Tektronix 7A26 User manual

PLEASE CHECK

CHANGE INFORMATION FOR
AT THE REAR O THIS MANUAL .
INSTRUCTION MANUAL
Tektronix,
Inc .
P
.O .
Box
500
Beaverton,
Oregon

97077

Serial
Number
COMMITTED TO EXCELLENCE
070-1484-01

First Printing OCT 1972
Product
Group 42

Revised FEB 1983

Copyright v 1972 Tektronix, Inc .
All rights reserved .
Contents
of this publication may not be reproduced
in any
form
without the written permission of Tektronix, Inc .
Products of
Tektronix, Inc . and its subsidiaries are covered
by U .S . and
foreign patents and/or pending patents .
TEKTRONIX,
TEK, SCOPE-MOBILE, and

°
.A

are
registered trademarks
of Tektronix, Inc . TELEQUIPMENT
is a registered
trademark of Tektronix U .K . Limited .
Printed in U .S .A . Specification
and price change privileges
are reserved .
INSTRUMENT SERIAL
NUMBERS
Each instrument has a serial number on a
parW insert, tag,
or stamped on the chassis . The
first number or letter
designates the country of
manufacture
.
The last five digits
of the serial number are
assigned sequentially and are
unique to each instrument
.
Those
manufactured in the
United States have
six
unique
digits . The country of
manufacture is identified as
follows
:
B000000

Tektronix, Inc ., Beaverton,
Oregon,
USA
100000

Tektronix Guernsey, Ltd .,
Channel Islands
200000

Tektronix United Kingdom, Ltd ., London
300000

Sony/Tektronix, Japan
700000

Tektronix Holland, NV, Heerenveen,
The Netherlands

REVSEP
1981
TA LE O
CONTENTS
PAGE
SECTION 1
OPERATING INSTRUCTIONS
7A26 Features ........ . ...............
. .....................
1-1
Installation . . ........................ .
. . ............... . .... .
1-1
General Operating
Information ..
. ....... . . ............... . .....
1-1
7A26
Front-panel Controls and Connectors
........ . ...... . . .....
1-2
Basic Applications . . ....... . . ........ . .........
. ....... . .....
1-4
SECTION 2 SPECIFICATION
Introduction .... . . .......
. ..................
. ....... . ....... . 2-1
Electrical Characteristics
. . ........ . . ................ . ....... . .
2-1
Environmental
Characteristics ..... . . ........................... 2-2
Physical Characteristics
. ......... . . ....... . ....... . ...........
2-2
WARNING
w
The remaining
portion of this Table of
Contents lists the servicing
instructions .
These servicing instructions are for
use by qualified per-
sonnel only .
To avoid electrical shock, do not
perform any servicing
other than that called
out in the operating
instructions unless qualified
to doso .
SECTION 3 THEORY OF
OPERATION
Introduction . . ....... . .........
. . ..........................
. . 3-1
Block Diagram ..... . . ..........
. ............... .
............ 3-1
Detailed Circuit
Description .... . ................. . . ....... . .... 3-1
SECTION
4 MAINTENANCE
Introduction ...... . .......
. .......... . ....... . .......
. ....... 4-1
Preventive Maintenance
... . . .......................... . ....... 4-1
Troubleshooting ......... .
. ............
. ....... . ....... . .......... . ....... 4-1
Corrective Maintenance
........... . ....... . . ............... . . .
4-3
Troubleshooting Chart ..........
. .......... . .............. . . . .
4-5
Repackaging for Shipment
...... . ................. . ........ . . .
4-8
SECTION5
CALIBRATION
Introduction ....... . . ..........................
. .............
5-1
Performance Check ........ . ........ . ........................
5-1
Test Equipment Required .... . .......
. . ............... . ....... .
5-1
Calibration Procedure .... . ......................... . . ....... .
5-3
Index
to Calibration Procedure ...............
. ...... . .....,. . . .
5-3
SECTION 6
ELECTRICAL PARTS LIST
Parts Ordering Information
Abbreviations and Symbols
SECTION 7
DIAGRAMS AND CIRCUIT BOARD
ILLUSTRATIONS
Symbols
and Reference Designators
SECTION 8
MECHANICALPARTS LIST
Mechanical Parts List Information
Index
of Mechanical Parts Illustrations
Mechanical Parts List
CHANGE
INFORMATION & TEST EQUIPMENT

Fig . 1-1 . 7A26 Dual Trace Amplifier .

