Philips PM 3266 Instruction Manual page 131

136
3.2.9.
Z-AMPLIFIER
AND
FOCUS AMPLIFIER
The
current that determines
the
intensity of the spot
on
the
c.r.t.
screen
is
applied to the emitter of V2001
and
is
derived
from
several sources
as follows:
the current
from
this resistor
is
influenced by
the
setting of the
INTENS
control (R14)
via
V2002.
In
addition,
it is
influenced
via
V2003
by
a signal
applied to the
EXT
Z MODulation
input X8.
the
signal applied to
this resistor
is
derived from
the storage
logic.
the
signal
applied
to this resistor
is
derived
from
the
chopper
oscillator (chopper
blanking).
the
signal applied to
this resistor
is
derived
from
the time-base unit for display blanking
if
the
time-base does not run (time-base unblanking).
The
circuitry
in
the input
circuit of the Z-amplifier
is
arranged
in
such
a
way
that
-
depending on
the selected
display function
-
the
maximum
Z-pulse
is
limited.
The
Z-pulse
is
limited to
such
a
value that
maximum
intensity
will
normally not damage
the
CRT.
However,
to be completely
safe
the
intensity control must
be operated with
care
!
WARNING:
IN
ORDER
TO PREVENT DAMAGE
TO THE
C.R.T.
THE
INTENSITY
MUST
NOT
BE
TOO
HIGH.
The
circuit that
limits
the Z-pulse
amplitude
is
located
on
a
small additional unit above
the time-base
unit.
The
Z-pulse
is
controlled
in
the following way:
-
The
maximum
Z-pulse
in
the
X EXT
mode
is
limited to
35
Volt because VI
51 7
on
the time-base
(unit 8)
is
switched
off.
Now
the
transistor
on
the additional unit
is
switched
off
and
R2009
on
the Z-amplifier (Unit 11)
does not
receive any
current.
In
the
MTB
positions
1
s/DIV
...
5
ms/DIV
the
transistor
on
the additional unit
is
switched
off.
Via
a
contact
of the
MTB
switch the junction of R927
and
R928
is
applied
to -1-5.2
V
and
a
diode
on
the additional
unit
becomes
conductive
so that the
transistor switches
off.
—
If
the instrument
is
not used
in
the
two
situations described above
the
max.
Z-pulse
is
75
Volt
in
the
SINGLE
and
FAST
modes.
In
the
SINGLE
mode
an
earth potential coming from S8b (TRIG) and
S8a
(AUTO)
is
applied to the
junction
of
R965
and
R983
on
the time
base
unit.
Therefore, the
transistor
on
the additional unit
can
become
conductive and
supplies current
to
R2009
on
the
Z
amplifier.
In the
FAST
mode
an
earth potential
is
applied to the junction of R965
and
R983
via
diode
V958.
The
current applied
to the emitter of
V2001
is
used
to
determine
the intensity
and
the focusing of the
c.r.t.
electron beam. As
the focusing
is
influenced by
the
INTENS
setting, the
c.r.t.
screen
will
show
a
well-focused
spot over nearly the whole
of the intensity range.
The
signal
from V2001
is
applied to
a differential amplifier comprising
V2007
and
V2008. The
signals
on
the
collectors of these
transistors have
a
phase
difference of
180
degrees.
The
signal present on
the collector of
V2008
is
used
to drive the focus
amplifier consisting of V2031, V2032,
V2028, V2023
and
emitter-follower V2026. The
focus
amplifier receives
its
input
signal via
a
correction
network
with zener diode
V2005
and
resistor
R2024.
This
network
gives
a
non-linear relationship between
the
INTENS
control pulses and
the focusing control
pulses
which
results
in
optimum
focusing over the whole
INTENSity
range.
The
output
signal of the Z-amplifier
is
present on the emitter of V2013;
this signal comprises
h.f.,
I.f.,
and
d.c.
components. The
h.f.
component
is
applied to the Wehnelt
cylinder
G1
of the
c.r.t.
via d.c. blocking capacitor
C2077. Grid
G1
is
at a potential of
about
—3
kV.
The
I.f.
and
d.c.
components
are applied to
a
modulator
circuit
V2038,
V2037
and V2033. The
trimming
potentiometer
R2064
permits
intensity
adjustment
in
order
to
compensate
for
c.r.t. tolerances.
The modulated
signal
is
connected
to the
-3 kV
level
at
G1
via d.c. blocking capacitor C2076.
Demodulation
is
achieved by V2076,
V2077
and matching
capacitors.
The demodulated
I.f.
and
d.c.
components
are
added
to the
h.f.
component
on
the Wehnelt
cylinder G1.
The
c.r.t.
has
two
focusing
electrodes:
—
G3
is
used
for horizontal focusing and
the voltage
is
adjustable by R17.
—
G5
is
used
for
vertical
focusing and
the voltage
is
adjustable
by R19.
-
R2006:
-
R2000:
-
R2002:
-
R2001: