Circuit Description; Receiver Circuits; Hf/50 Mhz Rf Circuit; Vhf And Uhf Rf Circuits - Icom IC-706MKIIG Service Manual

SECTION 3

3-1 RECEIVER CIRCUITS

3-1-1 HF/50 MHz RF CIRCUIT

(FILTER, MAIN AND HPF BOARDS)
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 8 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 (FILTER board; RL1) and low-pass
filter (FILTER board; L1, L2, C1–C5), and are then applied
to the MAIN board via J1 (FILTER board).
• Used RF low-pass filter (FILTER board)
Frequency Control Entrance
(MHz) signal coil
0.03–2 MHz L1 RL15
2–4 MHz L2 RL13
4–8 MHz L3 RL5
8–15 MHz L4 RL9
The signals from the FILTER board are applied to or bypass
the 20 dB attenuator (R122, R125, R126). The signals pass
through the high-pass filter (L132, L133, C132–C137) to
suppress strong signals below 1.6 MHz and are then applied
to the HPF board via the "SAF" terminal.
(1) 0.03–2 MHz and 30–40 MHz
The signals pass through a low-pass filter (L101, L102,
C101–C105), and then applied to the preamplifier circuit on
the MAIN board.
(2) 2–30 MHz
The signals from the low-pass filter (L101, L102,
C101–C105) are applied to one of 6 high-pass filters as at
right above and are then applied to the preamplifier circuit
on the MAIN board.
(3) 40–60 MHz
The signals pass through the low-pass filter (L172, L173,
C172–C176) and the high-pass filter (L174, L175,
C177–C181) via D171 and are then applied to the preampli-
fier circuit on the MAIN board.
• RECEIVER CONSTRUCTION
FILTER BOARD
[ANT1]
HPF
0.03–60 MHz
BOARD
LPF
HPF
PA UNIT
[ANT2]
60–470 MHz
LPF
BPF

CIRCUIT DESCRIPTION

Frequency Control Entrance
(MHz) signal coil
15–22 MHz L5 RL11
22–30 MHz L6 RL7
30–60 MHz L7 RL3
1st LO:
69.0415 MHz–
530.0115 MHz
Pre-amp.
FI511
IC151
Crystal
amp.
filter
1st mixer
69.0115 MHz
D271
MAIN BOARD
• Used RF high-pass filter (HPF board)
Frequency Control
signal
(MHz)
0.03–2 MHz THH
2–4 MHz L2H
4–8 MHz L3H
8–15 MHz L4H
3-1-2 VHF AND UHF RF CIRCUITS (PA UNIT)
The VHF and UHF RF circuits pass and amplify only the
desired band signals and suppresses any undesired band
signals. The both RF circuits have a preamplifier and band-
pass filters respectively.
• VHF RF CIRCUIT
The VHF RF signals from the [ANT2] connector pass
through the low-pass filter (L263–L265, C274–C276) and
antenna switching circuit (D291–D293). The signals are
applied to the bandpass filter (D403, D409, D408), and are
then amplified at the preamplifier circuit (Q403). The ampli-
fied signals are then applied to the another bandpass filter
(D407, D406, D410).
• UHF RF CIRCUIT
The UHF RF signals from the [ANT2] connector pass
through the high-pass filter (L316, L317, C326–C327), low-
pass filter (L313–L315, C322–C324) and antenna switching
circuit (D341, D342, D343). The signals are amplified at the
preamplifier circuit (Q453) between the 2 bandpass filters
(D454, D453 and D454, D456).
The filtered signals are applied to the MAIN board via J481
(PA unit) and are then applied to the preamplifier circuit.
D403, D408, D409, D406, D407, D410, D454, D453, D454
and D456 are varactor diodes that tune the ceinter frequen-
cy of an RF passband for wide bandwidth receiving and
good image response rejection. On the VHF band, receiving
signals are above 129 MHz, the switching diodes (D404,
D405) are turned off by the control signal "2MBL" from PLL
unit, then the varactor diodes (D408, D407) are disconnect-
ed.
2nd LO:
60.0 MHz
FI561
Crystal NB
gate
filter
2nd
9.0115MHz
mixer
D511
NB
circuit
to WFM detector
circuit (IC631)
3 - 1
Entrance Frequency Control
coil (MHz) signal
D111 15–22 MHz L5H
D121 22–30 MHz L6H
D131 30–40 MHz THH
D141 40–60 MHz B7H
FI671
SSB, CW
filter
FI681
AM, FM-N
filter
Optional
filter-1
Optional
filter-2
Entrance
coil
D151
D161
D111
D171
to AM demod.
circuit (D761)
to BFO circuit
(IC1041)
to FM demod.
circuit (IC791)