7A26 Features
OPE
The 7A26 Dual Trace Amplifier
plug-in unit is designed
for use with
TEKTRONIX
7000-Series Oscilloscopes . The
7A26
is a dual-channel wide-bandwidth
amplifier . Internal
gain
and compensation
circuits are automatically
switched
to correspond to the
setting of the VOLTS/DIV switch .
Channel 2 can be inverted
for differential measurements .
Installation
Introduction
PRELIMINARY INFORMATION
The 7A26
is calibrated and ready for use as
received . It
can be installed
in any compartment of
TEKTRONIX
7000-series
oscilloscopes, but is intended
for principal use
in vertical plug-in
compartments . To install,
align the upper
and lower
rails of the 7A26 with the
oscilloscope tracks and
fully insert it .
The front will be flush with
the front of the
oscilloscope
when the 7A26 is fully inserted,
and the latch
at the
bottom-left corner of the 7A26
will be in place
against
the front panel . See Fig . 1-2 .
To remove the
7A26, pull on the latch
(which is
inscribed with
the unit identification
"7A26") and the
7A26 will
unlatch . Continue
pulling on the latch to slide
the 7A26
out of the oscilloscope
.
GENERAL
OPERATING
INFORMATION
For
single-trace operation,
either of the two identical
amplifier channels can be used
independently by setting the
DISPLAY
MODE and TRIGGER
SOURCE switches to CH
1 or CH 2
and connecting the
signal to be observed to the
appropriate
input . In the discussions
to follow, single-trace
operations
using CH 1 only apply
equally to CH 2 only
.
Signal
Connections
In
general, probes offer
the most convenient means
of
connecting a signal to
the input of the 7A26 .
A 10X
attenuation probe
offers a high input impedance
and allows
the circuit under test to
perform very close to
normal
operating conditions .
Any TEKTRONIX probe,
with a readout coding ring can
be
used with TEKTRONIX
7A-series amplifier units
equipped
with readout . 'The readout
coding ring on the
probe
connects to a circuit
in the amplifier unit
which
automatically corrects the
readout displayed on the
CRT to
the
actual deflection factor at
the tip of the probe
being
ATIN
INSTRUCTIONS
used . For probes to be
used with amplifier units with or
without readout, see the
Tektronix, Inc . catalog .
Vertical Gain Check
and Adjustment
To check the gain of
either channel, set the
VOLTS/DIV
switch to 10 mV and
connect 40 millivolts, 1
kilohertz
signal from the
oscilloscope calibrator to the input
connec-
tor of the
channel being checked .
The vertical deflection
should
be exactly four divisions .
If not, adjust the
front-panel GAIN for exactly four
divisions of deflection .
The GAIN adjustment is
engaged by pressing in the
GAIN
control knob and
turning the knob with a
narrow-blade
screwdriver
(see Fig . 1-3 Front
Panel Controls and
Connectors)
. 'Turn the knob clockwise,
then counter-
clockwise,
until the GAIN control is
engaged . When the
GAIN control is engaged, the vertical
deflection will change
as
the knob is turned . Turn the
GAIN control knob with
the screwdriver until the
deflection is set to exactly
four
divisions, then remove the screwdriver
.
Input Coupling
The Channel 1 and
Channel 2 coupling
(AC-GND-DC)
switches allow a
choice of input coupling
methods . The
type of display
desired and the applied signal will
determine
the coupling to
use .
The DC
coupling position must be used to
display the
DC component of
the signal . It must also be used to
display
AC signals
below about 30 hertz (ten hertz with
a 10X
probe) and square
waves with low-frequency components as
these signals are
attenuated in the AC position .
Fig . 1-2
. Release latch .
Section
1--7A26

Operating Instructions-7A26
CH
1 Input
Connector
Provides signal
connection to CH 1 . ---,
CH 2 POLARITY
Provides means of inverting the
CH 2 display .
Provides a means of limiting
the
upper bandwidth .
CH 2 POSITION
Controls position of the CH 2
trace only Idisabled in ADD)
.
Deflects trace about 0 .2 division
for trace identification . In instru-
ments with Readout, also replaces
readout with the
word
"IDENTIFY" .
CH 1 POSITION
Controls position of the CH 1
trace and "ADD" display mode
trace .
CH
2
Input Connector
Provides signal connection to Ch 2 .
Fig . 1-3 . 7A26 front-panel controls
and connectors .
Selects source of the trigger
signal .
Selects the mode
of operation .
Provides continuously variable
uncalibrated settings between
calibrated steps .
When the VARIABLE control is
pushed in,
it becomes a front
panel screwdriver
adjustment for
calibration
of deflection factor .
AC-GND-DC (same as CH 1)
Selects calibrated deflection
factors from 5 mV/Div to
5
V/Div
; ten steps in a 1-2-5
sequence .
Selects signal input coupling
mode .

