Circuit
Description-Type
453
/ R453
Input Stage
The
Channel 1 signal
from
the Input Attenuator
is
con-
nected to the Input Stage through the network
Cl
7-Cl
8-
C20-Rl
6-Rl 7-Rl
8-Rl
9-R20-R21.
Rl
6,
Rl
7 and
R20
provide
the input resistance for
this
stage. These resistors are part
of the attenuation network
at
all
CH
1
VOLTS/DIV
switch
positions. Variable capacitor Cl 7 adjusts the basic input
time
constant for a nominal value of one megohm X
20
pF.
The
divider action of
Rl
6-Rl
7-R20
allows about 98% of
DC
and low-frequency signals to pass
to
the gate of
FET
(field-
effect transistor) Q23A. Cl 8
with
the stray capacitance·
in
the circuit
forms
an
AC
divider which maintains
this
same
voltage division for high-frequency signals.
Rl
8
limits
the
current drive to the
gate
of Q23A. Diode
Dl
8 protects the
circuit by clamping the
gate
of Q23A
at
about
-12.5
volts
if
a high-amplitude negative signal
is
applied
to
the Channel
1
INPUT
connector. Over-voltage protection
for
high-ampli-
tude positive signals
is
provided by forward conduction of
Q23A.
The
current path
is
through
R23,
L23,
D36
and
D37.
FET
Q23B
is
a constant current source for Q23A and also
provides temperature compensation for Q23A.
The
STEP
A
TTEN
BAL
adjustment,
R30,
varies the gate
level
of
Q23B
to
provide a zero-volt
level
at
the emitter of Q34
with
no
signal applied. With a zero-volt
level
at
the emitter of Q34,
the trace position
will
not change when switching between
the
5,
10
and
20
mV
positions of the
CH
1
VOLTS/DIV
switch.
DC
and low-frequency signals are connected
from
the
source of Q23A
to
the Feedback Amplifier through
R23,
L23,
Q33 and
R39.
L23
isolates the base of Q33
from
the source of
FET
Q23A.
Diodes
D34-D35
and
D36-D37
limit
the dynamic range of
the signal
at
the base of Q33 and prevent the following
stages
from
being damaged by a large voltage swing
at
the source of Q23A.
The
signal path for high-frequency
signals
is
through
C23,
Q43 and
C39.
High-frequency signals
at
the emitter of Q43 are connected to the base of Q33
through
C38.
This
allows Q33
to
be driven
at
high
fre-
quencies while preventing the base circuitry of Q33
from
capacitively loading the input
FET,
Q23A.
C38
is
selected
to
provide the same amplitude
AC
and
DC
signal
at
the
base of Q33.
C24
couples high-frequency information
to
the junction of
R25-R26,
thereby reducing the loading
at
the
base of Q43.
Feedback Amplifier
The
Feedback Amplifier, Q34 and Q54, changes the over-
all
gain of the Channel 1 Vertical Preamp to provide the cor-
rect deflection factor
in
the 5 and
10
mV
positions of the
CH
1
VOLTS/DIV
switch. Gain of
this
stage
is
determined
by
the
ratio of
R46-R50
to
R43,
R44
or
R45.
In
the 5
mV
position of
the
CH
1
VOLTS/DIV
switch, the network
C43A-C43B-C43C-
C43D-C43E-L43A-R43A-R43C-R43E
is
connected into the emit-
ter circuit of Q34.
The
ratio between
R46-R50
and
R43
pro-
vides a gain of about
10.
C43A,
C43C,
L43A
and
R43C
are
adjustable to provide high-frequency peaking
for
the net-
work.
In
the
10
mV
position, conditions are the same
ex-
cept that the network
C44A-C44B-C44C-L44A-R44A-R44B-
R44C
is
connected into the circuit
in
place
of
the previous net-
work.
The
ratio between
R46-R50
and
R44
provides a gain of
about 5
times
in
this
CH
1
VOLTS/DIV
switch position.
C44A,
C44C
and
R44C
provide high frequency peaking
for
this
network.
