Icom IC-730 Maintenance Manual page 12

The signal amplified by 04 and 05 is fed to the FI L (low-
pass filter) unit throu{ll L8 impedance matching trans-
former.
A thermal switch is mounted on the case of 04 and turns
ON when the case temperature exceeds 70 degrees C. This
changes the speed of the cooling fan from low to high. This
cooling fan rotates at low speed during transmit in the
normal condition. At this time, 8 volts is supplied to the
fan motor through R22, R23 and 07. When the thermal
switch is turned ON, 13.8 volts is supplied to the fan motor
through R22 only, and the cooling fan rotates at high speed
not only in the transmit mode but also in the receive mode.
The transmit signal which has passed the low-pass filter is
fed to the ANT (antenna) connector on the rear panel
through the SWR detector coil L 15 in the FI L unit.
3 - 2 - 4 ALC CIRCUITS
The foward power voltage, SWF and reflected power
voltage, SW8, detected at the F I L unit, are fed to the
MAIN unit through J20. The foward power voltage, SWF,
is fed to the negative input terminal of IC28 and amplified
as an ALC voltage. This ALC voltage is fed to the 2nd gates
of 09 in the MAIN unit and 07 in the RF unit.
In the SS8 or CW mode, the attack time of the ALC
voltage is determined by R70, C43 and R 169 and the release
time is determined by C43 and R88, and a peak voltage is
put out. -
In the AM mode, 013 and 014 are turned ON, and the
attack time is determined by R70 and C42, and the release
time by R68 and C42, and an average voltage is put out.
To the positive terminal of IC28, a voltage from the RF
POWER control on the front panel is applied to control
the ALC voltage and the output power can be adjusted
between 10 watts and 100 watts by adjusting the RF
POWER control.
The ALC voltage is also fed to the negative terminal of
IC38 through R88 and amplified to indicate ALC level on
the meter. The swing of the meter is adjusted by R91.
The voltage applied to the negative terminal of IC28 is
controlled by the RF POWER control on the front panel.
When the control is turned fully counterclockwise, the
voltage is minimum and the output power is also miminum
(10 watts). This voltage (output power) can be adjusted
by R149.
When the control is turned fully clockwise, the voltage is
the maximum and the output power is also the maximum
(SSB and CW: 100 watts, AM:4O watts). This voltage is
divided from 8 volts by R147 and R150, and can be ad-
justed by R 150.
In the AM mode, 019 is turned ON and R151 is connected
with R150 in parallel, and the maximum power is reduced
to 40 watts.
3-5
S3 is a switch to reduce the maximum power toOO watts.
When S3 is switched ON, a voltage is applied to the base
of 019 through 028 and R153 and 019 is turned ON the
same as in the AM mode. In addition, a voltage is applied
to the R F POWE R control through R 168 and 029, and the
maximum power is adjusted to 50 watts by R168. In the
AM mode, 020 is turned ON and R168 is shunted to
ground and the maximum power is 40 watts as usual.
3 - 2 - 5 VOX CIRCUITS
Audio signal from the microphone is fed to pin 5 (positive
input terminal) of IC6, comparator, through J3 and R137,
VOX GAIN control. To pin 5, a bias voltage which is
divided by R132 and R135, and R136 is applied through
R137. To_ pin 6 (negative input terminal), a reference
voltage which is divided by R132, and R135 and R136 is
applied. The difference voltage between the two terminals
is only the voltage across R135. Thus the sensitivity of the
comparator is very high and the VOX circuit actuates with
a small input level. When a voice signal presents, the output
terminal (pin 7) of IC6B becomes at High level and this
voltage charges C63. The charged voltage is discharged
through R128 and R129 and it decides the VOX holding
time. The VOX holding time can be adjusted by R129.
This High level voltage is applied to pin 5 (positive input
terminal) of IC5B, VOX control.
In the CW mode, IC5A is used as a break-in control.
In the key-up condition, the same bias voltage is applied to
both pin 2 (negative input terminal) and pin 3 (positive
input terminal), and pin 1 (output terminal) is low level.
In the key-down condition, pin 2 is grounded through
R117 and pin 1 becomes at high level. This voltage charges
C62 and is applied to pin 5 of IC5B. The holding time is
decided by C62, and R128 and R129.
To pin 6 of IC5B, a bias voltage divided by R125 and R139
is applied as a reference voltage. When the voltage at pin 5
becomes higher than that one at pin 6, pin 7 (output
terminal) puts out high level voltage. This turns on 017
and 018, and grounds the SEND line, when the VOX
switch is turned on, to set the radio in the transmit mode.
3 - 2 - 6 ANTI VOX CIRCUITS
A part of the receiver audio signal put out from IC1 is fed
to pin 3 (positive input terminal) of IC6A, comparator,
through the ANTI VOX level control, R142. When a re-
ceiver audio is present, pin 1 of IC6A puts out high level
voltage. This voltage is applied to pin 6 of IC5B, and in-
creases the reference voltage and cuts off IC5B. Thus the
VOX circuit does not function with an audio from the
speaker.
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