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 (Q5, Q6), 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
L6
15--22
ВЕЗЕТ ‫دت‬‎ E
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 D8 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 IC10 pin 3. The "POC" voltage controlled
by the [RF PWR] contro! is also applied to ІС11 pin 3 as
the reference voltage.
When the "FOR" voltage exceeds the "РОС" voltage, ALC
bias voltage from 1С11 pin 1 controls the IF amplifiers
to reduce the output power until the "FOR" and "POC"
voltages are equalized.
іп AM mode, 1С11 operates as an averaging АС
amplifier, because a capacitor on the optional AM - FM
UNIT (C51) is connected to the cathode of D76. Q54
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 АРС 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 Q56. When the "REF" voltage
exceeds the reference voltage, determined by R203 and
R204, Q56 turns ON and the "POC" voltage is shifted
for 12 W output power.
The "ICH" and "ICL" voltages are applied to the Іс АРС
amplifier (IC10, pins 5 and 6) and then to the ALC bias
voltage line to prevent excessive current flow.
= 42-8 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 Q38
charge C252 and Q26 is turned ON, turning ON Q52.
Q52 grounds the SEND line for transmitting. Тһе [DELAY]
control (R244) adjusts the transmit release delay time.
The 8 V from Q38 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 Q38 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.
. ALC CIRCUIT
一 5V
FOR voltage ALC
MAIN UNIT voltage
2 ш, D76 Q53
[RF PWR] POC line Raoa 3 с 0 -5V
controi m 5 8 Г
© О55 ё
& PS 5 d 2 1 d External
Q56 + P е ALC
REF
R203 4
R206
R209
voltage
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‫ ו‬‎ UNIT 1
Fig. 5 1 1
4—5