Circuit Description—475
cies within the bandwidth of the instrument, the Input
Attenuators are designed to maintain the same input RC
characteristics for each setting of the V O L T S /D IV switch.
Each attenuator contains an adjustable series capacitor to
provide correct attenuation at high frequencies and an
adjustable shunt capacitor to provide correct input capac
itance.
N O TE
Each attenuator is a hybrid encapsulated plug-in
assembly; therefore, replacement o f individual com
ponents within the attenuator is not possible. Should
defects occur, the attenuator must be replaced as a
unit.
First Cascode Amplifier
The first amplifier stage in the Channel 1 Preamplifier
circuit is hybrid circuit U 120. U120 basically consists of an
integrated emitter-coupled, push-pull, cascode amplifier cell
and two discrete field-effect transistors (FET) mounted on
a ceramic substrate with the thick-film resistors. The stage
is configured as a paraphase amplifier and converts the
single-ended input signal to push-pull output signals. C R 104
and CR107 provide protection for the input to U120 if
large negative-going signals or DC levels are applied to the
CH 1 OR X input connector.
This input amplifier stage contains the CH 1 VA R
V O L T S /D IV control. This control provides the ability to
obtain continuously variable (uncalibrated) vertical deflec
tion factors between the calibrated positions of the
V O L T S /D IV switch. With the V A R control in its calibrated
detent (wiper at ground), the output transistors of U 120,
whose collectors connect to pins 5 and 9 are conducting;
the output transistors whose collectors connect to pins 6
and 8 are biased off. Thus, the current available to the
following amplifier stage is that flowing from pins 5 and 9.
When the V A R control is rotated out of its calibrated
detent, the transistors whose collectors connect to pins 6
and 8 begin to conduct. The current they conduct is robbed
from the other output transistors. This causes two things to
occur:
1.
The current flowing in the collectors of the transis
tors connected to pins 5 and 9 is less now than when the
V A R control was in its calibrated detent.
2.
The current available to the following amplifier stage
is the algebraic sum of the currents in the collectors tied
together (i.e., pins 8 and 9 and pins 5 and 6). Since the two
collectors tied together have signals of opposite polarity,
the signal available to the following amplifier stage is less
than that available when the VA R control was in its
calibrated detent. The component values selected for the
variable function provide a variable attenuation ratio of
approximately 2.5 to 1. The Channel 1 Variable Balance
adjustment R110 adjusts for no trace shift in the display
when rotating the V A R control. The Channel 1 UNCAL
light indicates when the Channel 1 VA R control is out of
its calibrated detent. The components connected between
pins 2 and 3 of U120 provide high-frequency compensation
for the stage.
Second Cascode Amplifier
The second amplifier stage in the Channel 1 Preamplifier
circuit is U 140. U 140 is the same integrated emitter-
coupled, push-pull, cascode amplifier as the one used in
U120. It is used as a push-pull amplifier and contains 2 mV
and 5 mV gain adjustments that determine the overall gain
of the Channel 1 Preamplifier circuit. The output signals
available to the next stage for amplification are taken from
pins 5 and 9 of U 140.
The gain of U140 is determined by biasing the output
transistors connected to pins 6 and 8 to conduct more or
less current. As more current is conducted through the
transistors connected to pins 6 and 8, that much less is
conducted through the transistors connected to pins 5 and
9. The current conducted by the transistors connected to
pins 5 and 9 in the 2 mV position of the V O L T S /D IV
switch is approximately 2.5 times the current conducted by
the same transistors in all of the other V O L T S /D IV switch
positions. This results in increased gain and a DC level shift
at the signal output terminals of U 140. The V O L T S /D IV
switch compensates for a shift in the DC level by shorting
out part of the common-mode resistance (R157) when
operating with 2 mV sensitivity. The Gain Switch Balance
adjustment R135 adjusts the DC balance of the stage, so
there is no baseline shift in the C RT display when switching
between the 2 mV and 5 mV positions of the CH 1
V O L T S /D IV switch. The remainder of the components
connected between pins 2 and 3 of U140 provide high-
frequency compensation for the stage. Resistor-thermistor
R T149 reduces in value with increases in temperature to
compensate for losses in gain associated with increases in
ambient temperature.
Third Cascode Amplifier
The third amplifier stage in the Channel 1 Preamplifier
circuit is a discrete component cascode amplifier made up
of Q 172, Q 178, Q182, Q 184, and Q188. Q184 is a
relatively constant current source for Q172 and Q182,
Q172 and Q182 convert the input voltage signals into
current signals which are in turn converted back to voltage
signals by Q178 and Q188 respectively. R172 and R182
provide thermal compensation and C172 and C182 AC-
couple the signal around R172 and R182 to reduce Miller
effect. C l 75 and R175 are variable high-frequency com
pensation adjustments while C R170, C R174, and R T170
®
3-4
Summary of Contents for 475
Page 6: ...475 475 Oscilloscope ...
Page 20: ......
Page 46: ......
Page 151: ...A B C D E F G Fig 7 4 P 0 A4 Vertical Mode Switch circuit board ...
Page 152: ...n m m m 1 t siumm c ii im ...
Page 154: ...Diagrams 475 A B C D E F G ...
Page 155: ...GRID LOCATOR Fig 7 8 P O A3 Vertical Preamp circuit board ...
Page 157: ...Diagrams 475 ...
Page 158: ...GRID LOCATOR 2 3 4 5 6 7 8 9 1 10 11 1 12 Fig 7 10 P 0 A3 Vertical Preamp circuit board ...
Page 160: ...Diagrams 475 GRID LOCATOR Fig 7 11 A5 Vertical Output circuit board ...
Page 163: ...Diagrams 475 GRID LOCATOR Fig 7 12 P O A8 Trigger Generator And Sweep Logic circuit board ...
Page 167: ... 415 OSCILLOSCOPE B T R IG G E R G E N E R A T O R g B TR IG G E R GEN ER A TO R ...
Page 168: ...Diagrams 475 GRID LOCATOR ...
Page 172: ...SWEEP G E N ER A TO R S ...
Page 175: ...TIM IN G HORIZONTAL DISPLAY SWITCHING ...
Page 177: ...A 011 HORIZONTAL AMPLIFIER 8 l j k J Ll J Ol Z o N C O X U 1 C L O o v O o _J V v P O in r ...
Page 179: ... 476 OSCILLOSCOPE ...
Page 180: ...POWER SUPPLY ...
Page 182: ...CRT CIRCUIT ...
Page 185: ...C A L IB R A T O R A N D F A N C I R C U I T 4 C A LIB R A TO R FAN MOTOR C IR C U IT ...
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Page 190: ...4 T 475 OSCILLOSCOPE FIG 1 FRO NT PANEL ...
Page 191: ...475 OSCILLOSCOPE FIG 2 MAIN FRAME ...
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Page 200: ...FIG 3 RIGHT SIDE l 475 OSCILLOSCOPE ...
Page 201: ...V S X O t l B O T T OVJ 8 8 ...
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Page 210: ...5 ...
Page 211: ...4 4 75 O S C ILLO S C O P E FIG 5 CAB IN ET ...
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Page 241: ... 1 I R I II9A po S IT I o n f i n e I R l 130 PAGE 4 OF 5 R 1234 Ml 9 91 9 473 ...