Philips pm3232 - Circuit description

3. Service
data
3.1.
Circuit
description
3.1.1.
CHANNEL
Ya
VERTICAL
AMPLIFIER
3.1.
1.1.
General
The
vertical
amplifier
provides control
of the
input
coupling,
Y deflection
coefficient and
gain.
Furthermore,
a
drift-reduction
circuit
and a
trigger
pick-off stage
are
part
of the
amplifier circuitry.
Fig.
3. 1. Vertical
amplifier, channel
3.1.
1.2.
Input coupling
Input
signals
connected
to
Ya
socket BUS
can
be a.c.
coupled, d.c.
coupled or
internally disconnected.
In
position
DC
of AC-O-DC
switch SK16,
the input
signal
is coupled direct to
the step attenuator.
In the
AC
position,
there
is
a
capacitor
(C26)
in
the
signal path.
This
capacitor
prevents the d.c.
component
of the input
signal
from
being
applied to the amplifier.
In
position
0,
the
signal
path between the
input socket and
the
amplifier input
is interrupted, the
latter
being
earthed.
At the
same time,
blocking capacitor C26
is discharged
via resistor R26, to
prevent
damage
of
the
circuit
under
test by a
possible
high
charge.
3.1.
1.3.
Input
attenuator
The input
attenuator
is
a
frequency-compensated,
high-ohmic
voltage
divider with
twelve positions.
Both
attenuator
sections contain
trimmers
which are
adjusted
for optimum
square-wave
response
and
others
that
maintain
the
same
input capacitance
(20
pF)
for
each
position
of AMPL.
switch SK17.
The
input
capacitance
of both
channels
can be
equalized
using
trimmer
C235.
The
attenuator
works
in
conjunction
with a
gain
control
in the
pre-amplifier
(see also section
3.
1.1.
5.).
The
gain of
the
pre-amplifier is selected
together
with
the
relevant attenuator
section, by
means of
AMPL.
switch
SKI 7.
3.1.
1.4.
Input
stage
Input
source-
follower TS51
and
emitter
follower
TS52
provide
a
high
input
impedance
to
the
step
attenua;or
and
a
low-impedance
source
to
the
pre-amplifier.
The
input
source-follower
Is
protected by
diodes GR53
and GR54.
If
there
is
a
maximum
positive-going
voltage
change,
the
field-effect transistor
can deal
with
it.
If the
chanje