Circuit Description - Vertex Standard VXA-700 Service Manual

Receive Signal Path
Narrow FM and AM mode
Incoming RF from the antenna jack is passed through a
low-pass filter and high-pass filter consisting of coils
L1028, L1030, L1035, L1036, L1041, and L1045, capacitors
C1142, C1145, C1147, C1153, C1155, C1159, C1164, C1167,
C1180, C1182, C1185, and C1186 and antenna switching
diodes D1016 (RLS135), D1017 (HSU275), and D1019
(RLS135), and then proceeds to the receiver front end sec-
tion.
Signals within the frequency range of the transceiver are
applied to the receiver front end which contains RF am-
plifier Q1043 (2SC5555) and a varactor-tuned band-pass
filter consisting of coils L1022, L1023, L1026, L1027, L1037,
L1038, L1043, and L1044, capacitors C1130, C1134, C1135,
C1140, C1143, C1157, C1168, C1175, C1176, C1179, and
C1181, and diodes D1014, D1018, D1024, and D1026 (all
HVC350), and then applied to the 1st mixer Q1040
(2SC5555).
Buffered output from the VCO is amplified by Q1017
(2SC5555) to provide a pure 1st local signal between 143.4
and 199.4 MHz for injection to the 1st mixer. The 35.4 MHz
1st mixer product then passes through monolithic crystal
filter XF1001 (35M15A1, 7.5 kHz BW) which strips away
unwanted mixer product;, the 1st IF signal is then ampli-
fied by mixer post-amp Q1033 (2SC4215Y).
The amplified 1st IF signal is applied to the FM IF sub-
system IC Q2039 (TA31136FN), which contains the 2nd
mixer, 2nd local oscillator, limiter amplifier, noise ampli-
fier and FM detector.
The 2nd local signal is generated by PLL reference/2nd
local oscillator Q2030 (2SC4116GR) from the 17.475 MHz
crystal X2001. The 17.475MHz signal is doubled by Q2036
(2SC4400) to produce the 450 kHz 2nd IF when mixed
with the 1st IF signal within Q2039. The 2nd IF signal then
passes through the ceramic filter CF2002 (ALFYM450) and
CF2003 (CFWM450D) to remove all but the desired sig-
nal.
In the FM mode, the 2nd IF signal from ceramic filters
CF2002 and CF2003 is applied to the limiter amplifier sec-
tion of Q2039, which removes amplitude variations in the
450 kHz IF before detection of the speech by the ceramic
discriminator CD2001 (CDBM450C24). Detected audio
from Q2039 is passed through the de-emphasis network,
consisting of resistors R2051, R2053, R2054, and R2101,
capacitors C2048, C2049, C2053, and C2077, and Q2020-2
(NJM2904V), then applied to the AF amplifier Q2010
(TDA7233D).
In the AM mode, the 2nd IF signal from ceramic filters
CF2002 and CF2003 is amplifed by Q2039 and Q2043 (both
2SC4116GR), then amplifed signal is applied to the AM
detecter D2028 (RB751S). Detected audio from D2028 is

Circuit Description

passed through the audio amplifier Q2020-1 (NJM2904V)
and ANL circuit, then applied to the AF amplifier Q2020-
2 (NJM2904V). When impulse noise is received, a portion
of the AM detector output signal from D2028, including
pulse noise, is rectified by D2012 (BAS316). The result-
ing DC is applied to the ANL MUTE gate Q1019 (UMG2N),
which briefly interrupts the signal flow, thus reducing the
level of the pulse noise.
The processed audio signal from Q2020-2 passes through
the audio mute gate Q2014 (DTC143ZE) and the volume
control to the audio power amplifier Q2010 (TDA7233D),
providing up to 0.4 Watts to the headphone jack or 8 W
loudspeaker.
A portion of the AF signal from the FM IF subsystem
Q2039 converted into DC voltage within the IC, and then
passes through the AGC amplifier Q2026 (UMW1), Q2027
(2SA1602A), and Q2045 (TC75S51F) to the inversion am-
plifiers Q1035 and Q1042 (both 2SC5555). These amplifi-
er reduce the amplifier gain of the IF amplifier Q1033 and
the RF amplifier Q1043 when receiving a strong signal.
Wide FM mode
Incoming RF from the antenna jack is passed through a
low-pass filter consisting of coils L1030, L1035, L1036, and
L1042, capacitors C1142, C1145, C1147, C1153, C1155,
C1159, and C1167, and antenna switching diodes D1016
(RLS135), D1017 (HSU275), and D1019 (RLS135), and then
proceeds to the receiver front end section.
The RF signal is applied to the wide FM receiver front
end which contains RF amplifiers Q1041 (2SC5555) and
Q1046 (2SC5555) and a varactor-tuned band-pass filter
consisting of coils L1017, L1018, L1024, L1025, L1033,
L1034, L1039, and L1040, capacitors C1125, C1128, C1129,
C1131, C1136, C1148, C1158, C1162, C1163, and C1169,
and diodes D1013, D1015, D1022, and D1025 (all
HVC359), then applied to the 1st mixer Q1039 (2SC5555).
Buffered output from the VCO is amplified by Q1017
(2SC5555) to provide a pure 1st local signal between
133.68 and 153.65 MHz for injection to the 1st mixer. The
45.65 MHz 1st mixer product then passes through band-
pass filter consisting of coils L1010 and L1011, and capac-
itors C1075, C1078, C1081, C1083, C1087, C1092, and
C1093.
The 1st IF signal is applied to the wide FM IF subsystem
IC Q2023 (TA7792F), which contains the 2nd mixer, 2nd
local oscillator, limiter amplifier and FM detector.
The 2nd local signal is generated by PLL reference/2nd
local oscillator Q2030 (2SC4116GR) from the 17.475 MHz
crystal X2001. The 17.475MHz signal is doubled by Q2036
(2SC4400) to produce the 10.7 MHz 2nd IF when mixed
with the 1st IF signal within Q2023. The 2nd IF signal then
passes through ceramic filter CF2001 (SFECV10.7MS2)
to strip away unwanted mixer products.
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