Circuit Description; Receiver Circuits - Icom IC-706 Service Manual

SECTION 3 CIRCUIT DESCRIPTION
3-1 RECEIVER CIRCUITS
3·1·1 HF/50 MHz RF CIRCUIT (FILTER AND
MAIN UNITS, HPF BOARD)
HF/50 MHz RF filters pass only the desired band signals
and suppress any undesired band signals. The HF/50 MHz
RF circuit has 7 low-pass filters and 6 high-pass filters for
specified band use.
HF/50 MHz RF signals from the [ANT1] connector, pass
through one of 7 low-pass filters as below, the transmit!
receive switching relay (RL1) and low-pass filter (L1, L2,
C1-C5), and are then applied to the MAIN unit via P1
(MAIN unit J1).
Used RF low-pass filter (FILTER unit)
Band
Control Input
Band
Control Input
signal relay signal relay
0.03-2 MHz L1 RL15
15-22 MHz L5 RL11
2-4 MHz L2 RL13 22-30 MHz L6 RL7
4-8 MHz L3 RL5 30-60 MHz L7 AL3
8-15 MHz L4 RL9
The signals from the FILTER unit are applied to or bypass
the 20 dB attenuator (A3, R4, 01, 01). The signals pass
through the high-pass filter (L3-L7, C3-C7, R5-R7) to
suppress strong signals below 1.6 MHz and are then
applied to the HPF board via the "TOAT" terminal.
(1) 0.03-2 MHz and 30-40 MHz
The signals pass through a low-pass filter (L23, L24, C38-
C42), bypass the high-pass filters via 05 and are then
applied to the preamplifier circuit.
(2) 2-30 MHz
The signals pass through the low-pass filter (L23, L24,
C38-C42). The filtered signals are applied to one of 5
high-pass filters as at right above and are then applied to
the preamplifier circuit.
• RECEIVER CONSTRUCTION
(3) 40-60 MHz
The signals pass through the high-pass filter (L29-L31,
C47-G51) via 012 and are then applied to the preamplifier
circuit.
Used RF high-pass filter (HPF board)
Band
Control Input
Band
Control Input
signal diode signal diode
0.03-2 MHz TH 05 15-22 MHz H5 021
2-4 MHz H2 016 22-30 MHz H6 07
4-8 MHz H3 020 30-40 MHz TH 05
8-15 MHz H4 017 40-60 MHz BV, T8 012
3·1·2 VHF RF CIRCUIT (PA UNIT)
VHF AF bandpass filters pass only the desired band signals
and suppress any undesired band signals. The VHF AF
circuit has 2 bandpass filters.
VHF AF signals from the [ANT2] connector, pass through a
low-pass filter, antenna switching circuit (04, 014, 015) and
one of 2 bandpass filters. The signals within 144-148 MHz
pass through one bandpass filter (L29, L30, C45-C49) and
the signals within 60-144 MHz or 148-200 MHz pass
through another bandpass filter (L25, L26, L40, L49, C52-
C57, C151-C154).
The filtered signals pass through the AF attenuator (A27,
024), are amplified at the AF amplifier (IC1), applied to the
MAIN unit via P2 (MAIN unit J3) and are then applied to the
preamplifier circuit in the HPF board.
3·1·3 PREAMPLIFIER CIRCUIT (HPF BOARD)
The preamplifier circuit in the IC-70a has approx. 15 dB
gain over a wideband frequency range.
_
.......••
------~
.. _.
__
._--~_
..
----_
..
-
_-
I
IANT2
J
PA unit
60-200 MHz
a----
AMP>-----...J
FILTER
unit :
MAIN unit
2nd LO;
aO.OMHz
ToWFM
detector (IC9)
BFO;
9.0115 MHz
~ TOAF
selector
switch
Product
(IC1S)
detector
lela
To FM detector
(IC1S)
To AM detector
(044)
3 -1