In
the
20
mV
and higher
CH
1
VOLTS/DIV
switch
3-4
positions, the gain of the Feedback Amplifier
is
about
2.5
as established
by
the ratio between
R46-R50
and
R45.
Ad-
justable capacitor
C45A
provides high-frequency peaking
for the Feedback Amplifier stage.
C49
and
R49
provide
high-frequency damping for the circuit.
As
mentioned pre-
viously, the
STEP
A
TTEN
BAL
adjustment
is
set
to
provide
zero volts
at
the emitter of Q34 when the input
is
at
zero
volts.
Since
there
is
no
voltage differe·nce across the emit-
ter resistors,
R43,
R44
or
R45,
changing the value of the
resistance does not change the current
in
the circuit. There-
fore, the trace position does not change when switching be-
tween the 5
mV,
10
mV
and
20
mV
positions of the
CH
1
VOLTS/DIV
switch
if
the
STEP
ATTEN
BAL
control
is
cor-
rectly adjusted.
Vertical position of the trace
is
determined
by
the setting
of the
POSITION
control,
R40.
This
control changes the
cur-
rent into the emitter of Q34, a low-impedance point, which
results
in
negligible voltage change
at
this
point.
How-
ever, the change
in
current
from
the
POSITION
control pro-
duces a resultant
DC
voltage
at
the output of the Feedback
Amplifier stage
to
change the vertical position of the trace.
The
CH
1 Position Center adjustment,
R55,
is
adjusted
to
provide a centered display when the Channel 1
POSITION
control
is
centered
(with
a zero-volt
DC
input
level).
Zener diode
D53
provides a low-impedance source
for
Q54. Variable capacitor
C54
provides feedback
from
the
collector
to
the base of Q54 for amplifier stabilization.
The
output signal
from
the Feedback Amplifier stage
is
con-
nected
to
the Paraphrase Amplifier stage and the Channel 1
Trigger
Pickoff
stage.
Channel 1 Trigger Pickoff
The
signal
at
the collector of Q54
in
the Feedback
Ampli-
fier stage
is
connected
to
the Channel 1 Trigger
Pickoff
stage through
D58
and
R59.
This
sample of the Channel 1
input signal provides internal triggering
from
the Channel 1
signal or
X-axis
deflection
for
EXT
HORIZ
operation. Q63
is
conneced as an emitter follower
to
provide isolation be-
tween the Trigger Preamp circuit and the Feedback
Ampli-
fier stage.
It
also provides a
minimum
load
for
the Feed-
back Amplifier stage and a
low
output impedance
to
the
Trigger Preamp circuit.
D58
provides thermal compensation
for Q63.
The
CH
1 Trigger
DC
Level
adjustment,
R60,
ad-
justs
the
DC
level
at
the base of Q63
for
a zero-volt
DC
output
level
from
the Trigger Preamp circuit when the Chan-
nel
1 trace
is
centered vertically. Output
from
the Channel 1
Trigger
Pickoff
stage
is
connected
to
the Trigger Preamp
cir-
cuit through the
TRIGGER
switch,
SW230B.
Paraphase Amplifier
The
output signal
from
the Feedback Amplifier stage
is
connected
to
the Paraphase Amplifier stage through the
VAR-
IABLE
control,
R75.
When the
VARI
ABLE
control
is
set
to
the
CAL
position
(fully
clockwise),
R75
is
effectively by-passed
and
maximum
signal current reaches the base of Q84.
Switch
SW75, ganged
with
the
VARIABLE
control,
is
open
and the
UNCAL
neon bulb
is
disconnected.
As
the
VARIABLE
control
is
rotated counterclockwise
from
the
CAL
detent,
SW75
is
closed and the
UNCAL
light,
B75,
ignites
to
in-
,,,,,.
dicate that the vertical deflection
is
uncalibrated.
The
sig-
nal
applied
to
the base of Q84
is
continuously reduced as the
VARIABLE
control
is
rotated counterclockwise.
®
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