Heathkit SB-201 - Page 51

Geguearuxire
When transmitting, the Antenna Relay jack is
grounded by the VOX relay of the exciter. This allows
current to flow through the coil of relay RL1 and
actuate the relay. When actuated, relay RL1 connects
the RF Output jack (antenna) to the output circuit of
the Linear Amplifier. Grounding the Antenna Relay
jack also grounds the grids of tubes V1 and V2
through RFC2, and resistors R15 and R16. This re-
moves all but —2 volts from the grids of the tubes.
This —2 volts is operating bias, which limits resting
plate current to approximately 90 milliamperes.
When operating with the Linear Amplifier turned
OFF, there is no bias voltage to operate relay RL1 and
the
RF
Input
jack
remains
connected
to
the
antenna,
through RF Output jack. This permits low power op-
eration directly from the exciter into the antenna
without
changing
any
cables.
Because
silicon
rec-
tifiers and instant-heating-filament tubes are used in
the Linear Amplifier, you can go to high power opera-
tion as soon as the Linear Amplifier is turned ON.
METERING CIRCUITS
RIS
Figure 1-1
GRID— Figure 1-1
In
the
Grid
position
of
the
meter
switch,
the
meter
is
connected
to
measure
the
voltage
across
grid
resistor
R15.
The
voltage
across
this
resistor
is
directly
prop-
ortional
to
the
grid
current
which
flows
through
it:
therefore,
the
meter
will
indicate
grid
current.
Since
grid
current
in
tubes
V1
and
V2
should
not
exceed
100
mA,
only
half
(white
area)
of
the
0-200
mA
meter
scale
is
used
for
this
measurement.
PLATE
—
Figure
1-2
Figure
1-2
Plate
current
is
read
on
the
top
scale
of
the
meter.
This
scale
is
divided
into
20
mA
divisions
from
0
to
1000
mA.
Plate
current
readings
are
obtained
by
measuring
the
voltage
across
resistor
R12,
through
series
resistor
R14.
Page 51
REL PWR
Figure 1-3
REL PWR And SWR — Figure 1-3
Relative
Power
and
Standing
Wave
Ratio
are
mea-
sured
with
a
bridge
circuit,
consisting
of
coils
L7,
L8,
and
L9,
diodes
CR1
and
CR2,
resistors
R19
and
R20,
and
capacitors
C20
and
C23.
Relative
Power
Sensitiv-
ity
contro]
R13
sets
the
sensitivity
of
the
meter.
The
REL
POWER
position
is
used
when
adjusting
the
TUNE
and
LOAD
controls
for
maximum
output
of
the
Linear
Amplifier;
this
meter
function
is
also
used
to
establish
a
“‘set’’
meter
level
for
making
SWR
mea-
surements.
Because
coil
L7
of
the
bridge
circuit
is
connected
in
series
between
the
RF
Output
jacks
when
the
Linear
Amplifier
is
turned
OFF,
the
meter
can
be
used
to
measure
REL
PWR
and
SWR
when
operating
the
exci-
ter
alone.
This
method
of
checking
SWR
is
recom-
mended
for
accurately
determining
antenna
perfor-
mance.
In
the
bridge
circuit,
RF
current
is
inductively
and
capacitively
coupled
from
L7,
to
L8
and
L9.
The
RF
currents
in
L8
and
L9
are
rectified
by
diodes
CR1
and
CR2,
and
then
filtered
by
capacitors
C20
and
C23.
For
REL
PWR
measurements,
the
rectified
RF
voltage
from
L8
is
applied
to the
meter.
For
SWR
(reflected
power)
measurements,
the
rectified
RF
voltage
from
L9
is
applied
to
the
meter.