Icom IC-730 Maintenance Manual page 9

shunts the RF signals to ground. This grounds the anodes of
D20 and D21, and turns them OFF. Thus the noise pulse is
not fed to the following circuits.
The signal which passes the NB gate circuit is then fed to the
2nd mixer consisting of D1 - D4 diode DBM.
The 2nd local oscillator consisting of Q9 and X1 oscillates at
30.71901MHz - 30.720MHz with 10Hz steps. D19 varactor
diode provides this frequency variation. A control voltage
generated in the LOGIC unit and DC-amplified in the MAIN
unit is applied to D19. The oscillation frequency can be
adjusted by L1 and the voltage applied to D19. The 2nd local
oscillator signal is about +2dBm and is fed to the L6 center
tap in the 2nd mixer circuit to convert the 1st IF signal to 2nd
IF (9.0115MHz) signal.
The voltage CSW is fed to the filter selection pins consisting
of P3, J7 and JS, through pin 1 of J4 and D17. P3 is
connected to J7 when the set has been shipped. When the
optional SSB crystal filter (this provides PBT function) is
installed, P3 should be connected to JS.
The voltage CWN is fed to the filter selection pin consisting of
P2, J5 and J6, through pin 2 of J4 and 01S. P2 is connected
to J5 when the set has been shipped. When the optional CW
crystal filter is instal led, P2 should be connected to J6.
When any optional filters are not installed (the filter selection
pins are original connections), the bases of 04 and 05 are not
applied with any bias voltages, so 04 and 05 are turned OFF
and both emitters are OV. Thus D9 and D13 are turned OFF
and 06 is turned ON, and D10 and D11 are turned ON and
Fl1 is selected in any modes.
When the optional SSB crystal filter is installed and P3 is
connected to JS, 05 is turned ON and RSV is applied to D7
and DS, and D7 and DS are turned ON. Thus the SSB
The 2nd I F signal is f ed to Fl 1 monolithic crystal filter
installed as standard. When optional SSB crystal filter, FL-30,
and/or CW crystal filter, FL-45 is installed, the suitable crystal
filter is selected by the MODE switch.
Fl1 has 3kohm input/output impedance, so LS and L9 work as
step-up and step-down matching transformers respectively.
Fl1 has lower insertion loss than other optional filters, so R15
and R2S are inserted as an attenuator. L9 is tuned by D12
varactor diode, which gives more isolation when other filters
are selected. The filtered 2nd IF signal is then fed to the
MAIN unit through P1.
Filter selection is made by voltages CWN (applied in CW-N
mode) and CSW (applied in other modes) from the MODE
switch through J4.
crystal filter is selected. At the same time, R8V is applied to
the base of Q6 through D13 and turns OFF Q6, thus Fl1 is
isolated from the circuit.
When the optional CW crystal filter is installed and P2 is
connected to J6, 04 is turned ON and RSV is applied to D5
and D23, and D5 and D23 are turned ON. Thus the CW
crystal filter is selected. At the same time, RSV is applied to
the base of 06 through D9 and turns OF F 06, thus Fl 1 is
isolated from the circuit the same as when SSB crystal filter is
installed.
D6 is turned ON when the optional CW filter is selected, and it
shunts to ground the optional SSB filter input terminal to
prevent signal leakage in the pass band of the SSB filter.
These optional filters are selected only when the set is in the
receive mode.
3 – 1 – 3 MAIN UNIT
The receive signal from the 2
3 – 2
nd
IF unit P1 is ready to J16 on