Heathkit SB-1000 - Page 66

Heathkit
One
of
the
red
power
transformer
leads
is
connected
to
the
junction
of
capacitors
C404
and
C405.
During
the
half
cycle
when
this
lead
is
Positive,
capacitors
C401
through
C404
are
charged.
During
the
other
half
cycle,
the
other
red
power
transformer
lead
is
Positive
and
capacitors
C405
through
C408
are
charged.
These
two
capacitor
strings
are
in
series
across
the
load,
and
the
voltages
of
each
group
add
together.
Resistors
R401
through
R404
are
discussed
under
“Metering
Circuits."
Choke
RFC3
and
capacitor
C23
keep
RF
energy
out
of
the
power
supply
circuits.
FILAMENT
AND
BIAS
In
addition
to
the
larger
secondary
winding
just
described,
power
transformer
Tl
has
two
smaller
secondary
windings.
One
of
these
windings
supplies
5
VAC
at
15
amperes
for
the
amplifier
tube
filaments.
The
filament
voltage
is
fed
to
tube
V1
through
RFC2,
which
is
a
bifilar-wound
coil
on
a
ferrite
core.
This
coil
forms
a
choke
to
raise
the
tube
filaments
above
RF
ground
so
that
the
driving
voltage
is
not
short
circuited.
Capacitors
C21
and
C22
keep
RF
out
of
the
filament
supply
circuits.
In
the
transmit
mode,
the
center
tap
of
the
filament
winding
is
connected
to
ground
through
7.5-volt
zener
diode
D2,
lugs
8
and
4
of
relay
K1,
and
resistor
R203.
The
plate
current
through
the
zener
diode
developes
the
7.5-volts
DC
operating
bias
for
tube
V1
and
limits
the
idling
plate
current.
In
the
receive
mode,
a
positive
DC
voltage
is
formed
across
resistor
Ri.
This
voltage
is
applied
to
the
tube
filaments,
in
addition
to
the
AC
voltage,
to
bias
the
tube
grid
beyond
cutoff
so
that
no
plate
current
flows.
The
remaining
power
transformer
secondary
winding
is
fed
to
a_full-wave
rectifier
circuit
on
the
meter
circuit
board
which
supplies
the
voltage
needed
to
operate
relay
K1,
pilot
lamps
PL1
through
PL4.
This
voltage
also
provides
12
VDC
(at
100
milliamperes)
at
rear
panel
jack
J5
for
accessories.
Relay
Relay
K1
has
three
sets
of
single-pole,
double-throw
contacts.
In
the
receive
mode,
the
relay
coil
is
not
energized.
Switch
SW1
allows
you
to
keep
the
relay
in
a
standby
state.
Approximately
12
volts
DC
is
xX
Page 69
ervX—
BC
OD
connected
to
one
side
of
the
relay
coil
at
lug
11. Lu;
10
connects
the
other
side
of
the
relay
coil
to
RLY
jack
J3__on
the
rear
panel.
This
jack
is
usually
connected
to
normally-open
Telay
contacts
in
the
exciter
(such
as
a
VOX
or
PTT
relay).
When
these
contacts
close,
they
must
connect.
the
Telay
coil
circuit
to
ground
(0
VDC).
The
relay
will
then
close
to
place
the
contacts
in
the
transmit
mode.
NOTE:
A
transistor
switch
may
not
allow
relay
K1
to
operate
Properly,
because
this
type
of
switch
does
not
go
to
Zero
volts.
The
function
of
relay contacts
4
and
8
were
discussed
earlier
in
the
"Filament
and
Bias
Supply”
section.
Relay
contact
9
is
connected
to
RF
IN
jack
J1.
In
the
teceive
mode,
the
incoming
signal
is
transferred
directly
to
the
RF
output
through
relay
contacts
7,
1,
5,
and
9.
In
the
transmit
mode,
the
RF
input
voltage
is
connected
through
relay
contacts
9
and
6
to
the
wiper
of
the
Band
switch
SW2A.
The
amplified
RF
output
is
connected
through
relay
contacts
2
and
7
to
RF
Out
jack
J2.
RF Circuits
INPUT
An
input
impedance-matching
pi
network
circuit
for
each
band
is
switched
into
the
circuit
by
Band
switch
SW2A.
After
the
signal
Passes
through
the
matching
circuit,
the
RF
driving
power
is
coupled
through
switch
SW2B
and
capacitors
C18
and C19
to
the
tube
filaments.
TUBE
The
amplifier
tube
is
connected
in
a
class
AB
2
grounded
grid
circuit.
RF
driving
power
is
applied
to
the
filaments
in
‘the
normal
cathode-driven
configur-
ation.
As
mentioned
in
the
"Power
Supply”
section,
choke
RFC2
holds
the
filaments
above
RF
ground.
Pins
2,
3,
and
4
of
the
tube
are
internally
connected
to-
gether
and
are
connected
directly
to
ground.
Parasitic
choke
PC1
is
connected
into
the
plate
lead
to
suppress
any
VHF
parasitic
oscillations.
The
positive
side
of
the
power
supply
is
connected
to
the
tube
through
RFC3.
Fan
BI
circulates
cooling
air
around
the
tube.