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Shutting down
the transmitter, the
plate connection
is
again nade
to the
8008 rectifier
tube.
High voltage may then be re- applied,
and the
transmitter adjusted
to the
final loading condition.
Typical readings are given
in
Section IV
of
this
instruction book.
If
a
severe overload occurs with the application of high voltage,
note
if the
modulator current reads excessive.
With
a
voltmeter,
measure the voltage
on the
modulator
grids.
This should be
nega-
tive
50 to 60
volts
to
ground
if the
bias potentiometers
are
turned
to
the maximum bias voltage position.
If
the bias
is
higher than
30
volts, the
overload may be caused by
a
reversed audio feedback
loop causing high frequency oscillations.
Removal
of the
connec-
tions
to
the plates of the
modulator tubes will stop
the overload.
The remedy
is
the
reversal
of the
connections
to
terminals
81 and
83,
or
a
temporary correction can be made by reversing the grid
or plate connections of the 810
modulator
tubes.
It is
difficult in these instructions
to
give loading information
as
various types
of
loads
are
applied in almost each broadcasting
installation.
Where
a
70
ohm transmission line
is
provided, the
line current
meter
Ml is
usually
0
-3
amperes, while for
a
250 ohm
transmission line meter Ml
is
usually
0 -2
amperes.
The formula
of
I2R
is
usually employed
in
computing
the
proper line current
where the line impedance
is
known, and
of
course,
is
likewise
employed
whrlre
the antenna resistance
is
known
in the case of
direct coupling.
A dummy antenna
of
known value
is
usually pre-
ferable in
the
initial tune -up
so
that
it
can be determined
whether
the
transmitter
is
functioning properly
and thus, full
attention can be given
to
the
antenna loading problems for final
successful operation.
The Ohmite D250 dummy antenna for 73 ohms
operates excellently where
the
transmission
line
is 73 ohms.
Or,
in the case of 250 ohm
transmission
lines, four of these
dummy
antennas may be wired in series which will give slightly higher
resistance than the open wire transmission
line, but
usually will
be satisfactory
as the
resistance
is
known.
As
a
less
satisfactory dummy antenna,
150 watt lamp
bulbs may be
employed which have
a
resistance
of about 100 ohms,
but are not
dependable
as
their resistance will vary with intensity.
Two
may be paralleled
to
be in series
to
give
with
two in
parallel
9/11/63
give
a
50
ohm dummy antenna or three
may
a
300
ohm dummy antenna.
One in series
will give
approximately
75 ohms.
Two series
-11-
BC- 250GY
-1
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