Vertex Standard VXA-150, Service Manual

11

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, L1024, L1026, L1027, L1030, & L1031, capacitors
C1237, C1239, C1242, C1245, C1247, C1248, C1249, C1250,
C1251, C1252, C1255, & C1257 and antenna switching di-
odes 

D1037

 and 

D1039

 (both 

RLS135

) to the receiver front

end section.

Signals within the frequency range of the transceiver

are applied to the receiver front end which contains RF
amplifier 

Q1058

 (

2SC5226

) and varactor-tuned band-pass

filter consisting of coils L1013, L1017, L1020, L1021, L1028,
& L1029, capacitors C1198, C1204, C1205, C1212, C1216,
C1217, C1222, C1228, C1232, C1243, & C1253, and diodes

D1032

, 

D1034

, 

D1036

 & 

D1042

 (all 

HVC350

), then applied

to the 1st mixer 

Q1056

 (

2SC5226

).

Buffered output from the VCO is amplified by 

Q1029

(

2SC5226

) 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 monolithic crystal
filter 

XF1001

 (

35S15A

, 7.5 kHz BW) which strips away all

but the desired signal, which is then amplified by mixer
postamp 

Q1050

 (

2SC4215Y

).

The amplified 1st IF signal is applied to the AM/FM IF

subsystem IC 

Q1049

 (

TK10931V

), which contains the 2nd

mixer, limiter amplifier, and AM/FM detector.

A 2nd local signal is generated by PLL reference/2nd

local oscillator 

Q1045

 (

2SC4116GR

) from the 17.475 MHz

crystal 

X1002

. The 17.47 MHz signal is doubled by 

Q1048

(

2SC4116GR

) to produce the 450 kHz 2nd IF when mixed

with the 1st IF signal within 

Q1049

. The 2nd IF then pass-

es through the ceramic filter 

CF1001

 (

ALFYM450F=K

) to

strip away unwanted mixer products.

In the FM mode, a 2nd IF signal from the ceramic filter

CF1001

 applied to the limiter amplifier section of 

Q1049

,

which removes amplitude variations in the 450 kHz IF
before detection of the speech by the ceramic discrimina-
tor 

CD1001

 (

CDBM450C24T

). Detected audio from 

Q1049

is passed through the de-emphasis, consisting of the re-
sistors R1089, R1095, R1100, & R1149, capacitors C1104,
C1105, C1107, & C1151, and 

Q1028-2

 (

NJM2904V

).

In the AM mode, detected audio from 

Q1049

 is passed

through the audio amplifier 

Q1028-1

 (

NJM2904V

) and

ANL circuit, then applied to the AF amplifier 

Q1028-2

(

NJM2904V

). When ANL is on, the ANL MUTE gate

Q1027

 (

UMG2

) goes high, the low-pass filer/limiter con-

sisting of capacitor C1124, resistors R1105, R1111, R1112,
R1115, & R1119 and diode 

D1023 

(

BAS316

) is activate,

thus reducing the pulse noises when impulse noise re-
ceived.

The processed audio signal from 

Q1028-2

 passes

through the audio mute gate 

Q1008

 (

DTC143ZUA

) and

the volume control to the audio power amplifier 

Q1009

(

TDA7233D

), providing up to 0.4 Watts to the headphone

jack or 8 

W

 loudspeaker.

A portion of the AF signal from the AM/FM IF sub-

system 

Q1049

 converted into DC voltage within the IC,

and then passes through the AGC amplifier 

Q1054

(

2SA1602A

) and 

Q1055

 (

UMW1

) to the inversion ampli-

fiers 

Q1051

 and 

Q1057

 (both 

2SC5226

). These amplifiers

reduce the gain of the IF amplifier 

Q1050

 and the RF am-

plifier 

Q1058

 while receiving a strong signal.

Squelch Control

When a signal is received, a DC squelch control volt-

age appears at pin 15 of AM/FM IF subsystem 

Q1049

 ac-

cording to the receiving signal strength. This DC is ap-
plied to pin 2 of microprocessor 

Q1014

.

The DC squelch control voltage is compared with the

SQL threshold level by the microprocessor 

Q1014

. If the

DC squelch control voltage is higher, pin 49 of 

Q1014

 goes

low. This signal disable the AF MUTE gate 

Q1008

(

DTC143ZUA

), thus activating the AF audio.

Also, the microprocessor stops scanning, if active, and

allows audio to pass through the AF MUTE gate 

Q1008

.

Transmit Signal Path

Speech input from the microphone is passed through

the microphone sensitivity potentiometer VR1001 and
microphone amplifier 

Q1005-3

 (

NJM2902V

), then applied

to the ALC amplifier 

Q1007

 (

AN6123MS

). The amplified

speech signal is passed through the high-pass filter 

Q1005-

1

 (

NJM2902V

) and low-pass filter 

Q1005-2

 (

NJM2902V

)

which AM modulate the Tx frequency with speech sig-
nal.

When using the optional headset, the SIDETONE sig-

nal from 

Q1011

 (

UMG2

) becomes “HIGH”, turning 

Q1012

(

2SC4116GR

) on, therefore a portion of the speech signal

applied to the AF power amplifier 

Q1009

 as a monitor

signal.

The carrier signal from the VCO 

Q1023

 (

2SC5226

)

passes through the buffer amplifier 

Q1029

 (

2SC5226

) and