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Circuit Description — Type
3A74
pooceend
VOLTS/DIV.
/
/
(Set to .05)
3F
/
/
/
INPUT
oo
C407B
~~
Switched Portion
Cc
1 Portion——*}
X2.5
ATTENUATOR
Fig.
4-1.
Simplified
circuit
diagram
showing
the
most
important
components
involved
when
calculating
the
X2.5
attenuation
factor.
going overload signal is inadvertently applied to the input
connector.
Since
R417
alone
would
deteriorate
the
high-
frequency ac response of the stage, C417 is added to pass
the high-frequency ac components around
R417.
To protect
transistor Q424
from
the same
overload
signal, diode
D424
is provided.
This diode conducts during the overload interval,
thus limiting the reverse bias of Q424.
Negative-going
overload
signals
do
not
damage
V423
because
V423
is protected
by B419.
If V423
goes too far
below cutoff, neon B419 ignites and clamps the grid of V423
at about —60 volts.
The clamping action prevents the grid
from
going
further
negative
and
prevents
grid-to-cathode
arcing.
Current through transistor Q424 for the duration of
the
overload
signal
is limited
to 8ma
by
its long-tailed
circuitry.
Input Amplifier Q424/Q434.
This stage is a transistor-
ized
emitter-coupled
paraphase
amplifier.
It converts
the
single-ended input signal at the base of Q424 to differential
current signals in the collector circuits. Both emitters are long-
tailed (through R425 and R435} to the +125-volt supply for
greater stability with
respect to transistor
parameters
and
temperatures.
The
no-signal
current
through
each
of the
transistors is nominally 2.5 ma.
There
are two
gain
controls
located
in the common-emit-
ter circuit
of Q424
and
Q434.
One
is the
VAR.
GAIN
control R426 and the other is the GAIN
ADJ. control R436,
a front-panel screwdriver-adjust potentiometer.
Both controls
vary the emitter degeneration
and
thus affect the gain of
the stage.
With R426 set to the CAL. position, R436 is ad-
justed so that the total amplifier gain provided by this ad-
justment and the CALIB. adjustment R477 (in the Output Amp.
2nd Stage) causes the crt deflection to agree with the setting
of the VOLTS/DIV. switch.
The value of collector resistors R424 and R434 is chosen
to provide
proper base-emitter junction temperature
comp-
4-2
ensation
for
their
respective
transistors.
C424
and
C434
provide increased high-frequency response to compensate for
the response losses introduced by R424 and R434.
DC Balance CF V433.
This cathode follower stage pro-
vides a low-impedance
drive for ac balancing the Q424/-
Q434 emitters of the Input Amplifier stage.
To balance the
emitters
the
DC
BAL.
control
R427,
located
in
the
grid
circuit of V433,
is used to set the dc level of Q434.
When
correctly
adjusted,
the
emitter
level
of Q434
will
be the
same as the emitter level of Q424 under no-signal conditions.
(The usual method for quickly establishing a no-signal con-
dition
is to set the AC-GND-DC
switch to GND.}
With
the
emitters at the same voltage there will be no current through
the VAR.
GAIN
control.
Thus, the VAR.
GAIN
control
can
be rotated without affecting the de level of the succeeding
stages and thus not affect the vertical position of the beam.
Mode
Switch.
if the MODE
switch is set to the NORM.
position,
the
signal
passes
through
diode
switches
D440
and
D443
to the Output
Amplifier.
A normal
display will
appear on the crt; that is, the display will have the same
polarity as the input signal.
If the input signal is positive
going,
for example,
the displayed
waveform
will also be
positive going.
However, when the MODE switch is set to the
INV. position, the display is inverted because the switch re-
verses the signal
leads to the diode switches.
For example,
a positive going input signal will be displayed as a negative-
going waveform.
When
the MODE
switch
is set to the OFF
position, the
output leads from the Input Amplifier stage are shorted to-
gether (refer to the block or schematic diagrams to see how
this is accomplished).
Therefore
no differential current sig-
nals from the Input Amplifier stage or the positioning circuit
is applied to the Output Amplifier.
If all the MODE switches
are set to OFF, a trace will appear centered on the crt. This
is a quick,
convenient
method
for checking
the de balance
®
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