Audio Circuits; Keying Circuit; Receiver Muting; Side Tone - Heathkit HW-9 User Manual

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Heathkit
AUDIO
CIRCUITS
After
being
filtered at
the output
of
product
detector
U303,
the audio
signal
is
coupled
to the inverting
input
(pin
6)
of operational amplifier
U304B.
This
stage
makes up
a low-pass
filter
with an approximate
cutoff
frequency
of 1
kHz
at
the 6
dB
point.
The
amplified
audio
signal
at
pin
7 of
U304B
is
coupled
through
C347
to Selectivity
switch
SW3,
and
then
through
R352 and C339
to the
input
(pin 2 of
U304A)
of the
audio-active
filter.
In addition
to
U304A,
U304C
makes up
the audio-
active
filter.
These
operational amplifier sections
are
cascaded and allow
a certain
amount
of
audio
fre-
quencies
to
pass through.
The bandwidth
for this
filter is
300
Hz
(between
the
6dB
points)
at
a center
frequency
of
700 Hz. This
filter
provides sharp
selec-
tivity
and
permits
excellent rejection
of
unwanted,
adjacent audio
signals in the receive
mode. The
out-
put of the
audio-active
filter is
coupled through
C346
to Selectivity
switch
SW3. The
setting of this
switch determines
which
signal
is
applied
to
AF
Gain
control R3.
The
audio
signal
from
the
wiper arm
of
AF
Gain
control
R3
is
coupled through R369
to
the input (pin
8)
of
audio power
amplifier U306.
A
signal
is
coupled from
the output
(pin
8)
of sidetone
oscil-
lator
U304D
to this input. In the transmit
mode,
Q303 mutes
this
input by connecting
it
to
ground.
The
output
signal
from U306
is
coupled through
C359
to
speaker
J3
or
phone
jack
J2.
KEYING
CIRCUIT
Q407 through Q409
provide timing and
switching
of the receive
and
transmit functions
in the Trans-
ceiver.
The
keying
circuit
is
made up
of
Q408
(
+ T
transmit
request)
and
Q409
(
+ R
receive request).
The
outputs
of these transistors are activated
when
the
CW
key
is
depressed.
Q411 and Q412
in the receiver
mute
delay
circuit are activated in the
same
manner.
When
the
key
is
depressed, the base
of
Q408 goes
low.
This causes
it
to
conduct, thus
establishing a
+
12
volt
output
for
the
"
+
T"
request
lines.
At
the
same
time,
Q407
conducts
and
turns
off
Q409. C574
through C5 77 provide
the
proper timing
of these
lines as
they
are
turned
on and
off.
RECEIVER
MUTING
Q411 and Q412 provide
a variable
delay
for
the
re-
ceiver
muting
circuit.
The
emitter of
Q411
is
con-
nected
to
ground
when
the
key
is
being depressed.
This
effectively
puts the
collector of
Q411
at
ground
potential, causing
mute
drive transistor
Q412
to
con-
duct
and
provide
+12
volts.
The muting
of the
re-
ceive line
ceases
when
the base voltage
of
Q412
reaches the
+B
voltage.
R445 and C578 provide an
adjustable time constant
for
the
muting
circuit.
SIDE
TONE
When
the
key
is
depressed, pin 9
of
sidetone
oscil-
lator
U304D
goes high
and
the
oscillator
is
activated.
The
700
Hz
signal
at
the
output
(pin
8)
of
U304D
is
coupled through
C351 and
applied
across
Sidetone Level control R368.
From
the
wiper
of
R368,
the sidetone
is
coupled
to
the
input
(pin
8)
of
audio
amplifier
U306.
RELATIVE
POWER
In the
transmit
mode, C568
couples
a
portion
of the
RF
voltage across voltage divider
R428 and
R429.
D408
rectifies
the positive half cycle of the voltage
across
R429.
C569
filters this
positive voltage,
which
is
coupled
to
meter
Ml
through
current-limiting
re-
sistors
R331 and
R431.
S-METER
In the receive
mode,
D306
conducts
when
the
AGC
voltage across
C317 exceeds
a
predetermined
level.
A
portion of this voltage
is
coupled through
R314
to
the gate of
Q302.
The
resulting,
ramped
voltage
at
the source
of
Q302
is
coupled
to
meter
Ml
through
current-limiting
resistor
R331.
A
regulated
+9
volt
supply powers
the
meter
circuit.
Meter
zero control
R333
allows
you
to set
the
meter
to zero.