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72
3. 1.6.2.
C.r.t.
circuit
By means
of
INTENS
potentiometer
RIO
the brightness of the display
can be continuously
controlled.
The
maximum
beam
current
is
set
by
potentiometer R801.
The
control range of the
INTENS
potentiometer can be
preset
by
means
of
potentiometer
R802.
A
brightness
modulation
facility, a.c.
coupled,
is
provided
on
Z
MOD
input socket
BU7.
The
display
can be focused
by means
of
FOCUS
potentiometer R9.
In a
split-beam
c.r.t.
as
used
in
this
oscilloscope,
focusing has
certain
consequences.
These consequences and
their
remedies
are
discussed
in
section
3. 1.6. 3.
Astigmatism
is
corrected
on
a
low-impedance
level
by means
of
potentiometer
R809
via
emitter follower
TS801.
With
the
aid of
potentiometer
R812
barrel
and
pin-cushion
distortion
is
corrected.
Potentiometer
R813
provides
trace rotation control
and
allows the time-base
line
to
be accurately
aligned
with
the horizontal
graticule
lines.
3. 1.6.3.
Focusing
in
the split-beam
c.r.t.
In
the split-beam
c.r.t.
the electron
beam
is
split
into
two
parts
which form
the
traces
of the
Y
and
Yg
channel.
Both
traces are
controlled
independently
by
the controls of
both
channels.
The
electron
beam
is
focused,
however,
before
It is
split
and, therefore, there
is
only one focus
control.
This
mono-knob
control of
the focus has the following
consequence.
Assume
a
focus
setting
as
illustrated
in
Fig.
3.7.1.
Then
the focusing
point
is
exactly
on
the
screen.
When
the
FOCUS
potentiometer
R9
is
rotated, the
focusing point
is
displaced to
a
point
either side
of the
screen
(Figs. 3.7.2.
&
3.7.3.).
This
results in
both
traces
moving away from
each
other.
The more
clockwise
or anti-clockwise the
FOCUS
control
is
turned, the further the
traces
move
apart.
(
1
)
(
2
)
(3)
Fig. 3.7.
Effect
of
the
focus control
MA7288
To
compensate
for
this
trace
shift,
an
extra
shift
voltage
is
fed to the
vertical
amplifier
channels
via
transistor
TS56.
The
magnitude
of
this
voltage
is
controlled
by potentiometer
R9'
which
is
part of
FOCUS
tandem
potentiometer
R9.
The
control
is
such, that the
displacement caused by operation
of the
FOCUS
control
is
compensated
for.
The
compensation
voltage for
channel
is
obtained
from
the emitter of
transistor
TS56
and
that for
channel
Yb
from
the
collector.
The
compensation
is
so
effective
that the
maximum
displacement
of
each
trace,
cav'^^ed
by
rotating
FOCUS
potentiometer
R9
from
fully
anti-clockwise to
fully
clockwise,
does not exceed 2
mm.
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