Vertex Standard VX-146, Service Manual

Page: 5 / 24

5
Circuit Description
Receive Signal Path
Incoming RF from the antenna is delivered to the RF Unit
and passes through a low-pass filter consisting of coils
L1003, L1004 & L1005, capacitors C1002, C1016, C1017,
C1018, C1019, C1020, C1021 & C1023, and antenna switch-
ing diode D1005 (RLS135).
Signals within the frequency range of the transceiver en-
ter a band-pass filter consisting of coils L1013 & L1015,
capacitors C1071, C1072, C1073, C1074, C1075, C1076,
C1077, C1095 & C1096, then amplified by Q1012
(
2SC5006
) and enter a band-pass filter consisting of coils
L1018 & L1021, capacitor C1109, C1110, C1111, C1112,
C1114, C1128, C1129, C1130 & C1131 before first mixing
by Q1031 (
3SK320
).
Buffered output from the VCO is to provide a pure first
local signal between 401.75625 and 401.84357 MHz for
injection to the first mixer Q1031. The 44.25 MHz first
mixer product then passes through monolithic crystal fil-
ter XF1002 (HDF0042, 5.5 kHz BW) to strip away all but
the desired signal, which is then amplified by Q1034
(
2SC4215Y
).
The amplified first IF signal is applied to FM IF subsystem
IC Q1045 (
TA31136FN
), which contains the second mix-
er, second local oscillator, limiter amplifier, noise ampli-
fier, and S-meter amplifier.
A second local signal is generated by PLL reference/sec-
ond local oscillator of 14.60 MHz crystal X1001 to pro-
duce the 450 kHz second IF when mixed with the first IF
signal within Q1045 .
The second IF then passes through the ceramic filter
CF1001 (ALFYM450G=K) to strip away unwanted mixer
products, and is then applied to the limiter amplifier in
Q1045 , before detection of the speech by the ceramic dis-
criminator CD1001 (
CDA450C24
).
Detected audio from Q1045 is applied to the audio high-
pass filter, and then passed via the volume control to the
audio amplifier Q1041 (
NJM2070M
), which provides up
to 0.5 Watts to the optional headphone jack or a 4-Ohm
loudspeaker.
Squelch Control
The squelch circuitry consists of a noise amplifier and
band-pass filter within Q1045 , and noise detector D1019
(
1SS355
).
When no carrier received, noise at the output of the de-
tector stage in Q1045 is amplified and band-pass filtered
by the noise amplifier section of Q1045 and the network
between pins 7 and 8, and then rectified by D1019 .
The resulting DC squelch control voltage is passed to pin
37 of the microprocessor Q1011 (
M37515M6-121HP
). If
no carrier is received, this signal causes pin 24 of Q1011
to go low and pin 20 to go high. Pin 24 signals Q1043
(
2SC4154E
) to disable the supply voltage to the audio
amplifier Q1041 , while pin 20 hold the green (Busy) half
of the LED off, when pin 24 is high and pin 20 is high.
Thus, the microprocessor blocks output from the audio
amplifier, and silences the receiver, while no signal is be-
ing received (and during transmission, as well).
When a carrier appears at the discriminator, noise is re-
moved from the output, causing pin 37 of Q1011 to go
low and the microprocessor to activate the “Busy” LED
via Q1016 .
The microprocessor then checks for CTCSS or CDCSS code
squelch information, if enabled, or for DTMF data on the
optional DTMF Unit. If not transmitting and CTCSS or
CDCSS is not activated, or if the received tone or code
matches that programmed, allows audio to pass through
the audio amplifier Q1041 (
NJM2070M
) to the loudspeak-
er by enabling the supply voltage to it via Q1037 .
Transmit Signal Path
Speech input from the microphone is amplified by Q1017
(
NJM2902V
), after pre-emphasis by C1066 and R1070, the
audio passes another section of Q1017 .
The processed audio may then be mixed with a CTCSS
tone generated by Q1011 (
M37515M6-121HP
) then de-
livered to D1002 (
HVU350
) for frequency modulation of
the PLL carrier (up to ±5kHz from the unmodulated car-
rier) at the transmitting frequency.
If a CTCSS code is enabled for transmission, the code is
generated by microprocessor Q1011 and delivered to
D1004 (
HVC350B
) for CTCSS modulating.
The modulated signal from the VCO Q1003 (
2SK508-K52
)
is buffered by Q1007 (
2SC5005
). The low-level transmit
signal is then passes through the T/R switching diode
D1009 (
DAN235E
) to the predriver amplifier Q1013
(
2SC5227
) and driver amplifier Q1015 (
2SK2596
) and
Q1022 (
2SK2974
), then amplified transmit signal is ap-
plied to the final amplifier Q1022 up to 500 mill watts
output power.
The transmit signal then passes through the antenna switch
D1005 (
RLS135
) and is low-pass filtered to suppress har-
monic spurious radiation before delivery to the antenna.