Yaesu VX-450 series Service Manual Download

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VX-450 Series UHF Band Service Manual

Circuit Description (Version A & D)

1. Circuit Configuration by Frequency

The receiver is a Double-conversion Super-hetero-
dyne with a first intermediate frequency (IF) of 67.65
MHz and a second IF of 450 kHz. Incoming signal
from the antenna is mixed with the local signal from
the VCO/PLL to produce the first IF of 67.65 MHz.
This is then mixed with the 67.2 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 the
PLL VCO, and modulated by the signal from the
microphone. It is then amplified and sent to the an-
tenna.

2. Receiver System

2-1. Front-end RF amplifier

Incoming RF signal from the antenna is delivered to
the MAIN Unit and passes through Low-pass filer,
antenna switching diode, high pass filter and re-
moved undesired frequencies by varactor diode

D1006

D1007

D1009

, and

D1010

(tuned band-pass

filer: all

1SV325

The passed signal is amplified in

Q1007

(

BF1212WR

)

and moreover cuts an image frequency with the
tuned band pass filter and comes into the 1st mixer.

2-2. First Mixer

The 1st mixer consists of the

Q1015

(

3SK293

). Buff-

ered output from the VCO is amplified by

Q1014

(

2SC5005

) to provide a pure first local signal be-

tween 332.35 and 402.35 MHz (Version “A”) or 382.35
and 452.35 MHz (Version “D”) for injection to the
first mixer.
The IF signal then passes through monolithic crystal
filters

XF1001

(± 7.5 kHz BW) to strip away all but

the desired signal.

2-3. IF Amplifier

The first IF signal is amplified by

Q1025

(

2SC5226

The amplified first IF signal is applied to FM IF sub-
system IC

Q1033

(

NJM2591V

) which contains the

second mixer, second local oscillator, limiter ampli-
fier, noise amplifier, and RSSI amplifier.
The signal from reference oscillator

X1001

becomes

4 times of frequencies in

Q1033

, it is mixed with the

IF signal and becomes 450 kHz.

The second IF then passes through the ceramic filter

CF1001

CF1002

to strip away unwanted mixer

products, and is applied to the limiter amplifier in

Q1033

, which removes amplitude variations in the

450 kHz IF, before detection of the speech by the ce-
ramic discriminator

CD1001

(

ECDA450C24

2-4. Audio amplifier

Detected signal from

Q1033

(

NJM2591V

) is input-

ted to Audio Processor IC

Q1013

The signal which appeared from

Q1001

is in high

pass filter

Q1041

(

NJM12902V

The signal which passed

Q1041

goes to AF volume

(VR1001). And then the signal goes to audio ampli-
fier

Q1048

(

TDA2822L-50B

).The output signal from

J1006

is in audio speaker.

2-5. Squelch Circuit

There are 16 levels of squelch setting from 0 to 15.
The level 0 means open the squelch. The level 1
means the threshold setting level and level 14 means
tight squelch. From 2 to 13 is established in the
middle of threshold and tight.
The bigger figure is nearer the tight setting. The level
15 becomes setting of carrier squelch.

2-5-1. Noise Squelch

Noise squelch circuit is composed of the band path
filter and noise detector of

Q1033

When a carrier isn’t received, the noise ingredient
which goes out of the demodulator

Q1033

is ampli-

fied in

Q1037

through the band path filter, is detected

to DC voltage with

D1031

and is inputted to 15 pin

(the A/D port) of the

Q1035

(CPU).

When a carrier is received, the DC voltage becomes
low because the noise is compressed.
When the detected voltage to CPU is high, the CPU
stops AF output with

Q1049

“OFF” by making the

90 pin (CPU) “L” level.
When the detection voltage is low, the CPU makes

Q1049

ON with making 90 pin “H” and the AF sig-

nal is output.