Vertex Standard VX-800 Service Manual Download

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Circuit Description

Overview

The VX-800U is a UHF/FM hand-held transceiver
designed to operate in the frequency range of 400 to
512MHz.

Circuit Configuration by Frequency

The receiver is a double-conversion superhetero-
dyne with a first intermediate frequency (IF) of 44.25
MHz and a second IF of 450 kHz.

Incoming signals from the antenna are mixed with
the local signal from the PLL to produce the first IF
of 44.25 MHz. The first IF signal is subsequently
mixed with the 43.8 MHz second local oscillator
output (from the 14.6MHz reference crystal) to pro-
duce the 450 kHz second IF. The second IF is then
demodulated to produce audio output.

The transmit signal frequency is generated by the
PLL VCO, and is modulated by the signal from the
microphone. It is then amplified and sent to the an-
tenna.

Receive Signal Path

Front-end RF amplifier

Incoming RF energy from the antenna jack is deliv-
ered to the RF Unit and passes through a low-pass
filter and high-pass filter consisting of coils L1004,
L1005, L1006, L1030, L1001, L1028, L1029, L1002, and
L1003, capacitors C1013, C1014, C1015, C1017, C1019,
C1020, C1282, C1001, C1005, C1006, C1283, C1007,
C1008, and C1009, and antenna switching diode
D1020 (

HVU131

Signals within the frequency range of the transceiver
are then amplified by Q1001 (

2SC5226

) and enter a

varactor-tuned band-pass filter consisting of coils
L1008, L1010, and L1011, capacitors C1041, C1042,
C1044, C1045, C1046, C1047, C1068, C1069, C1070,
C1071, C1072, C1073, C1074, and C1174, and diodes
D1018, D1023, and D1024 (all

HVC350

) before de-

livery to the first mixer.

First Mixer

Buffered output from the VCO is amplified by Q1030
(

2SC5226

) to provide a pure first local signal be-

tween 355.75 and 467 MHz for injection to the first
mixer D1030 (

GN2011-Q

:Lot. 1~50,

SPM5001

:Lot.

51~). The 44.25 MHz first IF product then passes
through monolithic crystal filters XF1001 and XF1002

(44-11BF-P2, ±5.5 kHz BW) to strip away unwanted
mixer products.

IF amplifier

The first IF signal is amplified by Q1037
(

2SC4215Y

:Lot. 1~3,

2SC5226

:Lot. 4~).

The amplified first IF signal is then applied to FM IF
subsystem IC Q1046 (

TA31136FN

) which contains

the second mixer, second local oscillator, limiter am-
plifier, noise amplifier, and S-meter amplifier.

A second local signal is generated by Q1022
(

NJM2902V

) using the 14.6 MHz crystal (X1002) as

a reference, producing a 43.8 MHz signal; this yields
a 450 kHz second IF when mixed with the first IF
signal within Q1046.

The second IF then passes through the ceramic fil-
t e r C F 1 0 0 1 (

P B F C 4 5 0 R 1 5 D

: L o t . 1 ~ 3 9 ,

E L F Y 4 5 0 E

: L o t . 4 0 ~ ) o r C F 1 0 0 2

(

SFPCA450KG1A

:Lot. 1~50,

ELFC450G

:Lot. 51~:

“Narrow” channels) to strip away all but the desired
signal, and is applied to the limiter amplifier in
Q1046, which removes amplitude variations in the
450 kHz IF, before detection of the speech by the ce-
r a m i c d i s c r i m i n a t o r C D 1 0 0 1
(

CDBCB450KCAY24

:Lot. 1~50,

ECDA450C24

:Lot.

51~).

Audio amplifier

Detected audio from Q1046 is applied to Q1028
(

AK2345

) and an audio low-pass filter, and then fed

past the volume control (Q1025:

M62364FP

) to the

audio amplifier Q1017 (

TDA2822D

: external

speaker) or Q1063 (

TDA2822D

: internal speaker),

providing up to 0.5 Watts of audio to the optional
headphone jack or 16-Ohm 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 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 recti-
fied by D1053.