In the AC coupling position, the DC component
of the
signal is blocked by a capacitor in the input circuit . The AC
coupling position provides the best display of signals with a
DC component much larger than the AC components . The
precharge feature should be used with large DC
inputs
.
To
use this feature, first set the coupling to
GND .
Connect
the
probe to
the circuit
and
wait
about
two seconds for the
coupling capacitor to charge . Then set the coupling to AC .
The GND position provides a ground reference at the
input of the amplifier without externally grounding the
input connectors . However, the signals connected to the
inputs are not grounded, and the same DC load is
presented
to the signal source .
VOLTS/DIV
and VARIABLE Controls
The
amount of
vertical deflection producedby a signal is
determined by the signal amplitude, the attenuation factor
of the probe, the setting of the VOLTS/DIV switch, and
the setting of the VARIABLE control . Calibration deflec-
tion factors indicated by the settings of the
VOLTS/DIV
switch apply only when the
VARIABLE control is in the
calibrated (CAL
IN) position .
The VARIABLE control provides variable, uncalibrated
settings
between the calibrated steps of the VOLTS/DIV
switch . With the VARIABLE control fully counter-
clockwise and the VOLTS/DIV set to 5 volts/division the
uncalibrated vertical deflection factor is extended to at
least 12 .5 volts/division . By applying a calibrated voltage
source to the input connector, any specific deflection
factor can be set within the range of the
VARIABLE
control .
CH
2 POLARITY Switch
The
CH 2
POLARITY switch may be used to invert the
displayed waveform of the
signal applied to the CH 2 input .
This is particularly
useful in
added operation of the
7A26
when differential
measurements are
to be
made . The CH 2
POLARITY switch has two positions, +UP and INVERT . In
the +UP position, the displayed waveform will have the
same polarity as the applied signal and a positive DC voltage
will move the CRT trace up . In the INVERT position, a
positive-going waveform at the CH 2 input will be displayed
on the CRT in inverted form and a positive DC voltage will
move
the trace down
.
DISPLAY MODE Switch
For single-trace operation, apply the signal either to the
CH 1 input or the CH 2 input and set the DISPLAY MODE
switch to the corresponding position : CH 1 or CH 2 .
REV, B, MAR . 1975
Operating Instructions-7A26
To display a
signal in one channel independently
when a
signal is also applied to the other channel, simply select the
desired channel by setting the DISPLAY MODE switch to
the appropriate CH 1 or CH 2 position .
Alternate Mode . The ALT position of the DISPLAY
MODE switch produces a display which alternates between
channel 1 and channel 2 with each sweep on the CRT .
Although the ALT mode can be used at all sweep rates, the
CHOP mode provides a more satisfactory display at sweep
rates below about 0 .5 millisecond/division . At slow sweep
rates alternate mode switching becomes visually percep-
tible .
Add Mode . The ADD
position of the
DISPLAY MODE
switch can
be
used to display the sum or difference of two
signals, for
common-mode rejection
to
remove
an
undesired
signal
. The overall deflection factor in the ADD mode with
both VOLTS/DIV switches set to the same position is the
deflection factor indicated by either VOLTS/DIV switch .
However, if the CH 1 and CH 2 VOLTS/DIV switches are
set to different deflection factors,
the
resultant
amplitude
is difficult to determine from
the CRT display
.
In this case,
the
voltage
amplitude of the resultant display can be
determined accurately only if the amplitude of the signal
applied to one channel is known . In the ADD
mode,
positioning of the trace is controlled by
the channel 1
POSITION control
only .
Chop Mode . The CHOP position of the DISPLAY
MODE switch produces a display
which
is
electronically
switched between
channels
at
approximately a
500
kilo-
hertz rate (controlled by
mainframe) . In general the CHOP
mode provides the best display at
sweep rates slower than
about
0 .5 millisecond /division or whenever dual-trace,
non-repetitive phenomena is to be displayed .
TRIGGER SOURCE Switch
CH
1 . The
CH 1
position of the TRIGGER SOURCE
switch provides a
trigger signal obtained from the signal
applied to the
CH
1
input connector . This provides a stable
display of the
signal applied to the CH 1 input connector .
CH 2 . The CH 2 position of the TRIGGER SOURCE
switch provides a trigger signal obtained from
the signal
applied to the CH 2 input
connector . This provides a stable
display of the signal
applied to the CH 2 input connector .
MODE . In this
position of the TRIGGER SOURCE
switch, the trigger signal
for the time-base unit is dependent
on the setting of
the DISPLAY MODE switch . The trigger

