Icom IC-725 Service Manual page 12

4-2·5 RF FILTER CIRCUIT (PA UNIT)
The PA UNIT has 6 Chebyshev low-pass filters. The
signal from the power amplifier (05, 06), applied to one of
the low-pass filters depending on the transmit frequency
range, suppresses high harmonic components.
The filter switching voltage, obtained at the PLL UNIT, is
applied to the PA UNIT via P7.
FREQUENCIES AND APPROPRIATE FILTERS
FILTER
FREQUENCY RANGE (MHz)
L1
Below 2
L2
2-4
L3
4-8
L4 8-15
L5 15-22
L6 22-30
The filtered signal passes through the SWR detector
circuit (L51) and is then applied to the antenna connector.
The forward signal from L51 is detected at D7 and applied
to the MAIN UNIT as the "FOR" voltage. The reflection
signal from L51 is detected at DB and applied to the MAIN
UNIT as the "REF" voltage.
4·2·6 ALC CIRCUIT (MAIN UNIT)
The ALC (Auto Level Control) circuit stably controls the
RF output power using the [RF POWER] control.
The "FOR" voltage from the PA UNIT is applied to IC11
pin 2 and IC1 O pin 3. The "POC" voltage controlled
by the [RF PWR] control is also applied to IC11 pin 3 as
the reference voltage.
When the "FOR" voltage exceeds the "POC" voltage, ALC
bias voltage from IC11 pin 1 controls the IF amplifiers
to reduce the output power until the "FOR" and "POC"
voltages are equalized.
ALC CIRCUIT
REF
voltage
R203
S V from 041
MAIN UNIT
056
2SM
In AM mode, IC11 operates as an averaging ALC
amplifier, because a capacitor on the optional AM · FM
UNIT (C51) is connected to the cathode of D76. 054
turns ON and the "POC" voltage is shifted for 40 W AM
output power (maximum).
The. ALC bias voltage from IC11 pin 1 is also applied to
the inversion-amplifier (IC11 pin 6) to control an intensity
of the [TX] indicator, showing the ALC level.
An external ALC input from the [ALC] jack is applied to
the buffer amplifier (053). ALC operation is identical to
that of the internal ALC.
4·2·7 APC CIRCUITS (MAIN UNIT)
The APC circuits protect the final transistors from high
SWR and excessive current. The "REF" voltage from
the PA UNIT is applied to 056. When the "REF" voltage
exceeds the reference voltage, determined by R203 and
R204, 056 turns ON and the "POC" voltage is shifted
for 12 W output power.
The "ICH" and "ICL" voltages are applied to the le APC
amplifier (IC10, pins 5 and 6) and then to the ALC bias
voltage line to prevent excessive current flow.
4·2·8 CW KEYING CIRCUIT (MAIN UNIT)
When the CW key is closed, the "KEY" signal line
becomes "LOW." 038 outputs 8 V to control break-in
operation, sidetone signal and transmit signal.
When the [BK IN] switch is pushed IN, 8 V from 038
charge C252 and 026 is turned ON, turning ON 052.
052 grounds the SEND line for transmitting. The [DELAY]
control (R244) adjusts the transmit release delay time.
The 8 V from 038 charges C249 and D91 is turned OFF,
disconnecting C249 from 040. 040 then oscillates a
sidetone signal. R268 prevents sidetone click noise.
The 8 V from 038 is applied to a time constant and then to
the balanced modulator (IC6) to create a carrier signal.
R241 in the time constant adjusts a transmit delay timing
for 12 msec.
FOR voltage
-5V
_ _ _ _ _ _ ALC
voltage
R209
054
AMS
+
External
---~ALC
voltage
AMS
Fig. 5
4-5