Vertex Standard VXR-7000, Service Manual

Page: 25 / 146

25 VXR-7000 UHF Service Manual
Circuit Description
Receive Signal Path
Incoming RF from the RX antenna jack is delivered to
the RX Unit and passes through the protection diode
D3001
(
1SS302
) and a varactor-tuned band pass filter
consisting of coils L3002 and L3004, capacitors C3020,
C3021, C3024, C3025, and C3029, and diodes
D3004
and
D3007
(both
HVU350
). Signals are then applied to the
RF amplifier,
Q3005
(
2SC3357
). The amplified RF sig-
nal is applied through a varactor-tuned band pass filter
consisting of coils L3010 and L3012, capacitors C3048,
C3052, C3053, C3157 and C3158, and diodes
D3019
and
D3020
(both
HVU350
) to the first mixer
D3011
(
GN2011-Q
) along with the first local signal from the
PLL circuit.
The first local signal is generated between 376.65 MHz
and 406.65 MHz by the RX VCO, which consists of FET
Q3008
(
2SK508
) and varactor diodes
D3005
,
D3006
,
D3008
, and
D3009
(
HVU350
) according to the pro-
grammed receiving frequency; the local signal then pass-
es through buffer amplifier
Q3009
(
2SC5226
) and first
local amplifier
Q3011
(
2SC3357
) to the first mixer
D3011
.
The 73.35 MHz first IF signal is applied to monolithic
crystal filters
XF3001
and
XF3002
(both
73S10A
: ±5.0
kHz B.W.) which strip away unwanted mixer products,
and the IF signal is applied to the first IF amplifiers
Q3014
and
Q3017
(both
2SC5226
). The amplified first IF signal
is then delivered to the FM IF subsystem IC
Q3012
(
TA31136FN
), which contains the second mixer, second
local oscillator, limiter amplifier, noise amplifier, and FM
detector.
The second local oscillator signal, generated by the
72.895 MHz crystal
X3002
, produces the 455 kHz sec-
ond IF signal when mixed with the first IF signal within
Q3012
. The second IF signal passes through ceramic fil-
ter
CF3001
(
CFWM455G
: ±4.5 kHz B.W.) or
CF3002
(
CFWM455F
: ±6.0 kHz B.W.) which strips away all but
the desired signal, and then passes through the limiter
amplifier within
Q3012
to ceramic discriminator
CD3001
(
CDB455C7
), which removes any amplitude variations
in the 455 kHz IF signal before detection of speech. The
detected audio passes through the low pass filter, consist-
ing of R3066 and C3128, which rejects the 455 kHz IF
component, then delivers the audio to pin 12 of JP3001.
The audio signal from the RX Unit is delivered to the
CNTL Unit and passes through the audio amplifier
Q4014-
3
(
NJM2902
) to the active high pass filter section of
Q4024
(
FX-805
) which rejects the sub-audible frequen-
cy component. The filtered audio signal is delivered to
potentiometer
VR4001
, which adjusts the audio sensitiv-
ity to compensate for audio level variations, then passes
through audio amplifier
Q4014-1
(
NJM2902
), audio
switch
Q4030
(
NJU4066B
), a 20 dB attenuater consist-
ing of R4180 and R4211, and limiter amplifier
Q4018-4
(
NJM2902
), to the electronic volume control
Q4029
(
M51132FP
), where the maximum deviation is set. The
audio signal subsequently passes through the 3-section
active low pass filter consisting of
Q4019-1/-2/-3
(
NJM2902
) and audio amplifier
Q4019-4
to J4008’s
pin10, providing the repeater transmit audio.
A portion of the audio signal from the active high pass
filter section of
Q4024
is de-emphasized by
Q4020-1
(
NJM2902
), providing a flat audio response. The filtered
audio then passes through the active band pass filter
Q4016
(
NJM2902
) and audio mute gate
Q4036
(
DTC323TK
)
to audio power amplifier
Q4043
(
TDA2003H
), provid-
ing up to 2 Watts of audio power to the 8
Ω
loudspeaker.
A portion of the audio signal from the audio amplifier
Q4014-3
passes through the 3-section active low pass fil-
ter
Q4045-2/-3/-4
and the low pass filtering section of
Q4024
to separate the CTCSS tones from the received
audio signal.
Sub-Audible Signaling (Decoder)
A portion of the audio signal from the audio amplifier
Q4014-3
passes through the 3-section active low pass fil-
ter
Q4045-2/-3/-4
and the low pass filtering section of
Q4024
to separate the CTCSS tones from the received
audio signal. The CTCSS tones are sent to the CTCSS
decoder section of Q4024. When a CTCSS tone is re-
ceived, the CTCSS information is delivered to pin 20 of
the Main CPU
Q4012
from pin 4 and 8 of
Q4024
, which
compares the CTCSS tone with the programmed tone.
Another portion of the audio signal amplified by
Q4014-3
passes through the 3-section active low pass fil-
ter
Q4046-2/-3/-4
to separate the DCS codes from the re-
ceived audio signal. The low pass filtered signal passes
through the phase detector
Q4046-1
to pin 23 of the Main
CPU Q4012. When a DCS code is received, the Main CPU
Q4012
compares the DCS code with the programmed
code.
If the received CTCSS tone or DCS code matches
the programmed tone or code, pin 39 of the Main CPU
Q4012
goes low, turning on the squelch switch
Q4036
(
DTC323TK
) and passing the received audio signal to
the audio power amplifier
Q4043
.
Squelch Control
The squelch circuit consists of noise amplifier
Q3015
(
2SC4116
) and noise detector
D3015
(
MA143
) on the
RX Unit, and control circuitry within main microproces-
sor
Q4012
on the CNTL Unit.
When no carrier is received, noise at the output of the
audio detector stage of
Q3012
is amplified by
Q3015
(
2SC4116GR
), and then rectified by
D3015
(
MA143
)