Circuit Description - Vertex Standard VX-131 Service Manual

Receive Signal Path
Incoming RF signal from the antenna jack is delivered to
the MAIN Unit and passes through a low-pass filter con-
sisting of coils L1003, L1005, and L1007, capacitors C1014,
C1017, C1020, C1030, C1039, and C1042, and antenna
switching diode D1005 (RLS135).
Signals within the frequency range of the transceiver en-
ter a varactor-tuned band-pass filter consisting of coils
L1012 and L1014, capacitors C1072, C1074, C1080, and
C1086, and diodes D1011, D1012, D1013 and D1014 (all
HVC355B), then amplified by RF amplifier Q1015
(2SC5006) and enter a varactor-tuned band-pass filter con-
sisting of coils L1020 and L1022, capacitors C1101, C1103,
C1109, C1110, C1116, and C1118, and diodes D1016
(HVC355B), D1017 (HVC355B), D1019 (HVC355B),
D1022 (1SV280), and D1023 (HVC355B), before 1st mix-
ing by 1st mixer Q1029 (3SK320).
Buffered output from the VCO Q1004 (2SK508) is ampli-
fied by Q1008 (2SC5005) to provide a pure 1st local sig-
nal between 405.75 and 425.75 MHz for injection to the
1st mixer Q1029 (3SK320). The 44.25 MHz 1st IF signal
then passes through monolithic crystal filter XF1002 to
strip away all but the desired signal. The filtered 1st IF
signal is amplified by Q1036 (2SC4215Y).
The amplified 1st IF signal is applied to FM IF subsystem
IC Q1041 (TA31136FN), which contains the 2nd mixer,
2nd local oscillator, limiter amplifier, noise amplifier, and
RSSI amplifier.
A 2nd local signal is produced from the PLL reference/
2nd local oscillator of 14.60 MHz crystal X1001. The 14.60
MHz reference signal is tripled by Q1041 (TA31136FN),
capacitor C1182, and coil L1030, then resulting the 43.8
MHz second local signal delivered to mixer section of
Q1040 (TA31136FN) which produce the 450 kHz 2nd IF
mixed with the 1st IF signal.
The 2nd IF signal passes through the ceramic filter CF1001
(for "Narrow" bandwidth channels) or CF1002 (for
"Wide" bandwidth channels) to strip away unwanted
mixer products, and is then applied to the limiter amplifi-
er in Q1041 (TA31136FN), which removes amplitude vari-
ations in the 450kHz IF, before detection of the speech by
the ceramic discriminator CD1001.
Detected audio from CD1001 is applied to the audio high-
pass filter Q1030 (LM2902PW), and then passes through
the AF mute switch Q1040 (2SA1586Y), de-emphasis cir-
cuit consisting of resistor R1175 and Capacitor C1167, and
volume control to the audio amplifier Q1037 (NJM2070M),
which provides up to 0.5 Watts to the internal 4-W loud-
speaker or the optional headphone.

Circuit Description

Squelch Control
The squelch circuitry consists of a noise amplifier and
band-pass filter within Q1041 (TA31136FN) and noise
detector D1026 (1SS355).
When no carrier received, noise at the output of the de-
tector stage in Q1041 (TA31136FN) is amplified and band-
pass filtered by the noise amplifier section of Q1041
(TA31136FN) and the network between pins 7 and 8 of
Q1041 (TA31136FN), and then rectified by D1026
(1SS355).
The resulting DC squelch control voltage is passed to pin
37 of the microprocessor Q1013 (M37516F8HP). If no car-
rier is received, this signal causes pin 20 and 24 of Q1013
(M37516F8HP) to go high. Pin 24 signal disable the sup-
ply voltage of the audio amplifier Q1037 (NJM2070M).
Thus, the microprocessor blocks output from the audio
amplifier, and silences the receiver while no signal is be-
ing received (and during transmission, as well). Pin 20
signal hold the green (Busy) half of the LED D1021 (12-
22SURSYGC) to off.
When a carrier appears at the discriminator, noise is re-
moved from the receiver output, causing pin 37 of Q1013
(M37516F8HP) to go low and the microprocessor to acti-
vate the green (Busy) half of the LED D1021 (12-
22SURSYGC) via Q1013 (M37516F8HP).
The microprocessor Q1013 (M37516F8HP) checks for the
CTCSS or DCS code squelch information. If the CTCSS or
DCS is not activated, or the received tone or code matches
that programmed, allows audio to pass through the au-
dio amplifier Q1037 (NJM2070M) to the loudspeaker by
enabling the supply voltage to it via Q1033 (CPH6102).
Transmit Signal Path
Speech input from the microphone passes through a high-
pass filter Q1017 (LM2902PW-2) and AF mute switch
Q1011 (2SA1586Y) to Q1002 (M62364FP) which adjust
the microphone gain. The adjusted audio passes through
a pre-emphasis circuit consisting of resistor R1059 and Ca-
pacitor C1065 to the limiter amplifier Q1017 (LM2902PW-
3). The limited audio is applied to a low-pass filter Q1017
(LM2902PW-1/-4), then return to Q1002 (M62364FP)
which adjust the deviation level.
The audio signal is applied to varactor diode D1004
(HVC350) which frequency modulates the VCO Q1004
(2SK508). The modulated signal from the VCO Q1004
(2SK508) is buffered by Q1008 (2SC5005), then passes
through the T/R switching diode D1010 (DAN235E) to the
buffer amplifier Q1012 (2SC5005) and Q1014 (2SC3357).
The amplified signal is delivered to the driver amplifier
Q1014 (RD01MUS1), then amplified transmit signal is
applied to the final amplifier Q1023 (RD07MVS1) up to 2
watts output power.
9