Icom IC-706MKIIG Service Manual page 7

3-1-3 PREAMPLIFIER CIRCUIT (MAIN BOARD)
The preamplifier circuit in the IC-706MKIIG has approx.
15dB gain over a wide-band frequency range.
When the preamplifier is turned ON, the signals from the RF
circuit are applied to the preamplifier (IC151) via D2182.
Amplified or bypassed signals are applied to the 1st mixer
circuit (D271).
3-1-4 1ST MIXER CIRCUIT (MAIN BOARD)
The 1st mixer circuit mixes the receive signals with the 1st
LO signal to convert the receive signal frequencies to a 69
or 70.7 MHz 1st IF signal.
The signals from the preamplifier circuit, or signals which
bypass the preamplifier, are passed through a low-pass fil-
ter and then applied to the 1st mixer (D271).
The 1st LO signals
(69.0415–539.0115 MHz)
enter the MAIN board from
the PLL unit via J281. The
LO signal is amplified at
IC281, filtered by a low-
pass filter, and then,
applied to the 1st mixer.
3-1-5 1ST IF CIRCUIT (MAIN BOARD)
The 1st IF circuit filters and amplifies the 1st IF signals. The
1st IF signals are applied to a Crystal Filter (FI511) to sup-
press out-of-band signals.
The 69 MHz 1st IF signals (except WFM) pass through the
crystal filter (FI511), however, the 70.7 MHz 1st IF signal
(WFM) passes through a bandpass filter (L501–L505,
C501–C506). Then the filtered signals are applied to the IF
amplifier (IC521).
The AGC voltage is supplied to the transmit/receive switch-
ing circuit (D521, D522) and D521/D522 function as PIN
attenuators for AGC operation.
The amplified signals are then applied to the 2nd mixer cir-
cuit (D551) via the bandpass
C542–C545).
3-1-6 2ND MIXER CIRCUIT (MAIN BOARD)
The 2nd mixer circuit mixes the 1st IF signals and 2nd LO
signal (60.00 MHz) to convert the 1st IF to a 2nd IF.
The 1st IF signals from the band pass filter (L542–L546,
C542–C545) are converted to 9 MHz or 10.7 MHz 2nd IF
signals at the 2nd mixer (D551).
The 2nd IF signals are
applied to the bandpass fil-
ter (FI561) to suppress
undesired signals, such as
the 2nd LO signal, and are
then applied to the noise
blanker gate (D561,
D562).
• 1st IF frequency
Mode 1st IF
SSB/AM-N/FM 69.0115 MHz
AM/FM-N 69.0100 MHz
CW
69.0106 MHz
RTTY
69.0105 MHz
WFM 70.7000 MHz
filter (L542–L546,
• 2nd IF frequency
Mode 2nd IF
SSB/AM-N/FM 9.0115 MHz
AM/FM-N
9.0100 MHz
CW 9.0106 MHz
RTTY 9.0105 MHz
WFM 10.7000 MHz
While in WFM mode, the IF signals pass through the low-
pass filter (L601, C601–C603), IF amplifiers (Q601, Q611),
ceramic filter (FI611). The signals are then applied to the
WFM demodulator circuit (IC631).
3-1-7 NOISE BLANKER CIRCUIT (MAIN BOARD)
The noise blanker circuit detects pulse type noise, and turns
OFF the signal line when noise appears.
A portion of the signals from FI561 are amplified at the noise
amplifiers (Q621, Q632, amplifier section of IC631), then
detected at the noise detector (D632) to convert the noise
components to DC voltages.
The converted voltages are then applied to the noise
blanker switch (Q634, Q635). At the moment the detected
voltage exceeds Q634's threshold level, Q635 outputs a
blanking signal to close the noise blanker gate (D561, D562)
by applying reverse-biased voltage.
The detected voltage from D632 is also applied to the noise
blanker AGC circuit (Q631, Q633) and is then fed back to
the noise amplifier (IC631) as a bias voltage. The noise AGC
circuit prevents closure of the noise blanker gate for long
periods by non-pulse-type noise. The time constant of the
noise blanker AGC circuit is determined by R637.
The 2nd IF signals from the noise blanker gate are then
applied to the 2nd IF circuit.
3-1-8 2ND IF CIRCUIT
The 2nd IF circuit amplifies and filters the 2nd IF signals.
The 2nd IF signals from the noise blanker gate (D561,
D562) are amplified at the IF amplifier (IC571) via the Tx/Rx
switch (D572) and applied to a 2nd IF filter as shown below.
The filtered or bypassed signals are applied to the buffer
amplifier (Q721), IF amplifiers (Q731, Q741) and buffer
amplifier (Q751) to obtain a detectable level at the demodu-
lator circuit
• Used 2nd IF filter
Mode
SSB, CW, RTTY
AM nar.
AM, FM nar.
FM
SSB nar.
CW nar., RTTY nar.
SSB wide, CW wide,
RTTY wide
The amplified signals from the buffer amplifier (Q751) are
shared between the SSB/CW/RTTY detector (IC841), AM
detector (D761) and AGC detector (D771). Output signals
from the buffer amplifier (Q721) are applied to the FM IF IC
(IC791).
3 - 2
Used filter Control signal
FL-272 (FI671) 2F23
FL-94 (FI681) 2F80
Bypassed 2FTH
Optional FL-223 OP1 or OP2
Optional FL-100,
FL101, FL-223, OP1 or OP2
FL-232
Optional FL-103 OP1 or OP2