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When
the
power amplifier and
R.F.
Driver have been tuned, the
modulator currents may
be
set.
With
the two
bias potentiometers
R4 and R5
turned fully counter -clockwise, there should
be little
if
any current indicated on the
modulator current meter
M2.
Turn
one
potentiometer until this meter shows
30
milliamperes current,
then bring
the
other potentiometer
to
a
reading
of 60
milliamperes.
The
modulation monitor pick
-up loop
may now
be adjusted.
This
should be only enough
to
give just over the requirements of the
modulation monitor
so
that the
modulation monitor "Carrier Set"
control
is
almost
at
maximum.
Making Performance Measurements
To
make satisfactory ncise, distortion and
response measurements,
it is
necessary
to
have the proper measuring equipment.
The Gates
SA-131 Proof
of
Performance kit contains
the
required instruments
for making these measurements.
Checking noise
or
signal quality
by means
of
a
radio receiver in the sane room
as the
transmitter
is
generally not satisfactory,
as
quite often overloading of
sensitive circuits, or stray pickup, introduces
effects in the
receiver which
do not
indicate
the exact
performance obtained.
The use of the SA -131 kit
is
fully explained in the instruction
book for
the set.
The Gates
M
-5693 Modulation Monitor has provisions
for
making
performance measurements, other monitors
may
or
may
not
have this
provision.
Before accepting adverse readings when
using
a
modu-
lation monitor
as
the diode source,
determine
the
monitor
is
suitable for this
use.
For normal program monitoring, the pick-
up from the power amplifier tank coil
is
satisfactory, but
it
may be found that better results
are
obtained from
a
pick -up
loop
coupled
to
a
temporary turn
or two in the radio
frequency
output line.
The amount
of
coupling
to
the diode
may have con-
siderable effect.
Generally, the coupling should be such
that
with 100% modulation
the
distortion meter calibrate dial will
be
at
about one -half
to
two- thirds
of the scale.
Preliminary performance measurements need
be made on only three
frequencies,
50,
1000 and 5000 cycles.
50
cycles represents the
low frequency
end,
1000 cycles the
mid- frequency
and
best per-
formance, 5000 cycles the high frequency
end,
this
frequency
is
frequently higher
in
distortion than 7500 cycles.
9/11/63
-13-
BC- 250GY
-1
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