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RCA Manuals
Transmitter
BTA-250M
Instructions manual
RCA BTA-250M Instructions Manual page 9
Broadcast transmitter
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DESCRIPTION
The
Type
BTA-250M
Broadcast
Transmitter
is
a
complete,
self-contained
unit
that
provides
a
power
output
of
250
watts
at any
frequency
in the
535
to
1620
kilocycle
range.
Mounted
in a single
RCA
Type
BR-84
steel
cabinet,
as shown
in Figure
1, the
unit
utilizes
a temperature-controlled
crystal
for
close
regulation
of
the
operating
frequency.
All
operating
controls
are
accessible
from
the
front
of the
panel
through
the
access
openings
pro-
vided;
meters
are
mounted
above
the
hinged
front
door.
A power
source
of
1 17 volts,
50/60
cycles,
single
phase
capable
of
supplying
approximately
1,450
watts
is required
for
the
transmitter.
A
crystal
oscillator,
buffer-driver,
and
power-
amplifier
comprise
the r-f
portion
of the transmitter.
Referring
to
the
schematic
diagram,
Figure
12,
capacitor
Cl
is used
for
adjusting
the
oscillator
fre-
quency
within
narrow
limits,
and
is the only
variable
tuning
element
in this
stage.
Pilot
lamp
I1 0 1 serves
to
indicate
when
the
crystal
heater
is
energized,
while
an
extra
socket,
Xl
10,
provides
means
for
storing
a spare
crystal.
The
spare
crystal
heater
is
energized
at all times,
permitting
instant
substitution
of the spare
for
the
operating
crystal.
An
RCA
type
807
tube
is used
in
the
buffer-
driver
stage,
V 10 1.
The
plate
circuit
is shunt
fed
through
coil
L 10 1.
Taps
on L 10 1 enable
the buffer-
driver
stage
to be adjusted
for
optimum
output
over
a wide
frequency
range.
The
power
amplifier
is a plate-modulated
Class
"C"
amplifier
which
uses
two
RCA
type
8 13 tubes
connected
in parallel.
The
low-pass
"T"
filter
used
as output
in this stage
serves
to match
the high
plate
impedance
to
the
low
antenna
impedance
as well
as acting
to reduce
harmonic
radiation.
Since
screen
grid
tubes
are
employed,
neutralization
of
the
PA
stage
is not
required.
Two
push-pull
stages
comprise
the
transmitter
audio
system.
The
two
type
807
tubes
in the
first
stage
operate
Class
"A"
whi1.e
the
modulator
uses
two
type
8 13 tubes
as a Class
"AB,"
amplifier.
To
reduce
distortion,
approximately
14 db
of negative
feedback
is used
on
the
input
stage.
The
tapped
secondary
of
modulation
transformer
T102
makes
it
possible
to
modulate
the
screen
of
the
buffer-
driver,
Vi0
1, as well
as the
screens
and
plates
of
PA
tubes
VI02
and
VlO3.
Potentiometers
R155,
R 158,
and
R 159
provide
means
for
maintaining
the
overall
hum
at minimum
level.
D-c
supply
for
the
transmitter
is
furnished
by
two
rectifiers.
High-voltage
for
the
modulator
plates
and
the
PA
screens
and
plates
is supplied
by
two
type
866A
mercury
vapor
tubes,
V108
and
V109,
functioning
as a single-phase
full-wave
rectifier..
Selenium
rectifiers
SRI
02
to SRI
07
also
are
connected
in
a full-wave
rectifying
circuit
to
furnish
approximately
750
volts
for
the
type
80 7
tubes
and
a negative
200
volts
for
modulator
bias.
Power
to the
transmitter
is fed
through
the
main
circuit
breaker
Sl 0 1, the
trip
coils
of which
are con-
nected
in the
filament
circuit.
Thus
a filament
over-
load
would
remove
all
power
from
the
transmitter.
Control
of filament
voltage
is provided
by
rheostat
R 149,
in conjunction
with
meter
rectifier
SR 10 1 and
meter
M 104.
Pilot
light
I1 02
serves
to
indicate
the
status
of the
filament
circuit.
When
filament
power
is applied
by the closing
of
S 10 1, time
delay
relay
K 10 1 is energized.
Relay
K 10 1 closes
in 'approximately
30
seconds
de-ener-
gizing
the
trip
coil
of
plate
breaker
S104.
Plate
power
may
then
be
applied
manually
by
closing
S 104,
energizing
the rectifier
plate
circuits.
Circuit
breakers
S 102
and
S 103
are connected
in
the
cathode
circuits
of
the
modulator
and
power
amplifier,
respectively,
protecting
these
stages
in
case
of overload.
Milliammeter
M 104 and
selector
switch
S 105 pro-
vides
facilities
for
checking
all
stages
in
the
trans-
mitter.
Switch
positions
along
with
the
various
circuits
involved
are
listed
in
Table
3, under
IN-
STALLATION.
For
frequency
monitoring,
receptacle
J 10 1, con-
nected
to the cathode
of buffer-amplifier
V 10 1, pro-
vides
means
for
sampling
the
voltage
at this
point.
Output,
approximately
ten
volts
at
approximately
75
ohms
impedance,
may
be
varied
by
potenti-
ometer
Rl
12.
R-f
output
for
modulation
monitoring
is obtained
from
transformer
T103
which
is connected
to
J 102.
Output
of this
circuit
is designed
to supply
approxi-
mately
ten
volts
at approximately
75 ohms
imped-
ance.
Taps
on
the
secondary
of
T103
provide
means
for
adjusting
the
secondary
voltage.
Table of
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This manual is also suitable for:
Es-28937
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