Vertex Standard VXA-300 Pilot III, Руководство по обслуживанию

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7
Circuit Description
Receive Signal Path
Incoming RF from the antenna jack is passed through a
low-pass filter and high-pass filter consisting of coils
L1022, L1025, L1026, L1027, L1028, & L1029, capacitors
C1271, C1276, C1277, C1278, C1280, C1281, C1282, C1283,
C1285, C1286, C1289, C1290, C1293, C1294, & C1295 and
antenna switching diodes
D1039 and D1042 (both
RLS135 ) to the receiver front end section.
Signals within the frequency range of the transceiver is
applied to the receiver front end which contains RF am-
plifier
Q1076 ( 2SC5555ZD ) and varactor-tuned band-pass
filter consisting of coils L1014, L1017, L1018, L1019, L1023,
L1024, & L1031, capacitors C1241, C1243, C1245, C1246,
C1248, C1250, C1255, C1256, C1259, C1266, C1272, &
C1273, and diodes
D1034 , D1035 , D1038 , & D1041 (all
HVC350B ), then applied to the 1st mixer Q1068
( 2SC5555ZD ).
Buffered output from the VCO is amplified by Q1049
( 2SC5555ZD ) to provide a pure 1st local signal between
143.4 and 172.4 MHz for injection to the 1st mixer. The
35.4 MHz 1st mixer product then passes through mono-
lithic crystal filter XF1001 (7.5 kHz BW) which strips away
all but the desired signal, which is then amplified by mix-
er post-amp
Q1061 ( 2SC4915Y ).
The amplified 1st IF signal is applied to the AM/FM IF
subsystem IC
Q1058 ( TK10931VT1 ), which contains the
2nd mixer, 2nd local oscillator, limiter amplifier, noise
amplifier and AM/FM detector.
A 2nd local signal is generated by PLL IC Q1041
( MB15A01PFV1 ) from the 17.475 MHz crystal X1002.
The17.475MHz signal is doubled by Q1054 ( 2SC4617 ) to
produce the 450 kHz 2nd IF when mixed with the 1st IF
signal within
Q1058 ( TK10931VT1 ). The 2nd IF then pass-
es through the ceramic filter CF1001 to strip away unwant-
ed mixer products.
In the FM mode, a 2nd IF signal from the ceramic filter
CF1001 applied to the limiter amplifier section of
Q1058
( TK10931VT1 ), which removes amplitude variations in
the 450 kHz IF before detection of the speech by the ce-
ramic discriminator CD1001. Detected audio from
Q1058
( TK10931VT1 ) is passed through the de-emphasis, con-
sisting of the resistors R1144, R1152, R1155, & R1199, ca-
pacitors C1118, C1119, C1124, C1149, & C1151, and
Q1037-
2 ( NJM12904R ).
In the AM mode, detected audio from Q1058
( TK10931VT1 ) is passed through the audio amplifier
Q1037-1 ( NJM12904R ) and ANL circuit, then applied to
the AF amplifier Q1037-2 ( NJM12904R ). When impulse
noise received, a portion of the AM detector output sig-
nal from the AM/FM IF subsystem
Q1058 ( TK10931VT1 ),
including pulse noise is rectified by D1019 ( 1SS400 ). The
resulting DC is applied to the ANL MUTE gate Q1039
( UMG2N ), thus reducing the pulse noises.
The processed audio signal from Q1037-1 ( NJM12904R )
is passed through the AF mute and amplifier Q1037-2
( NJM12904R ) to the audio tone equalizer Q1021
( NJM12904R ), Q1018 ( HN1J02FU ), Q1023 ( HN1J02FU ),
Q1026 ( HN1J02FU ), and Q1020 ( NJM12904R ). The equal-
ized audio signal is passed through the volume control to
the audio power amplifier
Q1001 ( TDA2822D013TR ), pro-
viding up to 0.8 Watts to 16-ohm loudspeaker or audio
power amplifier
Q1005 ( TDA7233D ), providing up to 0.4
Watts to 8-ohm the headphone jack.
A portion of the AF signal from the AM/FM IF subsystem
Q1058 ( TK10931VT1 ) converted into DC voltage within
the IC, and then passes through the AGC amplifier Q1063
( 2SA1774 ) and Q1066 ( UMW1 ) to the inversion amplifi-
ers Q1075 ( 2SC5555ZD ) and Q1062 ( 2SC5555ZD ). These
amplifier reduce the amplifier gain of the IF amplifier
Q1061 ( 2SC4915Y ) and the RF amplifier Q1076
( 2SC5555ZD ) while receiving a strong signal.
Squelch Control
When signal is received, appear the DC squelch control
voltage at pin 15 of AM/FM IF subsystem
Q1058
( TK10931VT1 ) according to the receiving signal strength.
This DC is applied to pin 16 of microprocessor Q1024
( LC87F74C8A ).
The DC squelch control voltage is compared with the SQL
t h r e s h o l d l e v e l b y t h e m i c r o p r o c e s s o r
Q 1 0 2 4
( LC87F74C8A ). If the DC squelch control voltage is low-
er, pin 7 of Q1024 ( LC87F74C8A ) goes high. This signal
activates the AF MUTE gate Q1037-2 ( NJM12904R ), thus
disabling the AF audio.
Also, the microprocessor stops scanning, if active, and
allows audio to pass through the AF MUTE gate
Q1037-2
( NJM12904R ).