Standard Horizon HX471S, Service Manual

Page: 7 / 66

7
Circuit Description
The HX4710S consists of RF UNIT, CNTL UNIT, VCO
UNIT and AF UNIT. The RF UNIT contains the receiver
front end, PLL IC, power and switching circuits, and the
VCO UNIT for transmit and receive local signal oscilla-
tion. The CNTL UNIT contains the CPU, and audio ICs,
and the power circuitry for the LCD. The AF UNIT con-
tains the IF, and audio ICs.
Receiver Signal Flow
The HX471S includes three receiver front ends, each opti-
mized for a particular frequency range and mode combi-
nation.
Triplexer
Signals between 0.5 and 1.8 MHz received at the antenna
terminal pass through a first low-pass filter composed of
L1041, L1001, C1203 and C1007.
Received VHF bands signals, after passing through a low-
pass filter to the VHF T/R switch circuit composed of di-
ode switch
D1033 ( RLS135 ), D1034 ( 1SV307 ).
Received UHF bands signals, after passing through a low-
pass filter to the UHF T/R switch circuit composed of di-
ode switch
D1030
( RLS135 ) and
D1031
( 1SV307 ).
VHF Bands Reception
Received VHF bands signals pass through the Triplexer
circuit, low-pass filter/high-pass filter circuit, VHF T/R
switch circuit and protector diode
D1002 (
1SS362
) before
additional filtering by a band-pass filter prior to applica-
tion to RF amplifier
Q1002
( 2SC5555 ). The amplified RF
signal is pass through the band-pass filter to first mixer
Q1006
( 2SC5555 ). Meanwhile, VHF output from the VCO
UNIT is amplified by Q1011 ( 2SC5374 ) and applied
through diode T/R switch D1022 (
DAN222
) to mixer Q1006
as the first local signal.
The 47.25 MHz (WFM: 45.8 MHz) intermediate frequency
product of the mixer is delivered to the AF UNIT.
The TUNE-voltage from the CPU on the CNTL UNIT is
amplified by DC amplifier
Q3014 ( NJU7007F2 ) and ap-
plied to varactors D1008 and D1010 (both
HVC369B
),
D1007 , D1009 , D1011 , D1012 , D1013 , and D1019 (all
1SV325
) in the variable frequency band-pass filters. By
changing the electrostatic capacitance of the varactors,
optimum filter characteristics are provided for each spe-
cific operating frequency.
UHF Band Reception
Received UHF bands signals pass through the Triplexer
circuit, low-pass filter/high-pass filter circuit, UHF T/R
switch circuit and protector diode
D1001 ( 1SS362 ) before
additional filtering by a band-pass filter prior to applica-
tion to RF amplifier
Q1001 ( 2SC5555 ). The amplified RF
signal is pass through the band-pass filter, RF amplifier
Q1003 ( 2SC5555 ) and band-pass filter to first mixer Q1005
( 2SC5555 ). Meanwhile, UHF output from the VCO UNIT
is amplified by Q1010 ( 2SC5374 ) and applied through di-
ode T/R switch D1021 ( DN222 ) to mixer Q1005 as the first
local signal.
The 47.25 MHz intermediate frequency product of the
mixer is delivered to the AF UNIT.
0.5 - 1.8 MHz Reception
Received MW signals pass through the Triplexer circuit,
low-pass filter circuit, protector diode
D1005 ( 1SS362 ) be-
fore additional filtering by a band-pass filter prior to ap-
plication to RF amplifier
Q1004 ( 2SC4915 ). The amplified
RF signal is pass through the band-pass filter to first mixer
Q1007 ( 2SC4915 ). Meanwhile, MW output from the VCO
UNIT is amplified by Q1012 to mixer Q1007 as the first
local signal.
The 47.25 MHz intermediate frequency product of the
mixer is delivered to the AF UNIT.
The TUNE voltage from the CPU on the CNTL UNIT is
amplified by DC amplifier
Q3014 and applied to varac-
tors D1013 ( HVR100 ) in the variable frequency band-pass
filters. By changing the electrostatic capacitance of the var-
actors, optimum filter characteristics are provided for each
specific operating frequency.
First Intermediate Frequency (Narrow FM / AM)
The 47.25 MHz first intermediate frequency from first mix-
ers is delivered from the RF UNIT to the AF UNIT through
jacks J1003 and J2002. On the AF UNIT, the IF for AM and
FM-narrow signals is passed through NAR/WIDE switch
D2001
( DAP222 ) and 47.25 MHz monolithic crystal filter
(MCF) XF2001 to narrow IF amplifier Q2002 ( 2SC4915 )
for input to pin 16 of Narrow IF IC Q2013 (
TA31136FN
)
after amplitude limiting by D2002 ( DA221 ).
Meanwhile, a portion of the output of 11.7 MHz crystal
X1001 on RF UNIT is multiplied fourfold by Q2005
( 2SC4915 ) and Q2010 ( 2SC4154E ) to provide the 46.8
MHz second local signal, applied to the Narrow IF IC.
Within the IC, this signal is mixed with the 47.25 MHz first
intermediate frequency signal to produce the 450 KHz sec-
ond intermediate frequency.
This second IF is filtered by ceramic filter CF2002
(
ALFYM450F=K
) and amplified by the limiting amplifier
within the Narrow IF IC before quadrate detection by ce-
ramic discriminator
CD2001 (
CDBM450C7
).