Circuit Description - Vertex VX-920 Series Service Manual

1. Circuit Configuration by Frequency
The receiver is a double-conversion superhetero-
dyne type, with a first Intermediate Frequency (IF)
of 50.85 MHz and a second IF of 450 kHz. Incoming
signals from the antenna are mixed with the local
signal from the VCO/PLL to produce the first IF of
50.85 MHz. The signals are then mixed with the 50.4
MHz second local oscillator output to produce the
450 kHz second IF. This is detected to give the de-
modulated signal. The transmit signal frequency is
generated by the PLL VCO, and modulated by the
signal from the microphone. It is then amplified and
sent to the antenna.
2. Receiver System
2-1. Front-end RF amplifier
Incoming RF signals from the antenna are delivered
to the Main Unit and pass through a Low-pass fil-
ter, antenna switching diode D1011 (HVU131), and
a high pass filter, and undesired out-of-band signals
are then removed by a varactor-diode-tuned band-
pass filter D1014, D1015, D1049, and D1050 (all
1SV323). The residual signals are amplified by
Q1022 (2SC5226) and then pass through a varactor-
diode-tuned image-stripping band pass filter D1018
and D1019 (both 1SV323) prior to delivery to the
1st mixer.
2-2. First Mixer
The 1st mixer consists of Q1034 (SPM5001), T1001,
T1002, and T1003. Buffered output from the VCO is
amplified by Q1033 (2SC5226) to provide a pure
first local signal between 184.85 and 224.85 MHz for
injection to the first mixer. The IF signal then passes
through monolithic crystal filter XF1001 (±5.5 kHz
BW) to strip away undesired mixing products.
2-3. IF Amplifier
The first IF signal is amplified by Q1058 (2SC5226).
The amplified first IF signal is applied to FM IF sub-
system IC Q1067 (TA31136FN) which contains the
second mixer, second local oscillator, limiter ampli-
fier, noise amplifier, and S-meter amplifier. The sig-
nal from reference oscillator X1003 is multiplied by
three by Q1067 (TA31136FN), and then is mixed with
the IF signal to produce a 450 kHz second IF.
The second IF signal then passes through the ceramic
VX-920/-920E VHF series Service Manual

Circuit Description

filter CF1001 (wide channels), CF1002 (narrow chan-
nels) to strip away all but the desired signal, and is
applied to the limiter amplifier in Q1067
(TA31136FN), which removes amplitude variations
in the 450 kHz IF, before detection of the speech by
the ceramic discriminator CD1001.
2-4. Audio amplifier
Detected audio from Q1067 (TA31136FN) is applied
to Q1032 (AK2345) and is fed to the bandpass filter
inside Q1032 (AK2345).
If an optional signaling unit is installed, Q1078
(NJU4066BV) is set to "OFF," and the AF signal from
Q1032 (AK2345) is fed to the optional unit. If a sig-
naling unit is not installed, Q1078 (NJU4066BV) is
switched "ON," and the signal passes through
Q1078 (NJU4066BV). The signal then proceeds
through the de-emphasis stage and the expander at
Q1023 (LA8630M). When the expander function is
off, the signal will be bypassed via Q1016
(NJU4053BV). The output signal of the expander (or
a signal from de-emphasis) goes through AF mute
switch Q1078 (CD4066BV) and is amplified by
Q1077 (LM2902PWR). The signal then proceeds to
Q1030 (NJU4053BV) when the audio equalizer is
"OFF;" if the audio equalizer is "ON," the signal
from Q1077 (LM2902PWR) passes to the buffer am-
plifier Q1077 (LM2902PWR) for audio equalization
at Q1083 (LM2902PWR) and Q1025/Q1029 (both
CD4052BPWR). The signal then is fed to Q1030
( N J U 4 0 5 3 B V ) . T h e o u t p u t f r o m Q 1 0 3 0
(NJU4053BV) is amplified by the AF power ampli-
fier Q1015 (TDA2822) after passing through the AF
volume control Q1020 (M62364FP). The output of
Q1015 (TDA2822) drives the internal speaker.
2-5. Squelch Circuit
There are 13 levels of squelch setting, from "0" to
"12." The level "0" corresponds to an "open"
squelch. The level "1" is the lowest squelch thresh-
old setting level, and level "11" means tight squelch.
From "2" to "10" are intermediate, increasingly-tight
settings. The level "12" represents carrier squelch.
2-5-1. Noise Squelch
The Noise Squelch circuit is composed of the
bandpass filter at Q1067 (TA31136FN), noise ampli-
7