Philips PM 3266 - Page 92

97
3. Service
manual
3.1.
DESCRIPTION
OF
THE
BLOCK
DIAGRAM
3.1.1.
General
information
The
block
diagram
is
shown
in
Fig.
3.1.
The PM
3266
oscilloscope
comprises
the
following
parts:
—
a
dual-channel
vertical
system
—
a main
time-base
—
a delayed
time-base
—
a
display-mode
logic
stage
—
an
X-amplifier
—
a Z-stage
—
a c.r.t.
circuit
—
the storage
section
—
a stabilized
power
supply
3.1.2.
Dual-channel
vertical
system
Both
vertical
channels
contain
identical
circuits.
An
input signal
to one of
the
channels
is, via
a coupling
switch
AC/O/DC,
applied
to the
input
attenuator.
In
the AC
position
of the
coupling
switch
there is a
capacitor
in the
signal
path. In
the DC
position
the
coupling
is direct.
If the
coupling
switch
is set to
the
0
position,
the
connection between
the
input
socket and the
attenuator
input
is
interrupted,
the latter
being
earthed.
The input
attenuator,
which is
controlled
by
the AMPL
switch,
enables
the
adjustment
of the
vertical-deflection
sensitivity
in
calibrated
steps.
The
attenuator
is followed
by
a low-drift
impedance
converter which
gives
the
input
circuit a high
input
impedance.
The
impedance
converter
also contains
a
voltage
divider
which
works in
conjunction
with
the input
attenuator.
The
signal
that leaves
the
impedance
converter is
applied
to a balanced
amplifier
(D201
-Ya,
D301-Yb)
where
it is transformed
into a
push-pull
signal.
The
balanced amplifier
has
two outputs.
From
one of
these
outputs
the signals
are
applied
to a trigger
selector
stage
and from the
other
one to an
amplifier
stage (D202
-Ya,
D302-Yb).
This
stage
comprises
the switch
NORMAL/INVERT
by means
of which
the phase
of the
signal can
be inverted,
and
the controls
for
vertical
trace positioning.
The following
stage is a
channel
selector which
either
blocks or passes
the
signal as
dicated
by the vertical
display-mode
logic
and
switches.
In
the A, B,
ADD
and TRIG VIEW
modes
the channel
selector
is set by
means
of voltage
levels
(via the
display-logic
stage) and
in the
ALT and CHOP
mode
controlled by
pulses
(also via
the display-mode
logic
stage).
In the ALT mode
those
pulses
are supplied
by the sweep-gating
multivibrator
of the
main
time-base
generator
during
the fly
back
of the
sweep, so that
alternately
the complete signals of
channel
A,
channel
B and
the 3rd
channel TRIG
VIEW
are displayed.
In the CHOP
mode the drive
pulses
are provided by
an oscillator
which works
at a fixed frequency
of
approximately
1
MHz.
Those
pulses
cause the electronic
switches
in the display-mode
logic stage to be
successively
opened and
closed so
that
successively part of
the signal
of channel
A, channel
B and the 3rd
channel
TRIG VIEW are
displayed.
After
the
channel selector,
the following
circuits
are
common to the vertical
channels.
A delay line
that
delayes the
vertical signals to such
an
extent that the
steep leading
edges of fast
signals
are
still displayed,
a delay line driver
stage and a final
output
stage which feeds the
signals
to the vertical-deflection
plates.