is fed to AGC detector 01. The detected DC signal
is then fed to the base of 02. When the applied voltage is
over the threshold voltage, 02 is turned ON and a negative
voltage connected to the emitter, charges C6 through R9.
When the applied voltage becomes less than threshold
voltage, or zero, 02 is turned OFF and the voltage of C6
is discharged through RS (high value resistor). This provides
fast attack/slow release AGC. AGC voltage is taken from
the collector of 02 and supplied to each 2nd gate of the
I F amplifiers. On the AGC line, the RF gain control voltage
from the RF GAIN control is superimposed.
When the AGC switch on the front panel is pushed in, 010
is turned ON, and R13 and C10 (in series) are connected
in parallel with RS and the AGC time constant becomes
shorter.
3 - 1 - 6 AF POWER AMPLIFIER CIRCUIT
AF signal from the VOLUME control, RS-2, on the front
panel is fed to pin 1 of IC1 AF povver amplifier on the
MAIN unit. The signal is amplified with IC1 to get 2 watts
output power in an S ohm load. The output signal is fed to
the internal speaker through the PHONES jack and EX-
TERNAL SPEAKER jack. This IC is activated in the
transmit mode to produce the CW side-tone.
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