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1. Receiver Signal Flow
1-1. Front-end RF Amplifier
Received VHF bands signals pass through the low-pass
filter/high-pass filter circuit, T/R switch circuit composed
of diode switch D1040 (RLS135), D1041 (RLS135), and
protector diode D1042 (1SS400) before additional filter-
ing by a high-pass filter prior to application to RF ampli-
fier Q1043 (2SC5006). The amplified RF signal is passed
through the band-pass filter to first mixer Q1035 (3SK318).
Meanwhile, VHF output from the VCO is applied through
diode T/R switch D1024 (DAN235E) to mixer Q1035 as
the first local signal.
1-2. First Intermediate Frequency
The 47.25 MHz first intermediate frequency from first
mixer is passed through 47.25 MHz monolithic crystal fil-
ter (MCF) XF1001 to IF amplifier Q1042 (2SC4915-O) for
input to pin 16 of IF IC Q1040 (NJM2591V) after ampli-
tude limiting by D1037 (DA221).
Meanwhile, a portion of the output of 11.7 MHz crystal
X1001 is multiplied fourfold by Q1037 (2SC4915-O) to
provide the 46.8 MHz second local signal, applied to the
IF IC Q1040. Within the IC, this signal is mixed with the
47.25 MHz first intermediate frequency signal to produce
the 450 kHz second intermediate frequency.
This second IF is filtered by ceramic filter CF1001
(LTWC450F) and amplified by the limiting amplifier with-
in the IF IC before quadrate detection by ceramic discrim-
inator CD1001 (JTBM450CX24).
Demodulated audio is output from pin 9 of the IF IC
Q1040, and then is passed through a de-emphasis filter
Q1034 and band-pass filter Q1022 (both LM2902PW) to
the volume control IC Q1018 (BU2502FS).
The adjusted audio signal from pin 22 of the volume con-
trol IC Q1018 is applied to the audio power amplifier
Q1001 (TDA2822D), providing up to 700 mW of audio
power to the 16-ohm loudspeaker.
Circuit Description
1-3. Squelch Control
Signal components in the neighborhood of 15 kHz con-
tained in the discriminator output pass through an active
band-pass filter composed of resistors R1211, R1213, &
R1214, and capacitors C1172 & C1182 and the operational
amplifier between pins 7 and 8 within IF IC Q1040. They
are then rectified by D1031 and D1033 (both DA221) to
obtain a DC voltage corresponding to the level of noise.
This voltage is input to pin 16 of the microprocessor Q1017
(LC87F7BC8A-F56G2), which compares the input volt-
age with a previously set threshold. If the DC squelch
control voltage is "high", the microprocessor Q1017 con-
trols pin 22 of volume control IC Q1018 (BU2502FS) goes
"low", thus disabling the AF audio. Also, the micropro-
cessor stops scanning, if active, and allows audio to pass
through the volume control IC Q1018.
2. Transmitter Signal Flow
2-1. Transmit/Receive Switching
Closing PTT switch S1001 pulls the base of Q1002
(2SA1744-R) "low", causing the collector to go high. This
signal is input to pin 22 (PTT) of microprocessor Q1017,
allowing the microprocessor to recognize that the PTT
switch has been pushed.
When the microprocessor Q1017 detects closure of the PTT
switch, pin 31 (TX SW) goes "high". This control signal
switches Q1044 (UMD5N). At the same time, PLL divi-
sion data is input to PLL IC Q1019 (LV2105V) from the
microprocessor Q1017, to disable the receiver power sav-
er. Also, switching Q1041 (UMD5N) to disable the receiv-
er circuits. Then causing the red side of BUSY/TX lamp
D1016 (CL-165HR/YG-D-T) to light.
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