Receive Signal Path; Circuit Description - Standard Horizon HX600S Service Manual

1. Receive Signal Path

The HX600S includes two receiver front ends, each opti-
mized for a particular frequency range and mode combi-
nation.
Duplexer
Signals between 0.5 and 1.8 MHz received at the antenna
terminal pass through a first low-pass filter composed of
L1039, L1040, C1300, and C1301.
Received VHF bands signals, after passing through a low-
pass filter to the T/R switch circuit composed of diode
switch D1048 (RLS135), D1049 (1SV307).
VHF Bands Reception
Received VHF bands signals pass through the Duplexer
circuit, low-pass filter/high-pass filter circuit, T/R switch
circuit, and protector diode D1050 (1SS302) before addi-
tional filtering by a high-pass filter prior to application to
RF amplifier Q1065 (2SC5555). The amplified RF signal
is passed through the band-pass filter to first mixer Q1042
(3SK318). Meanwhile, VHF output from the VCO is ap-
plied through diode Band switch D1025 (1SS400) and T/
R switch D1026 (DAN222) to mixer Q1042 as the first lo-
cal signal.
The TUNE-voltage from the D/A converter Q1014
(M62364PF) is amplified by DC amplifier Q1037
(2SA1774-R) and Q1039 (UMW1), and applied to varac-
tors D1033, D1035, D1038, and D1039 (HVC369B), D1028,
D1029, D1036, D1037, D1040, and D1041 (1SV325) in the
variable frequency band-pass filters. By changing the elec-
trostatic capacitance of the varactors, optimum filter char-
acteristics are provided for each specific operating fre-
quency.
0.5 - 1.8 MHz Reception
Received MW signals pass through the Duplexer circuit,
low-pass filter circuit, protector diode D1057 (1SS302)
before additional filtering by a band-pass filter prior to
application to RF amplifier Q1079 (2SC4915-0). The am-
plified RF signal is pass through the band-pass filter to
first mixer Q1074 (2SC4617-R).
Meanwhile, MW output from the VCO is applied through
diode Band switch D1024 (1SS400) to mixer Q1074 as
the first local signal.
The TUNE-voltage from the D/A converter Q1034 is am-
plified by DC amplifier Q1037, Q1039 and applied to var-
actor D1055 (KV1520) in the variable frequency band-pass
filters. By changing the electrostatic capacitance of the
varactor, optimum filter characteristics are provided for
each specific operating frequency.

Circuit Description

First Intermediate Frequency (Narrow FM / AM)
The 47.25 MHz first intermediate frequency from first
mixers for AM and FM-narrow signals is passed through
NAR/WIDE switch D1056 (DAP222) and 47.25 MHz
monolithic crystal filter (MCF) XF1001 to narrow IF am-
plifier Q1072 (2SC4915-0) for input to pin 16 of Narrow
IF IC Q1063 (TA31136FN) after amplitude limiting by
D1045 (DA221).
Meanwhile, a portion of the output of 11.7 MHz crystal
X1001 is multiplied fourfold by Q1070 (2SC4617-R) and
Q1077 (2SC4915-0) to provide the 46.8 MHz second lo-
cal signal, applied to the Narrow IF IC. Within the IC, this
signal is mixed with the 47.25 MHz first intermediate fre-
quency signal to produce the 450 kHz second intermedi-
ate frequency.
This second IF is filtered by ceramic filter CF1001
(LTWC450F) and amplified by the limiting amplifier with-
in the Narrow IF IC before quadrate detection by ceramic
discriminator CD1001 (JTBC450C7).
Demodulated audio is output from pin 9 of the Narrow
IF IC through narrow mute analog switch Q1047 (2SJ364)
before de-emphasis at Q1053 (DTC144EE).
The demodulated audio signal from the Q1063 passes
through a band-pass filter and squelch gate Q1029
(NJM12902V).
The resulting audio is amplified by AF amplifier Q1009
(TDA2822D) and output through MIC/EAR jack J1001 to
internal speaker or an external earphone.
First Intermediate Frequency (Wide FM)
The 45.8 MHz first intermediate frequency from first mix-
ers for Wide FM signals is passed through NAR/WIDE
switch D1056 and IF amplifier Q1080 (2SC4915-0) and
second mixer Q1073 (2SC4915-0).
The 10.7 MHz intermediate frequency product of the mix-
er is delivered to the 10.7 MHz ceramic filter CF1002
passed through NAR/WIDE switch D1046 for input to pin
16 of IF IC Q1063 after amplitude.
Meanwhile, a portion of the output of 11.7 MHz crystal
X1001 is multiplied threefold by Q1082 (2SC4915-0) to
provide the 35.1 MHz second local signal, applied to the
second mixer Q1073. Within the second mixer, this signal
is mixed with the 45.8 MHz first intermediate frequency
signal to produce the 10.7 MHz second intermediate fre-
quency.
Meanwhile, a portion of the output of 11.7 MHz crystal
X1001 is amplitude by Q1070 and Q1077 to provide the
11.7 MHz third local signal, applied to the IF IC. Within
the IC, this signal is mixed with the 10.7 MHz second in-
termediate frequency signal to produce the 450 kHz third
intermediate frequency.
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