Operating Instructions-7A26
Trace Identification
When
the
IDENTIFY
button is pressed, the trace is
deflected
about 0 .2 division to identify the 7A26 trace .
This feature is particularly useful when multiple traces are
displayed . When the IDENTIFY
button is pressed on instru-
ments
with readout, the deflection factor readout is
replaced with the word "IDENTIFY" .
BW Switch
Provides a means of limiting the upper
bandwidth .
FULL : Allows the 7A26 to operate at full bandwidth .
20
MHz :
Reduces the upper bandwidth of the 7A26 to about
20 megahertz .
General
BASIC
APPLICATIONS
The following
information describes the procedures and
techniques for making
basic measurements with a 7A26 and
the
associated
TEKTRONIX
oscilloscope and time-base .
These
applications are not described in detail since each
application must be
adapted to
the
requirements of the
individual measurements . This instrument can
also be used
for many applications not described in
this manual
.
Contact
your local TEKTRONIX Field Office or
representative for
assistance in making specific measurements with
this
instrument .
Peak-to-Peak Voltage Measurements (AC)
To make peak-to-peak voltage measurements, use the
following procedure :
2 . Set the
DISPLAY MODE
and TRIGGER SOURCE
switches to
display the channel used .
1 . Apply the signal to either input connector .
3 . Set the coupling switch
to
AC .
5 . Set the time-base Triggering controls for a stable
display . Set the time-base unit to a sweep rate which
displays several cycles of the waveform .
6 .
Turn the
7A26 POSITION control so the lower
portion of the waveform coincides with one of the graticule
lines below the center horizontal line, and the top of the
waveform is within the viewing area . With the time-base
Position control, move the display so one of the upper
peaks lies near the center vertical line (see Fig . 1-4) .
7 . Measure the divisions of vertical deflection peak-to-
peak . Check that the
VARIABLE
(VOLTS/DIV) control is
in the CAL IN
position
.
NOTE
This technique can also be used to make measure-
ments between two points on the waveform, rather
than peak to peak .
8 . Multiply the deflection measured in
step 7 by the
VOLTS/DIV switch setting . Include the
attenuation factor
of the probe if used .
Position To-
T
Centerline
Fig . 1-4 . Measuring the peak-to-peak voltage of a waveform .
source for
as follows :
MODE
each position of the DISPLAY MODE switch is
TRIGGER SIGNAL SOURCE
NOTE
For low-frequency signals belowabout
30 hertz use
the DC position to prevent attenuation of the signal .
CH
1 Channel l
CH 2 Channel 2
ADD
Algebraic sum of channel 1 and channel 24
. Set the VOLTS/DIV switch
to display about
five
CHOP Algebraic
sum of channel 1 and channel 2
divisions of the waveform vertically .
ALT Alternates between
channel 1 and channel 2

EXAMPLE
: Assume
that
the peak
to
peak vertical
deflection is
4
.5
divisions (see Fig . 1-4) using a 1OX
attenuator probe,
and the VOLTS/DIV switch is set to 1 V .
Volts
vertical
VOLTS/DIV
probe
Peak to Peak

deflection

X

setting

X
attenuation
(divisions)

