HP406 Service Manual
Page 6 of 12
2.4.c Power control
Output power is controlled via the dual Op-Amp (
Q408
) , which is used as a differential amplifier and comparator.
Current is sensed by the voltage drop across
R421A
and
R422
. This voltage is compared to the one set by the 2-
watt Adjustment
RV401
. The power output is then reduced or increased by varying the
Q410
's output voltage
applied to the power amplifier
Q407
's pin 2.
2.4.d Transmitter Audio Circuits
The transmitter audio circuits consist of the audio processing circuits, the CTCSS circuits and the DCS circuits.
2.4.e Audio Processing
Transmit speed audio is providing by either the internal electric microphone
N101
or the external microphone.
The microphone audio is applied to MIC MUTE SW
Q235
, and Lo-pass filter
Q214A
,
Q214B
. The audio is pre -
emphasized by 6dB per octave by
C236
and
R284
, and then signal amplification. The gain is such that when a
signal greater than 20 dB.
Limiting the peak-to-peak output. Under these conditions, the MOD. ADJ. Pot
RV201
configured as a four-pole
active low-pass filter. The resulting signal is then limited when respect to side band splatter, and has an 18 dB
per octave roll-off above 3 KHz.
The audio is then applied through the 25 KHz/12.5 KHz channel spacing SW
Q215
to transmit VCO. By varying
the voltage on the varactor diode
D921
at an audio rate. The resonant frequency of VCO is varied. The result is
an oscillator output that is frequency -modulated at the audio frequency.
2.5 Receiver
2.5.a Receiver Front End
In the receive mode, the RF signal enters thorough the antenna, then through the low-pass filter,
C404
-
C409
and
L401
-
L402
. The diodes
Q401
and
Q402
are biased off so that the output of the low-pass filter is coupled (
C411
)
to the first band-pass filter
C811
-
C818
,
C822
,
L404
-
L407
and to the Front End RF overload protection diode pair
D9
. The signal from the band-pass filter is applied to the input of the RF amplifier
Q804
.
The output of the RF amplifier feeds the input to three more stages of band-pass filters consisting of
C826
-
C839
,
and
L804
-
L808, L411
. The output from the band-pass filter is applied to the mixer’s
Q405
/
L411
.
2.5.b Local Oscillator (LO)
The Receive VCO (
C989
-
C991
,
CV901
,
L926
-
L928
,
Q922
,
Q923, Q919
and
R953
) provides the LO signal. The
VCO is running at 45.1 MHz above the desired receive frequency and is applied to output Buffer
Q915
/
Q414
. The
output of the buffer through the low-pass filter
C433
-
C435
,
L432
-
L433
and applied to the mixer
Q405
/
L411/L412
.
2.5.c Mixer
The mixer is a DBM type (
L411,Q405,L412
). The mixer LO frequency is 45.1 MHz below the desired receiver
frequency.
When the receiver frequency is present, the mixer output will be a 45.1MHz signal. The mixer output is peaked
for 45.1MHz at
L434
,
C437
and
R413
, and the signal is filtered by crystal filter
F401A
and
F401B
and amplified
by
Q406
before being applied to the input of the IF IC
IC6
.
Inside
IC6
, the 45.1MHz IF signal becomes the input to a second mixer with a LO frequency of 44.645 MHz set
by
X201
. The 455 KHz ceramic filter
F201
or
F202
filters the second mixer’s output which is the second IF signal.
The mixer’s output is then fed to the internal limiting amplifier and then on to the FM decoder.
2.5.d FM Detector and Squelch
The FM detector output is used for squelch, decoding tones and audio output. The setting of the squelch
adjustment potentiometer
RV204
(for 25 KHz channel spacing) and
RV205
(for 12.5 KHz channel spacing) sets
the input to the squelch amplifier.
The squelch amplifier is internal to
IC6
and its output is fed to an internal rectifier and squelch detector.
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