Circuit Description - Vertex Standard VX-4200 Series Service Manual

1. Circuit Configuration by Frequency
The receiver is a double-conversion superheterodyne with a 1st
intermediate frequency (IF) of 50.85 MHz and a 2nd IF of 450
kHz. Incoming signal from the antenna is mixed with the local
signal from the VCO/PLL to produce the 1st IF of 50.85 MHz.
This is then mixed with the 50.4 MHz 2nd local oscillator out-
put to produce the 450 kHz 2nd IF. This is detected to give the
demodulated signal. The transmit signal frequency is generated
by the PLL VCO, and modulated by the signal from the micro-
phone. It is then amplified and sent to the antenna.
2. Receiver System
2-1. Front-end RF amplifier
Incoming FR signal from the antenna is delivered to the RF Unit
and passes through Low-pass filer, antenna switching diode
D1001 (UM9957F), and removed undesired frequencies by
varactor tuned band-pass filter consisting of capacitors C1041,
C1045, C1057, C1058, C1065, C1083, C1091, coils L1048,
L1049 and diode D1003, D1004 (Both 1SV279). The passed
signal is amplified by RF amplifier Q1022 (2SC3357), then
moreover cuts an image frequency with the other varactor tuned
band-pass filter consisting of capacitors C1359, C1360, C1361,
C1369, C1387, C1389, C1398 coils L1010, L1011and diode
D1008, D1009 (Both 1SV279). The filtered RF signal delivery
to the 1st mixer. The Q1020 (2SA1774) controls bias of Q1022
(2SC3357).
2-2. First Mixer
The 1st mixer consists of the Q1035 (SPM5001), T1004, T1005
and T1006. Buffered output from the VCO is amplified by Q1038
(2SC5226) to provide a pure 1st local signal between 399.15
and 461.15 MHz for injection to the 1st mixer. The output IF
signal is amplified by Q1046 (2SC3357), then passes through
monolithic crystal filter XF1001 (±5.5 kHz BW) to strip away
all but the desired signal.
2-3. IF Amplifier
The filtered 1st IF signal is amplified by Q1059 (2SC5226).
The amplified 1st IF signal is applied to FM IF subsystem IC
Q1079 (TA31136FN) which contains the second mixer, sec-
ond local oscillator, limiter amplifier, and noise amplifier. The
signal from reference oscillator X1003 (16.8 MHz) becomes 3
times of frequencies in Q1079 (TA31136FN), it is mixed with
the IF signal and becomes 2nd IF signal. The 2nd IF signal passes
through the ceramic filter CF1001 (for wide channels), CF1002
(for narrow channels) to strip away unwanted mixer products,
and is applied to the limiter amplifier in Q1091 (2SC4617),
which removes amplitude variations in the 2nd IF signal, before
detection of the speech by the ceramic discriminator CD1001.

Circuit Description

2-4. Audio amplifier
Detected signal from Q1079 (TA31136FN) is delivered to
Q1037 (AK2345) and is output through the band path filter
inside Q1037 (AK2345). When the optional unit is installed,
the Q1063 (BU4066BCFV) is made to "OFF" and the AF sig-
nal from Q1037 (AK2345) goes the optional unit. When the
optional unit is not installed, Q1037 (AK2345) is made to "ON"
and the signal goes through Q1063 (BU4066BCFV). The AF
signal then goes through AF mute switch Q1068
(BU4066BCFV), de-emphasis part and expander Q1021
(LA8630M). When the function of expander is off, the Q1021
(LA8630M) will be bypassed by Q1069 (TAR5S50). The out-
put signal of expander (or a signal from de-emphasis) is ampli-
fied by Q1066 (NJM12902V) and it is amplified with AF power
amplifier Q1005 (TDA1519CTH) after passing AF volume
Q1029 (M62364FP). The output from Q1005 (TDA1519CTH)
drives a speaker (it chooses the external speaker or internal
speaker in J6001).
2-5. Squelch Circuit
There are 13 levels of squelch setting from "0" to "12." The
level "0" means open the squelch. The level "1" means the thresh-
old setting level and level "11" means tight squelch. From "2"
to "10" is established in the middle of threshold and tight. The
bigger figure is nearer the tight setting. The level "12" becomes
setting of carrier squelch.
2-5-1. Noise Squelch
Noise squelch circuit is composed of the band-path filter of
Q1079 (TA31136FN), noise amplifier Q1091 (2SC4617), and
noise detector D1059 and D1061 (both DA221). When a car-
rier isn't received, the noise ingredient which goes out of the
demodulator Q1079 (TA31136FN) is amplified in Q1091
(2SC4617) through the band path filter Q1079 (TA31136FN),
is detected to DC voltage with D1059 and D1061 (both DA221)
and is inputted to pin 19 (A/D port) of the Q1083 (CPU:
LC87F74C8A). When a carrier is received, the DC voltage be-
comes "LOW" because the noise is compressed. When the de-
tected voltage to CPU is "HIGH," the CPU stops AF output
with Q1068 (BU4066BCFV) "OFF" by making the 40pin
"LOW" level. When the detection voltage is low, the CPU makes
Q1068 "ON" with making pin 40 to "HIGH" and the AF signal
is output.
2-5-2. Carrier Squelch
The pin 18 (A/D port) of Q1083 (CPU: LC87F74C8A) detect
RSSI voltage output from pin 12 of Q1079 (TA31136FN), and
controls AF output. The RSSI output voltage changes according
to the signal strength of carrier. The stronger signal makes the
RSSI voltage to be higher voltage. The process of the AF signal
control is same as Noise Squelch. The shipping data is adjusted
3dB higher than squelch tight sensitivity.
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