factor
Substituting the
given values :
Volts
Peak-to-Peak = 4 .5 X 1 X 10
The
peak-to-peak
voltage is 45 volts .
Instantaneous Voltage Measurements
(DC)
To measure the DC
level
at a
given point on a waveform,
proceed as follows :
1 . Connect the signal to either
input connector .
2 . Set the DISPLAY MODE and TRIGGERSOURCE
switches to
display the channel used .
3 . Set the VOLTS/DIV switch to display about five
divisions of the waveform .
4 . Set the coupling switch to GND and position the
trace to the bottom graticule line or other reference line . If
the voltage is negative with respect to ground, position the
trace to the top graticule line . Do not move the POSITION
control after this reference line has been established .
NOTE
To measure a voltage level with respect to a voltage
other than ground, make the following changes to
step
4 . Set the coupling switch to DC and apply the
reference voltage to the input connector . Then
position the trace to the reference line .
5 . Set the
coupling switch to DC . Theground reference
line can be
checked at any time by switching to the GND
position
.
6 . Set the time-base Triggering controls
for
a
stable
display . Set the time-base sweep rate for an
optimum
display of the waveform .
Operating Instructions-7A26
7 . Measure the distance in divisions between
the refer-
ence line and the point on the waveform at which
the DC
level is to be measured . For example, in Fig . 1-5 the
measurement is between the reference line and point
A .
8 . Establish the polarity of the waveform .
With the CH
2 POLARITY switch in the +UP position, any point
above
the reference line is positive .
9 . Multiply the distance
measured in step 7 by the
VOLTS/DIV setting . Include the attenuation factor of the
probe, if used .
EXAMPLE
:
Assume
the vertical distance measured is
3 .6
divisions (see Fig . 1-5) and the waveform is above the
reference line using a 1OX probe with a VOLTS/DIV setting
of 0 .5 V .
Using the formula :
Instan- vertical

VOLTS/ probe
taneous = distance X polarity X

DIV

X attenuation
Voltage (divisions)

setting factor
Substituting the given values :
The instantaneous voltage is 18 volts .
Instantaneous

3 .6 X +1 X 0 .5
V X 10
Voltage
Fig . 1-5 . Measuring
instantaneous voltage with respect to some
reference .

Operating Instructions-7A26
ComparisonMeasurements
In some applications it may be desirable to establish
arbitrary units of measurement other than those indicated
by the VOLTS/DIV switch
. This
is particularly useful when
comparing unknown signals to a reference amplitude . One
use for the comparison-measurement technique is to facili-
tate calibration of equipment where the desired amplitude
does not produce an exact number of divisions of deflec-
tion . The adjustment will be easier and more accurate if
arbitrary units of measurement are established so that the
correct adjustment is indicated by an exact number of
divisions of deflection . The following procedure describes
how to establish arbitrary
units
of
measure
for
comparison
measurements .
To establish an arbitrary vertical deflection factor based
upon a specific reference
amplitude,
proceed as follows :
1 . Connect the reference signal to the input connector .
Set the time-base unit sweep
rate to display several cycles
of the signal .
2 . Set the VOLTS/DIV switch and the VARIABLE
control to produce a display which is an exact number of
vertical divisions in amplitude . Do not change the
VARIABLE control after obtaining the desired deflection .
3 . To establish an arbitrary vertical deflection factor so
the amplitude of an unknown signal can be measured
accurately at any setting of the VOLTS/DIV switch, the
amplitude
of the reference signal must be known . If it is
not
known, it can be measured before the VARIABLE
VOLTS/DIV
control is set in step 2 .
4 . Divide the amplitude
of
the
reference signal (volts)
by the product of the vertical deflection (divisions)
established in step 2
and
the setting
of
the
VOLTS/DIV
switch . This
is
the vertical conversion
factor .
5 .
To
measure the amplitude of an unknown signal,
disconnect the reference signal and connect the unknown
signal to the input connector . Set the VOLTS/DIV switch
to a setting that provides sufficient vertical deflection to
make an accurate measurement . Do not readjust the
VARIABLE control .
Vertical
Conversion
Factor
reference signal
__amplitude (volts)

_
vertical VOLTS/DIV
deflection X switch
(divisions) setting
6 . Measure the vertical deflection in divisions
and
calculate the amplitude of the unknown signal using the
following formula .
Signal VOLTS/DIV
vertical vertical
Amplitude setting
X conversion X deflection
factor (divisions)
EXAMPLE : Assume a reference signal amplitude of 30
volts,
a
VOLTS/DIV setting of 5 V and the VARIABLE
control adjusted to
provide a vertical deflection of four
divisions . Substituting these values in the vertical conver-
sion factor formula (step 4) :
Vertical
Conversion
Factor
30 V_
4X5V
Then with a VOLTS/DIV setting of 2 V, the peak-to-
peak amplitude of an unknown signal which produces a
vertical deflection of five divisions can be determined by
using the signal amplitude formula (step 6) :
Signal

