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Distortion readings on these three frequencies can be
taken.
Then with
the
50
cycle frequency, hold the back door interlock,
apply the high voltage, and
set
for the distortion reading.
Then
adjust the bias potentiometers for distortion below
3 %,
but not
necessarily
the
minimum possible distortion,
as
this adjustment
may increase
the
noise and distortion on other frequencies.
After this adjustment
is
made, recheck the other two frequencies
and the noise level.
When setting
the
bias potentiometers,
ex-
treme caution should be used, with preferably
a
second person
helping.
If the
performance
is
within specifications,
a
complete
frequency run may be made.
In case the
distortion
or noise
is out of
specifications, several
remedies may be tried.
Different tubes may be tried, particularly
the
6L6G audio driver and 810 modulator tubes.
Because
a
tube
is
new
out of
a
carton
is no
guarantee that
it
has not received some
damage in shipping.
The polarity
of
the modulation tránsformer may be reversed by
inter -changing
the two
modulator tube leads on the primary
of the
transformer and connecting whichever way gives the best results
for all frequencies.
The
modulator grid connections could be
interchanged, and doing this requires that the feedback also
be
reversed.
Both reversals, that
is,
the
driver
and
modulator
could
be
tried
simultaneously.
In
each
case, the
modulator
bias should be adjusted for
50
cycle distortion
below
3%
if
obtain-
able.
Some noise improvement can be obtained by proper phasing
of the
audio system
by
the means just outlined
so
some
cancellation
is
obtained between
the audio and radio
frequency sections, but this
can easily upset the distortion performance.
Considerable noise
variations can be encountered in the tuning
of
the radio frequency
part
of the
transmitter.
Normally,
the
best noise reading will be
obtained when the 813 driver stage
is
tuned
to the
minimum plate
current.
The power amplifier neutralizing has some effect.
If
the radio frequency drive is low in any of the radio
frequency
stages, some grid
modulation
is
fed through
to
cause the noise.
The phasing mentioned
is
that the noise generated in the audio
section
is
opposed to.that generated in the radio section result-
ing in cancellation with the residual being
the
difference
of
the two
magnitudes.
BC- 250GY
-1
9/11/63
-14-
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