Icom IC-730 Maintenance Manual page 10

the MAIN unit. In the receive mode, D1 is turned ON and D10
is turned OFF 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 determined 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 D2, which is turned
ON in the receive mode. To the 2nd gate, 9.4665MHz local
oscillator signal for IF TUNE or PBT is applied, and the 2nd IF
signal is converted into 455kHz 3rd IF signal.
In SSB or CW mode, the 3rd IF signal is passed through Fl1
mechanical filter through D4 and D6, which are turned ON.
MF-455-11AZ or MF-455-11GZ is employed for Fl1. 11AZ 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 Fl2 ceramic
filter through D5 and D7.
The output from the 455kHz filters is fed to a balanced mixer
consisting of Q4 and Q5, 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 IF signal is fed to IF amplifier 06 through a
switching diode DS, then amplifier
07. Both amplifiers employ dual gate MOS FET respectively.
To the 1st gate, the IF signal is fed and to the 2nd gate, AGC
voltage is applied respectively. The amplified signal is fed to
the DET 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
adjustment 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 D13 varactor diode. This
voltage is varied by R13, PASS BAND SHIFT CONTROL on
the front panel, and applied to D13 through D12 in the receive
mode. In the transmit mode, a voltage adjusted by R66 is
applied to D13 through D11, 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 amplifier Q12.
When the Pass-Band Shift Control is set at the center
position, the VXO oscillates at 9.4665MHz, and 9.0115MHz,
the center frequency of the 2nd IF signal is converted into
455kHz, the center frequency of 3rd IF signal: This 455kHz
signal is converted into 9.0115MHz signal again with the
9.4665MHz VXO signal. This is the normal condition 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
(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 800Hz.
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 DET 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.
In the AM mode, the 4th IF signal is fed to 01, IF amplifier. A
part of the amplified signal is then fed to AM detector D5.
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 D1. 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 IF 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, RB-2, on the front panel
is fed to pin 1 of IC1 AF power 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 EXTERNAL
SPEAKER jack. This IC is activated in the transmit mode to
produce the CW side-tone.
3 – 3