=

2
VX
1 .5 X 5 = 15 volts
Amplitude
Dual-TracePhase Difference Measurements
Phase comparison between two signals of the same
frequency can be made using the dual-trace feature of the
7A26 . This method of phase difference measurement can
be used up
to the
frequency limit of the oscilloscope
system .
To
make the comparison, use the following
procedure
1 . Set the CH 1 and CH 2 coupling switches to the same
position, depending on the type of coupling desired .
2 . Set the DISPLAY MODE to ALT or CHOP . In
general, CHOP is more suitable for low frequencies and
ALT is more suitable for high frequencies . Set the
TRIGGER SOURCE to CH 1 .
3 . Connect the reference signal to the CH 1 input and
the comparison signal to the CH 2 input . Use coaxial cables
or probes which have similar time delay characteristics to
connect the signals to the input connectors .
4 . If the signals are of opposite polarity, set the CH 2
POLARITY switch to invert the channel 2 display . (Signals
may be of opposite polarity due to 180 ° phase difference ;
if so, take this into account in the final calculation .)

5 . Set the VOLTS/DIV switches and the
VARIABLE
controls of the two
channels
so the displays are equal and
about
five divisions in amplitude .
6 . Set the time-base unit to a sweep rate which displays
about one cycle of the waveforms . Set the Triggering
controls for a stable display .
7 . Center
the
waveforms on the graticule with the 7A26
POSITION controls .
8 . Adjust the time-base Variable Time/Div
control until
one cycle of the reference signal occupies exactly eight
horizontal divisions between the second and tenth vertical
lines of the graticule (see Fig . 1-6) . Each division of the
graticule represents 45 ° of the cycle (360 ° - : 8 divisions =
45 0 /division) . The sweep rate can now be stated in terms of
degrees as 45 ° /division .
9 . Measure the horizontal difference between corre-
sponding points on
the waveform .
10 . Multiply the measured distance (in divisions)
by
45 0 /division to obtain the exact amount of phase
difference .
EXAMPLE : Assume a horizontal difference of 0 .3
division with a sweep rate of 45 ° /division as shown in Fig .
1-6 .
Fig .
1-6 .
Measuring phase difference between
two
signals
.
Using the formula :
horizontal
Phase Difference

= difference
X

sweep rate
(divisions)
(degrees/division)
Substituting the given values :
Phase Difference = 0 .3 X 45 0
The phase difference is 13 .5 0 .
High Resolution Phase Measurements
More accurate dual-trace phase measurements can be
made by increasing the sweep rate (without changing the
Variable
Time/Div control) . One
of
the easiest
ways to
increase the sweep rate is with the time-base Magnifier
switch . Set the Magnifier
to
X10 and determine the
magnified
sweep
rate
by dividing the sweep rate obtained
previously
by
the amount of sweep magnification .
EXAMPLE : If the sweep rate is increased 10 times by
the Magnifier, the magnified sweep rate is 45 0 /division =10
= 4 .5 0 /division . Fig . 1-7 shows the same signals as used in
Fig . 1-6 but with the Magnifier set to X10 .
With
a
horizontal difference of 3 divisions, the phase
difference is :
horizontal
magnified
Phase Difference

= difference X

sweep rate
(divisions)
(degrees/division)
Substituting the given values :
Operating
Instructions-7A26
Phase Difference = 3 X 4 .5 0
The phase difference
is 13 .5 ° .
Fig . 1-7 . High resolution phase measurement using time-base
magnifier .

