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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.
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