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oduïator and sower ampliiier stages have overload relays in
the
filament return circuits.
These are normally open.
When
the current becomes sufficient, the
contacts close, completing
the circuit through an
auxiliary relay with normally closed
contacts
in series
with the high voltage contactor
coil.
Under
normal operation, modulation may provide sufficient current
to
cause the overload armature
to
pull open.
If
normally closed
contacts were used
to
operate directly in series with the plate
contactor
coil,
these modulation cycles would cause the contactor
to
chatter, or possibly drop
out,
although
no
overload actually
occurs.
Both overload relay coils
are
shunted by adjustable
resistors
for setting the overload current point.
The
normal
unshunted operating current
is 300
MA
and shunting
with resistors
gives
a
higher current rating.
Increasing
the
amount that
is
shorted out on the resistors R27 and
R28,
raises the current
required
to
trip the respective relay.
The relays should trip
at
about 400 milliamperes,
heavy enough
that the power amplifier
overload does
not trip
with normal tuning and the modulator over-
load does not trip with normal program.
In the case of the
mod-
ulators, the resistors also load the inductance
of the coil,
pre-
venting distortion through this element.
8008 Rectifier Tubes
The 8008
mercury vapor rectifier tubes
are
sensitive
to
temper-
ature.
If the
temperature
is too
high or
too low, this type of
tube has
a
tendency
to
"arc- back" or conduct current in the re-
verse direction, resulting in
a
high voltage short circuit.
When
the
transmitter
is
operated where temperature
is a
problem, such
as-unattended and in
a
unheated building,
or where the temperature
may become excessive, the mercury vapor rectifier tubes may be
directly replaced by
a
type using xenon or some other inert gas
and
which
is not
nearly
so
sensitive
to
temperature, or with
minor modifications by
semi
-conductor rectifiers.
Another solution
is to
install
a
heating element inside
the
transmitter cabinet with
a
thermostat control such
as
used for
furnaces, adjusted
so
that
when
the
temperature drops below
a
certain value such
as
75
degrees, the heating element will be
energized.
Similarly,
if
high temperature
is
a
problem, an ex-
haust fan could be installed in the top of the transmitter, with
the
thermostat operating when
the
temperature reached
a
value
of
around 95 degrees.
Operation
of
this equipment with mercury rec-
tifiers
is
NOT recommended above 120
°F or
below
45
°F
ambient.
Q
/11
/Fiji
-5-
Re-
25flGY
-1
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