Philips PM3208 - 4.3 CRT Circuit

4 CIRCUIT DESCRIPTION 4-5
4.3 CRT CIRCUIT
4.3.1 Z-axis amplifier
The Z amplifier provides intensity control of the CRT by driving the Wehnelt-cylinder g1
(pin 3), and focus control by means of the focus electrode g3 (pin 4). The voltage to the
Wehnelt can be influenced by several signals. Inputs to the Z-axis amplifier are :
- ADC control signal to R3026.
This voltage is 0 V in position "X DEFL" of the TIME/DIV switch and +5 V in
the other positions.
- "INTENS", intensity control to R3029.
This is a DC signal, 0 to +12 V, from the "INTENSITY" control RV3033. The
signal is intended to control the intensity of the beam.
- "MTB UNBLANK", unblanking signal MTB, to D3027.
During the MTB, this logic signal is HIGH, and the MTB can be displayed.
- "CHOP BLANK", blanking signal to D3029 and D3031.
This
logic
signal
is
HIGH
when
the
trace
must
be
displayed,
and
LOW
during
switching from channel A to channel B.
- "Z-AXIS INPUT", to R3001.
This
input
consists
of
external
signals,
if
any,
to
the
BNC
connector
at
the
rear-side of the instrument. A positive signal blanks the display; a signal of
about 3 V is sufficient to blank a display of normal intensity. All signals
mentioned above influence the voltage at gi and thus the trace intensity.
The
signals
are
input
to
the
amplifier
consisting
of
Q3001...3006.
Q3006
is
the
output
transistor
intended
to
drive
g1.
The
DC-level
of
Q3006-E
is
about
+50
V.
At
Q3006-E
the
Z-pulse is available. This is a positive pulse with an amplitude between 0 and 80 V,
depending
on
the
intensity
control.
In
position
"X
DEFL",
the
Z-pulse
is
a
DC
signal,
maximum
amplitude
80
V,
superimposed
on
the
DC
level
of
50
V.
The
Z-pulse
is
fed
to
g1
via
a
modulator
circuit.
This
circuit
is
necessary
as
the
Z-pulse
which
can
be
DC,
has
to
be
transferred
to
the
DC
level
of
g1,
about
—
2000
V.
The
modulator
is
a
peak-peak
detector,
consisting
of
D3024-3025
and
C3014-3018.
The
input
voltage
is
an
AC
voltage
which
is
clamped
between
about
+130
V
by
means
of
R3019-3020
and
D3019,
and
the
output
voltage
from
the
Z-amplifier,
via
D3018
and
R3014.
This
Z-output
can
be
+50
to
+130
V,
depending
on
the
intensity
controls.
So
the
input
voltage
to
the
peak-peak
detector
is
an
AC
voltage
with
a
positive
peak
of
+
130
V
and
a
negative
peak
between
+50
and
+130
V.
Thus,
the
output
from
the
detector
will
be
0
to
80
V,
depending
on
the
intensity.
This
voltage
is
superimposed
on
the
—
2000
V
and
thus,
the
voltage
at
g1
can
be
—
2000
V
to
—
1920
V.
For
good
pulse
behaviour,
the
Z-pulse
is
also
transferred
via
an
cans
AC
path,
©3023.
Focus control.
For
optimum
focussing,
the
voltage
at
the
focus
electrode
g3
(pin
4)
must
depend
on
the
intensity.
This
means
that
if
the
voltage
at
g1
is
increased,
the
voltage
at
g3
must
be
decreased.
This
is
called
"dynamic
focussing"
and
it
is
done
by
driving
g3
with
a
pulse
which
has
the
opposite
polarity
of
the
Z-pulse.
The
opposite
Z-pulse
is
available
at
Q3005-E
and
is
fed
via
a
similar
modulator
circuit
to
g3.
Focus
adjustment
is
obtained
by
the
voltage
of
potmeter
VR2011.
Focus
control
takes
place
by
means
of
the
"FOCUS"
potmeter
RV3035,
via
R3016
4.3 CRT CIRCUIT