Standard Horizon HX370S, Service Manual

Page: 7 / 44

7
1. Receive Signal Path
Incoming RF from the antenna jack is delivered to the RF
Unit and passes through a low-pass filter consisting of
coils L1023, L1026, and L1027, capacitors C1227, C1273,
C1276, C1270, C1257, and C1256, and antenna switching
diode
D1036 (
RLS135
).
Signals within the frequency range of the transceiver en-
ter a Varactor-tuned band-pass filter consisting of coils
L1022 and L1021, capacitors C1246, C1244, and C1242,
and diode
D1048 ( 1SV323 ), then amplified by Q1058
(
2SC5006
) and enter a Varactor-tuned band-pass filter
consisting of coils L1018, L1017, and L1016, capacitors
C1211, C1208, C1210, C1212, C1209, C1207, C1198, C1197,
C1196, C1195, and C1194, and diodes
D1032 , D1031 , and
D1029 (all
HVC350B
), before first mixing by Q1054
(
3SK320
).
Buffered output from the VCO is amplified by Q1024
(
2SC5374
) to provide a pure first local signal between
112.3 and 152.3 MHz for injection to the first mixer Q1054
(
3SK320
).
The 21.7 MHz first mixer product then passes through
monolithic crystal filter XF1001 / XF1002 (both
21R15AB
)
to strip away all but the desired signal, which is then am-
plified by
Q1046 (
2SC4400
). The amplified first IF sig-
nal is applied to FM IF subsystem IC Q1039 (
NJM2591V
),
which contains the second mixer, second local oscillator,
limited amplifier, noise amplifier, and RSSI amplifier.
A second local signal is produced from the PLL reference/
second local oscillator of
X1001 (21.25 MHz). The 21.25
MHz reference signal is delivered to mixer section of
Q1039 (
NJM2591V
) which produce the 450 kHz second
IF mixed with the first IF signal.
The second IF then passes through the ceramic filter
CF1001 (
LTWC450G
: for “Narrow” channels) or CF1002
(
LTC450FW
: for “Wide” channels) to strip away unwant-
ed mixer products, and is then applied to the limited am-
plifier in
Q1039 (
NJM2591V
), which removes amplitude
variations in the 450kHz IF, before detection of the speech
by the ceramic discriminator
CD1001 (
JTBC450C7
).
2. Audio Amplifier
The demodulated audio signal from the Q1039
(
NJM2591V
) passes through a band-pass filter and High-
pass filter to the Voice Scrambler Unit when the optional
Voice Scrambler Unit is installed, then applied to the de-
emphasis. Then passes through the audio volume and the
audio power amplifier
Q1010 (
TDA2822D
), providing up
to 400 mW of audio power to the 4-ohm loudspeaker.
3. Squelch Control
The squelch circuitry consists of a noise amplifier and
band-pass filter and noise detector within
Q1039
(
NJM2591V
) When no carrier received, noise at the out-
put of the detector stage in Q1039 (
NJM2591V
) is ampli-
fied and band-pass filtered by the noise amplifier section
of
Q1039 (
NJM2591V
) and the network between pins 7
and 8, and then rectified by detection circuit in Q1039
(
NJM2591V
).
The resulting DC squelch control voltage is passed to pin
52 of the microprocessor
Q1066 (
HD64F2266
). If no carri-
er is received, this signal causes pin 52 of Q1066
(
HD64F2266
) to go high and pin 30 to go high. Pin 11
signals of Q1053 (
CD4094BPW
) to disable the supply volt-
age to the audio amplifier Q1010 (
TDA2822D
), while pin
7 hold the green (Busy) half of the LED D1027 (
CL-165HR/
YG-D
) off, when pin 11 is high and pin 7 is high. Thus, the
microprocessor blocks output from the audio amplifier,
and silences the receiver, while no signal is being received
(and during transmission, as well).
When a carrier appears at the discriminator, noise is re-
moved from the output, causing pin 75 of the micropro-
cessor
Q1066 (
HD64F2266
) to go low which is activate
the ”Busy” LED D1027 (
CL-165HR/YG-D
) via Q1053
(
CD4094BPW
).
The microprocessor Q1066 (
HD64F2266
) then checks for
CTCSS or CDCSS code squelch information, if enabled. If
not transmitting and CTCSS or CDCSS is not activated,
or if the received tone or code matches that programmed,
allows audio to pass through the audio amplifier
Q1010
(
TDA2822D
) to the loudspeaker by enabling the supply
voltage via Q1014 (
FMMTL718TA
).
4. Transmit Signal Path
The speech input from the microphone MC1001 passes
through the audio amplifier Q1076 (
LM2902PW
) which
is adjusted the microphone gain. The speech signal pass-
es through pre-emphasis circuit of
Q1076 (
LM2902PW
)
which contains the IDC, and low-pass filter.
The filtered audio signal is applied to Q1033 (
M62364FP
)
which is adjusted the audio level, then is applied to var-
actor diode
D1006 (
HVC350B
) which frequency modu-
lates the VCO Q1012 (
2SC5231
). A portion of the audio
signal from Q1033 Q1033 (
M62364FP
) is applied to X1001
(21.25 MHz).
The processed audio may then be mixed with a CTCSS
t o n e g e n e r a t e d b y t h e m i c r o p r o c e s s o r
Q 1 0 6 6
(
HD64F2266
) for frequency modulation of the PLL carri-
er (up to ±5 kHz from the unmodulated carrier) at the
transmitting frequency.
Circuit Description