Circuit Description - Vertex Standard VX-800 Service Manual

Overview
The VX-800V is a VHF/FM hand-held transceiver
designed to operate in the frequency range of 134 to
174MHz.
Circuit Configuration by Frequency
The receiver is a double-conversion superhetero-
dyne with a first intermediate frequency (IF) of 22.05
MHz and a second IF of 450 kHz. Incoming signals
from the antenna are mixed with the local signal
from PLL to produce the first IF of 22.05 MHz.
This is then mixed with the 21.6 MHz second local
oscillator output to produce the 450 kHz second IF.
This is detected to give the demodulated signal.
The transmit signal frequency is generated by PLL
VCO, and modulated by the signal from the micro-
phone. It is then amplified and sent to the antenna.
Receive Signal Path
Front-end RF amplifier
Incoming RF from the antenna jack is delivered to
the RF Unit and passes through a low-pass filter and
high-pass filter consisting of coils L1004, L1005,
L1006, L1001, L1002, and L1003, capacitors C1013,
C1014, C1015, C1017, C1019, C1020, C1006, C1007,
C1008, and C1009, and antenna switching diode
D1020 (HSU277).
Signals within the frequency range of the transceiver
are then amplified by Q1001 (2SC5226-4/5) and en-
ter a varactor-tuned band-pass filter consisting of
coils L1008, L1010, L1011, and L1012, capacitors
C1042, C1044, C1045, C1046, C1047, C1068, C1069,
C1070, C1071, C1072, C1073, C1074, C1075, C1098,
C1099, and C1100, and diodes D1018, D1023, D1024,
and D1027 (all HVC350) before delivery to the first
mixer.
First Mixer
Buffered output from the VCO is amplified by Q1030
(2SC5226-4/5) to provide a pure first local signal
between 112.3 and 152.3 MHz for injection to the
first mixer, D1030 (GN2011-Q). The 22.05 MHz first
mixer product then passes through monolithic crys-
tal filters XF1001 and XF1002 (22-11B-FP2, 5.5 kHz
BW), to strip away all but the desired signal.

Circuit Description

IF amplifier
The first IF signal is amplified by Q1037 (2SC5226-4/5).
The amplified first IF signal is applied to FM IF sub-
system IC Q1046 (TA31136FN) which contains the
second mixer, second local oscillator, limiter ampli-
fier, noise amplifier, and S-meter amplifier.
The IF signal is then mixed with the second local/
PLL reference oscillator Q1022, derived from 21.6
MHz crystal X1002, to produce the 450 kHz second
IF when mixed with the first IF signal within Q1046.
The second IF then passes through the ceramic fil-
ter CF1001 (PBFC450R15D) or CF1002 (SFPC450G:
Narrow-band Channels) to strip away unwanted
mixer products, and is applied to the limiter ampli-
fier in Q1046, which removes amplitude variations
in the 450 kHz IF, before detection of the speech by
the ceramic discriminator CD1001 (CDBC450CX24).
Audio amplifier
Detected audio from Q1046 is applied to Q1028
(AK2345) and audio low-pass filter, and then past
the volume control Q1025 (M62364FP) to the audio
amplifier Q1017 (TDA2822D: external speaker) or
Q1060 (TDA2822D: internal speaker), providing up
to 0.5 Watts to the optional headphone jack or 16W
loudspeaker.
Attention : Audio output is BTL output.
Squelch Control
The squelch circuitry consists of a noise amplifier
and band-pass filter within Q1046, and noise detec-
tor D1053 (DA221).
When no carrier is received, noise at the output of
the detector stage in Q1046 is amplified and band-
pass filtered by the noise amplifier section of Q1046
and the network between pins 7 and 8, and then rec-
tified by D1053.
The resulting DC squelch control voltage is passed
to pin 19 of the microprocessor Q1048. If no carrier
is received, this signal causes pin 32 of Q1048 to go
low and pin 89 to go high. Pin 32 signals Q1008
(CPH6102), Q1062 (UMC5N), and Q1063 (UMC5N)
to disable the supply voltage to audio amplifiers
Q1017 and Q1060, while pin 89 makes Q1014
(UMG2N) hold the green (Busy) half of the LED off,
when pin 32 is low and pin 89 is high.
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