Receiver System; Circuit Description; Audio Amplifier - Vertex Standard VX-820 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 D1013 and D1015 (both 1SV323). The re-
sidual signals are amplified by Q1021 (MSG33001)
and then pass through a varactor-diode-tuned im-
age-stripping band pass filter D1020 and D1022
(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 Q1032 (2SC5005) to provide a pure
first local signal between 399.15 and 461.15 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 Q1056 (2SC5226).
The amplified first IF signal is applied to FM IF sub-
system IC Q1065 (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 Q1065 (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
filter CF1001 (wide channels), CF1002 (narrow chan-

Circuit Description

nels) to strip away all but the desired signal, and is
applied to the limiter amplifier in Q1065
(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 Q1065 (TA31136FN) is applied
to Q1031 (AK2345) and is fed to the bandpass filter
inside Q1031 (AK2345).
If an optional signaling unit is installed, Q1078
(CD4066BPWR) is set to "OFF," and the AF signal
from Q1032 (AK2345) is fed to the optional unit. If a
s i g n a l i n g u n i t i s n o t i n s t a l l e d , Q 1 0 7 8
(CD4066BPWR) is switched "ON," and the signal
passes through Q1078 (CD4066BPWR). The signal
then proceeds through the de-emphasis stage and
the expander at Q1022 (LA8630M). When the ex-
pander function is off, the signal will be bypassed
via Q1014 (CD4053BPWR). The output signal of the
expander (or a signal from de-emphasis) goes
through AF mute switch Q1083 (DTC114TE) and is
amplified by Q1082 (LM2902PWR). The output from
Q1082 LM2902PWR) is amplified by the AF power
amplifier Q2017 (TDA2822) after passing through
the AF volume control Q1020 (M62364FP). The out-
put of Q2017 (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 Q1065 (TA31136FN), noise ampli-
fier Q1069 (2SC4617), and noise detector D1049/
D1050 (both DA221). When a carrier is not being
received, the noise components passed from de-
modulator Q1065 (TA31136FN) are amplified by
Q1069 (2SC4617), fed through bandpass filter Q1065
(TA31136FN), detected as a DC voltage by D1049/
D1050 (both DA221), and applied to pin 16 (the A/D
port) of Q1067 (CPU: LC87F5BP6A). When a car-
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