Philips PM 3266 - 3.2.9. Z-AMPLIFIER AND FOCUS AMPLIFIER

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: