Yaesu VX-450 series, Service Manual

Page: 27 / 71

27 VX-450 Series UHF Band Service Manual
1. Circuit Configuration by Frequency
The receiver is a Double-conversion Super-hetero-
dyne with a first intermediate frequency (IF) of 67.65
MHz (Version “I”) or 50.85 MHz (Version “K”) 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 (Version
“I”) or 50.85 MHz (Version “K”).
This is then mixed with the 67.2 MHz (Version “I”)
or 50.4 MHz (Version “K”) 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-2 Unit and passes through Low-pass filer,
antenna switching diode, high pass filter and re-
moved undesired frequencies by varactor diode
D3006 , D3007 , D3009 , and D3010 (tuned band-pass
filer: all
1SV325
).
The passed signal is amplified in Q3007 ( 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 Q3015 (
3SK293
). Buff-
ered output from the VCO is amplified by Q3014
(
2SC5005
) to provide a pure first local signal be-
tween 282.35 and 322.35 MHz (Version “I”) or 249.15
and 289.15 MHz (Version “K”) for injection to the
first mixer.
The IF signal then passes through monolithic crystal
filters
XF3001 (± 7.5 kHz BW) to strip away all but
the desired signal.
2-3. IF Amplifier
The first IF signal is amplified by Q3025 ( 2SC5226 ).
The amplified first IF signal is applied to FM IF sub-
system IC
Q3033 ( NJM2591V ) which contains the
second mixer, second local oscillator, limiter ampli-
fier, noise amplifier, and RSSI amplifier.
Circuit Description (Version I & K)
The signal from reference oscillator X3001 becomes
4 times of frequencies in Q3033 , it is mixed with the
IF signal and becomes 450 kHz.
The second IF then passes through the ceramic filter
CF3001 or CF3002 to strip away unwanted mixer
products, and is applied to the limiter amplifier in
Q3033 , which removes amplitude variations in the
450 kHz IF, before detection of the speech by the ce-
ramic discriminator
CD3001 (
ECDA450C24
).
2-4. Audio amplifier
Detected signal from Q3033 ( NJM2591V ) is input-
ted to Audio Processor IC Q3013 .
The signal which appeared from Q3001 is in high
pass filter Q3041 (
NJM12902V
).
The signal which passed Q3041 goes to AF volume
(VR3001). And then the signal goes to audio ampli-
fier
Q3048 ( TDA2822L-50B ). In the 4 key & 16 key
Type, the output signal from Q3048 is fed through
J2002 on the DISPLAY Unit to the audio speaker. In
the Non key Type, the output signal from Q3048 is
fed through J3006 to the 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
Q3033 .
When a carrier isn’t received, the noise ingredient
which goes out of the demodulator
Q3033 is ampli-
fied in Q3037 through the band path filter, is detected
to DC voltage with D3031 and is inputted to 15 pin
(the A/D port) of the Q3035 (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
Q3049 “OFF” by making the
90 pin (CPU) “L” level.