ever
either
of
these
thresholds
is
exceeded,
current
flow
in-
creases in
the
associated
transistor
which
causes
0204
to
turn
on.
0157
acts
as a
clamp
to
prevent
the
Buffer
Ampli-
fier
output
from
going negative
enough
to
saturate
0156.
RMS
Converter
overrange
occurs
whenever
the
crest
factor
ratio
is
exceeded.
When
this
happens,
0195
turns
on
and
causes
0204
to
turn
on.
In
the
A/D
Converter,
R205
and
R206
set
a positive thres-
hold
for
0206.
Whenever
this
threshold
is
exceeded
by
the
positive
ramp
from
U220
there
is
increased
current
flow
in
0206
which
causes
0204
to
turn
on.
When
0204
turns
on,
pin 2
of
U250A
goes LO, pin 3 goes
HI,
and
0256
turns
on
which
causes a negative
unblanking
signal
to
appear
at
the
input
of
0390.
As long as
the
over-
range
condition
remains,
the
count
0000
reset pulse
from
pin 8
of
U260,
which
is
applied
to
pin 5
of
U250B,
cannot
OVERRANGE
STABLE
OCCURS
+
ON
I
0204
OFF
U250A
PIN 2
HI
LO
U2508
PIN 5
HIV
LO
HI
U250A
PIN 3
U2508
PIN 4
LO
U250A
PIN 1
HI
U250B
PIN 6
LO
ON
0256
OFF
Theory
of
Operation-213
Service
reset
the
overrange latch. As
soon
as
the
overrange condi-
tion
disappears,
pin 2
of
U250A
goes HI. On
the
next
reset
pulse, pin 1 goes HI, pin 3 goes LO,
0256
turns
off,
and
the
overrange
indication
disappears.
OSCILLOSCOPE FUNCTIONS 0
<9
Preamplifier
The
Oscilloscope Preamplifier
is
a simple
feedback
amplifier
consisting
of
O
170,
0172,
and O
174.
The
input
signal
is
de-
veloped across
the
1-2-5 gain switching resistor
network
R
128,
R
129,
and R
178.
Amplifier
gain
is
set
by R
130,
R132,
R133,
and
feedback
resistor
R174.
Vertical signal
positioning
is
controlled
by
current
from
R
135
through
R
134.
This
current
changes
the
voltage across R
174
thus
providing
position
control
without
affecting amplifier
gai.[i.
The
Preamplifier
output
drives
the
vertical ampl ifi.er section
and provides an internal vertical signal
to
the
trigger
circuit
when
selected
by
S315.
0
OVERRAN
GE
0
0
STOPS
0
0
+
0
0 0
I
I
V"
r
n
1481-14
Fig.
3-7.
Overrange
timing
diagram.
@
3-9
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