Vertex Standard VX-520U, Service Manual

Page: 13 / 52

13
Circuit Description
Refer to the block diagram when reading this descrip-
tion. For finer details, refer to the schematic diagram.
Receiver
In coming signals at the antenna are passed through a
low pass filter and T/R switching diode on the ANT SW
Unit before delivery to the front-end circuitry on the moth-
er board. Here the signal is band-pass filtered again by a
2-stage resonator, amplified by Q3201 (
2SC3356-R24 ),
and then further filtered by a 2-stage band-pass resona-
tor befor application first mixer FET Q3202 (
SGM2016M )
along with the first local signal from Local Amplifier
Q2406 (
2SC4226-R24 ) on the VCO Unit.
The 47.9 MHz product from the first mixer is deliv-
ered through 4-pole monolithic crystal filter XF3201 (± 6.0
kHz BW) to strip away all but the desired signal, which is
amplified by Q3203 (
2SC2620QB ) on the Main Unit. FM
receiver sub-system IC Q2302 ( BA4116FV ) on the IF Unit
includes local oscillator, mixer, IF limiter amplifier and
FM detector circuits. The amplified first IF signal is ap-
plied to mixer section, along with the second local signal
generated via 48.355 MHz crystal X2301, which produces
the 455 kHz 2nd IF when mixed with the 1st IF signal
within Q2302. The 2nd IF passes through ceramic filter
CF2301 (4.5 kHz BW) or CF2302 (7.5 kHz BW) to strip
away unwanted mixer products, and is then applied to
the limiter amp in Q2302, which remove amplitude vari-
ations in the 455 kHz IF before detection of the speech by
Q2302 via quadrature resonator CD2301.
Detected audio is delivered to the CTCSS IC Q1001
( FX165CLH ) and then passes through the de-emphasis
circuitry consisting of R1033 & C1015, via muting gate
Q1011 (
2SK160-K6 ) and volume control to audio power
amplifier Q2206 ( TDA2822D ) and Q2211 ( 2SC4116GR )
on the regulator unit, providing up to 0.5 W to the exter-
nal speaker jack or 16-
Ω
loudspeaker.
Squelch Control
The squelch control circuit consists of noise amplifier
Q2304 ( 2SC4116GR ) and band-pass filter and squelch trig-
ger within Q2302 on the IF Unit, and control circuitry within
microprocessor Q1017 (
M38067MC ) on the control unit.
When no carrier is received, noise at the output of the
detector in Q2302 is amplified by Q2304, and band-pass
filtered by the noise amplifier section of Q2302 and then
rectified by D2302 to provide a DC control voltage for the
squelch switching transistor Q2306 (
2SA1586Y ). With no
carrier, the emitter of Q2306 is high. The signal is buff-
ered by Q1013. This SCAN STOP signal is delivered to
the microprocessor on the Control Unit, and micropro-
cessor controlled through Q1003 (
FMG5 ) to the BUSY in-
dicator on the top panel, which remains off until a carrier
is received. The microprocessor causes audio mute gate
Q2207 (
DTC144EU ) & Q2202 ( 2SB1132Q ) to open the au-
dio power amplifier power source, thus disabling the au-
dio amplifier and silencing the receiver when no signal is
being received, and during transmission.
When a carrier appears at the discriminator, noise is
removed from the output, causing the emitter of Q2306
to go low, then Q1017 controls the signal high, which in
turn causes Q1003 to turn on the
BUSY indicator. The
microprocessor then checks for CTCSS tone information
from Q1001, plus Digital Code Squelch information form
Q1006 (
TA75S393F ). If not transmitting and no tone
squelch is programmed for the channel, or if the received
tone matches that programmed for the channel, the mi-
croprocessor switches Q2207 to allow operation of the
audio power amplifier.
Transmitter
When the PTT switch is depressed, audio from the mi-
crophone is delivered to the Control Unit, where it is high-
pass filtered by Q1018 (
2SC4116GR ), and by one section
of microphone audio processing dual opamp IC Q1008
(
NJM2904V ). After pre-emphasis by C1074 and R1030,
another section of Q1008 serves as an IDC (Instantaneous
Deviation Control) amplifier to prevent over-deviation
from excessive microphone levels, and the two remain-
ing states provide low-pass filtering to suppress out-of-
band modulation, and buffering.
Processed audio from the IDC Unit is delivered to VCO
Unit where it is applied, along with carefully filtered DC
from Q2407 (
2SC4116 ), to varactor diode D2403 ( 1SV230 )
to modulate (via the TX Line) VCO FET Q2404 ( 2SK508-
K52 , on the VCO Unit), which oscillates at the transmit
frequency. VCO output is buffered and amplified by
Q2406 on the VCO Amplifier Unit before returning to the
Main Unit. Buffered, modulated VCO output is applied
via T/R switch D3202 to driver Q3205 (
2SC3356 ), Q3214
( 2SC3357 ), and the transmit signal is delivered to RF Pow-
er Module Q3206 ( M68732H ).
The transmit signal is passed through T/R switching
diode D2001 on the APC Unit, and then low-pass filtered
by L2003~2005, and C2007, C2011 to suppress spurious
harmonics before application to the antenna.
Transmitter output is controlled by Q2001 ( 2SA1586Y )
and Q2002 ( FMW1 ) on the APC Unit. When the TX 5V
line (from the regulator Unit) is active, bias voltage and
driver collector voltage is applied to the RF Power Mod-
ule via Q2003 and Q2004 , turning it on. A sample of the
final transistor collector current in the RF Power Module
is taken via R3224 on the mother board, detected by Q3207
(
IMZ2 ), passed through RF Power potentiometer VR3201
on the main unit back to APC switch Q2001 ( 2SA1586Y )
via one half of Q2002 ( FMW1 ) on the APC Unit. Q2002
passes the Automatic Power Control voltage when en-
abled by the other (transmit sequencer) half of Q2002. This
circuit is also used by the PLL to disable the transmitter
when the PLL is unlocked, and by the microprocessor to
select low power output.