3Rd Mixer Circuit (Main Unit); 2Nd Mixer Circuit (Main Unit); Power Amplifier Circuit; Filter Circuit - Icom IC-7800 Service Manual

The converted analog IF signal passes through the low-
pass filter (IC601, pins 3, 6, IC651, pins 3, 6), and is then
applied to the IF amplifier (IC701, pins 2, 3) to convert to
unbalancing IF signal.
The 36 kHz IF signal from the IF amplifier (pin 6) is applied
to the 3rd mixer circuit.

3-2-3 3RD MIXER CIRCUIT (MAIN UNIT)

The modulated 36 kHz IF signal from the DSP-TX board
pass through the two low-pass filters (IC501, pins 3, 6,
IC502, pins 3, 6), and is then applied to the 3rd mixer
circuit (IC503, pin 3). The 36 kHz IF signal is mixed with
491 kHz 3rd LO signal (from RXPLL-A unit) to produce the
455 kHz 3rd IF signal.
The signal is amplified at the IF amplifier (Q501), and
passes through the one of the ceramic bandpass filters
(FI501; 20 kHz, FI502; 4 kHz). The filtered signal is
amplified at the other IF amplifier (Q504), and is then
applied to the 2nd mixer circuit via the attenuator (R545–
R547).

3-2-4 2ND MIXER CIRCUIT (MAIN UNIT)

The 3rd IF signal from the 3rd mixer circuit is applied to the
2nd mixer circuit (D505). The applied 3rd IF signal is mixed
with 64 MHz 2nd LO signal (from the RXPLL-A unit) to
produce the 64.455 MHz 2nd IF signal.
The signal passes through the bandpass filter (FI503;
Fc=64.455 MHz) to suppress unwanted signals, and is
then applied to the IF amplifier (Q506). The 2nd IF signal is
applied to the 1st mixer circuit.
3-2-5 1ST MIXER AND RF CIRCUITS (MAIN UNIT)
The 2nd IF signal from the 2nd mixer circuit is applied to
the 1st mixer circuit (D506). The 1st IF signal is mixed with
1st LO signal (from the RXPLL-A unit) to produce the RF
signal.
The RF signal passes through the low-pass filter (L518,
L519, C564–C568) and 3 dB attenuator (R578–R580). The
signal is then applied to the RF amplifier and bandpass
filter circuits.
3-2-6 RF AMPLIFIER AND BANDPASS FILTER
CIRCUITS (MAIN UNIT)
The RF signal is amplified at the RF amplifier (Q508), and
applied to the one of the 10 bandpass filters via the 5 dB
attenuator (R575–R577).
• TRANSMITTER CONSTRUCTION
MIC
FI503
BPF
20 kHz
36 kHz IF
IC503
DSP-TX
455 kHz
board
3rd LO
(491 kHz)
D506
Q506
IF
1st LO
The filtered signal is applied to the YGR amplifier (Q801),
and passes through the 3 dB attenuator (R805–R807). The
signal is applied to the power amplifier circuit via the J801.
3-2-7 POWER AMPLIFIER CIRCUIT (PA200W BOARD)
The signal from the attenuator (MAIN unit; R805–R807)
passes through the 3 dB attenuator (R101–R103) via the
J1, and is then amplified at the RF amplifier (Q101). The
signal is applied to the pre-drive (Q201) and drive amplifier
(Q301), and is then amplified at the power amplifier (Q401,
Q402) to obtain 200 W output power.
The output signal is applied to the FILTER circuit via the
J401.
3-2-8 FILTER CIRCUIT (FILTER UNIT)
The power amplified signal passes through the one of the
9 low-pass filters which is depended on the RF frequencies
to suppress the unwanted frequency components. The
filtered signal is applied to the CONTROL circuit via the
J904.
3-2-9 CONTROL CIRCUIT (CTRL BOARD)
The filtered signal from the FILTER unit is applied to the
SWR-DET circuit (D101, D102) to detect SWR level, and
then passes through the P-DET (D151, D152) and R-DET
circuit (D201, D202). The signal is applied to the T/R switch
(RL251), and is then applied to (or bypass) the antenna
tuner circuit (NETWORK board) via the antenna tuner
selector switch (RL252, RL253).
The tuned signal from J252 (or bypass signal) passes
through the CRNT DET circuit (D301, D302) to detect the
driving current, and is then applied to the antenna switching
circuit.
The detected signal at CRNT DET circuit is amplified at
the amplifier (IC301, pin 3), and is then applied to the
TUNER_CPU (IC1, pin 16).
3-2-10 ANTENNA SWITCHING CIRCUIT
(ANT-SW UNIT)
The signal from the CTRL board is applied to the ANT-SW
unit via the J1, and is applied to the one of the 4 relays
(RL51, RL61, RL71, RL81). The signal is output from the
one of the 4 antenna connectors, J51, J61, J71 or J81.
FI501
Ceramic
filter
Q501
20 kHz
IF
FI502
Ceramic
filter
4 kHz
Q508
to PA200W board
RF
BPFs
3 - 6
D505
Q504
64.455 kHz
IF
2nd LO
(64.00 MHz)