Heathkit HW-12 - Page 59

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RF FINAL AMPLIFIERS
RF
final
amplifiers
V6
and
V7
are
connected
in
parallel,
High
voltage
plate
connections
are
under
the
chassis,
The
grids
are
connected
by
a
long
foil
strip
on
the
circuit
board,
This
str
rip
is
b
py-
passed
at
its
ends
by
capacitor
C61
and
C71
to
Suppress VHF ign ra oa Cathode resistor
R71
is
a
meter
shunt
for
measuring
cathode
current when the meter switch is in the BIAS
SET position,
Tubes V6 and V7 are operated as linear ampli-
fiers, with high power sensitivity, Grid load
resistor R72 is connected to the Final Bias
control through R73, to allow adjustment of the
grid voltage for proper operation, The Final
Bias control is grounded through resistor R75
when transmitting, When receiving, this control
is
grounded
through
resistors
R76
and
R77
to
increase
the
grid
bias
to
cut
off
the
final
tubes.
AUTOMATIC LEVEL CONTROL
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No grid current is drawn by tubes V6 and V7
in normal linear operation; however, when
,
higher
than
normal
grid
drive
is
applied,
grid
current will Ho ow and change the bias voltage,
This higher-than- aera grid drive, caused
by too much audio signal, makes the bias voltage
change at an Adis rate,
hi
This varying bias
voltage
is
coupled
thro
ough
capacitor
C75
to
diode
D70,
which
rectifies
s
signal
to
develop
the
DC
negative
ALC
voltage,
which
is
applied
to
V2A,
v4,
and
V5,
neuer
som
R78
and
R79,
along
with
capacitor.
C74,
filter
this
voltage
and
provide
the
proper
time
delay
for
ALC
action,
The
entire
ALC
circuit
is
biased
above
ground
by
resistors
R76
and
R77
to
cut
off
the
transmitter section when receiving,
TRANSMITTER OUTPUT CIRCUIT
The plates of the RF amplifier tubes are con-
nected to the supply voltage by choke RFC61,
Their signal is coupled to the pi-section output
circuit through capacitor C67, Output coil L4
is tuned by Final Tune capacitor C65, Antenna
loading is fixed by capacitor C77, With the Func-
tion switch in the Tune position, a sampling of
the output voltage from resistors R61 and R62,
and diode CR60, gives a meter indication to
indicate proper adjustment of the Final Tune
capacitor,
Page 59
RELAY TRANSMIT-RECEIVE SWITCHING
Switching between transmitting
and receiving
is
done
by
the
relay,
Section
A
of
the
relay
switches the antenna, and section C switches the
bias voltages,
Section
B
of
the
relay
is
connected
to
the
Ext
Relay
socket
on
the
rear
of
the
Transceiver,
and
can
be
used
to
control
external
equipment,
such
as
a
linear
amplifier
or
antenna
relay.
Because
one
side
of
the
Ext
Relay
socket
is
grounded
to
the
Transceiver
chassis,
the
socket
must
not
be
used
to
switch
any
voltage
ina
circuit
that
operates
above
ground,
as
the
chassis
would
become
"hot,"
causing
a
possible
shock
hazard,
VOX AMPLIFIER Vi0 AND RELAY AMPLI-
FIER V2B
The
Transceiver
can
be
switched
from
receive
to
transmit
by
either
the
push-to-talk
or
the
VOX
method,
VOX
Amplifier
V10
is
normally
operated
in
a
saturated
condition,
that
is,
with
very
low
plate
voltage
and
maximum
plate
current,
Positive
half
cycles
of
the
voice
signals
from
V1A
have
no
effect
on
V10,
however,
the
negative
half
cycles
cause
the
plate
current
to
drop,
thus
increasing
plate
voltage,
This
in-
creased
plate
voltage
fires
neon
lamp
NE2,
pro-
viding
a
positive
switching
action,
The
voltage
from
the
neon
lamp
is
then
amplified
by
relay
amplifier
V2B,
which
operates
the
relay,
Ca-
pacitor
C105
and
resistor
R107
form
a
delay
network
that
establishes
the
time
the
relay
stays
closed
after
being
tripped,
The
length
aAuminna Aas thn antti VN
of time 3 is deter mined wy wic Ssétring of the VuUaA
Delay control,
ANTI-TRIP CIRCUIT
Because the VOX stages operate on both trans-
mit and receive, the speaker signals during
receive must be kept from tripping the relay
when receiving, This is done by taking a portion
of the audio signal from AF output stage V12A,
rectifying it with diode D100 to produce a posi-
tive voltage, and feeding this voltage to the grid
of V10 from the VOX control, This voltage tends
to increase the plate current of V10; signals
from the microphone (picked up from the speaker)
tend to decrease the plate current, Therefore