Operating Instructions-7A26
Common Mode Rejection
The ADD feature of the 7A26 can be used to display
signals which contain undesirable components . These
undesirable components can be eliminated through
common-mode
rejection
.
The procedure is as follows :
1 . Set the DISPLAY MODE switch
to
ALT
or CHOP
and the TRIGGER SOURCE switch to MODE .
2 . Connect the signal containing both the desired
and
undesired
information to the CH 1 input connector .
3 . Connect a signal similar
to
the
unwanted portion of
the CH 1 signal to the CH 2 input connector . For
example,
in Fig .
1-8 a line-frequency signal is connected to Channel
2
to cancel out the line-frequency component of the Channel
1
signal .
4 . Set both coupling switches to the
same setting, DC or
AC, depending on the applied signal .
5 . Set the VOLTS/D IV switches so the
signals are about
equal in amplitude .
6 . Set the DISPLAY MODE switch to ADD . Set the CH
2 POLARITY switch to INVERT so the common-mode
signals are of opposite polarity .
7 . Adjust the Channel 2 VOLTS/DIV switch and
VARIABLE control for maximum cancellation of the
common-mode signal . The signal which remains should
be
only the desired portion of the channel 1 signal .
EXAMPLE : An example
of this mode of operation is
shown in Fig . 1-8
.
The signal applied to
Channel 1 contains
unwanted line frequency
components (Fig . 1-8A) . A
corresponding line frequency signal is connected
to Channel
2 (Fig . 1-8B) . Fig . 1-8C shows the desired
portion of the
signal as
displayed when common-mode rejection
is used .
The above procedure can
also be used for examining a
signal superimposed onsome DC level
when DC coupling is
used . A DC voltage of the proper
polarity applied to
Channel 2 can be used to cancel out the
DC portion of the
signal applied to Channel 1 .
Fig
.
1-8 . Using the ADD mode for common-mode rejection
.
(A)
Channel 1 signal contains desired information along with line-
frequency component . (B) Channel 2 contains line frequency only .
(C) Resultant CRT display using common-mode rejection .

Introduction
The following electrical characteristics are valid over the stated environmental range for instruments calibrated at an
ambient temperature of +20 ° C to
+30
°C, and after a five-minute warmup unless otherwise noted .
Characteristic
Deflection Factor
Calibrated Range
Deflection Factor Accuracy
Uncalibrated (VARIABLE)
GAIN Range
BW FULL/20 MHz Gain Match
SPECIFICATION
TABLE 2-1
Electrical
Performance Requirement
5 mV/Div to 5 V/Div ; ten steps in a 1,2,5
sequence .
Within 2% with GAIN adjusted at 10 mV/Div .
20 MHz gain within 1% full gain .
Frequency Response
Upper Bandwidth
R iset i me
Maximum Input Voltage
DC Coupled
AC Coupled
REV D NOV 1979
Depends upon oscilloscope used .
Supplemental Inf ormatio n
Aberrations ; 5 mV and 10 mV . +4 .0%, -4 .0%,
total
not
to
exceed 6% P-P ;
20 mV to 5 V, not
to exceed 8% .
Continuously variable between calibrated steps ;
extends deflection factor to at least 12 .5 volts
per division .
Permits adjustment of deflection factor for
calibrated operation with all 7000-series oscillo-
scopes .
See the oscilloscope
mainframe specifications
or the current
Tektronix, Inc . catalog .
20 MHz, ±3 MHz .
21 ns maximum .
250 volts, (DC
+
Peak AC) ; AC
component500
volts peak-to-peak maximum, one kilohertz or
less .
less .
Section 2-7A26
500 volts, (DC
+
Peak AC) ; AC component 500
volts peak-to-peak maximum, one kilohertz or
50 :1 display ratio up to 200 megahertz .

Specification-7A26
2-2
SN 8208879-below
SN B208880-up
Capacitance
SN B208879-below
SN 8208880wp
Overdrive Recovery Time
Display
Modes
Delay Time Difference
Between Channels
Common Mode Rejection Ratio
Trigger Source Selection

Channel 1 only .
At least 10 :1, DC to 50 MHz .
Channel 1 only .
Dual-trace, alternate between channels .
Added
algebraically .
Dual-trace chopped between channels .
Channel 2 trace only .
Channel 2 only .
Size
Weight
Follows DISPLAY MODE selection .
TABLE 2-3
1 M E2

2% .
1 MA

2% .
TABLE 2-2
Environmental Characteristic
Refer to the specification for the associated mainframe
.
-----------
Fits all 7000-series plug-in compartments .
2 _ Pounds 9Ounces 1 .2 kiloqra ams~ . __ 0 _
20 pF ± 02 pF
22 pF ± 02 pF
0 .1 ms or less to recover to within one
division
after removal of overdrive signal of up to +75
divisions or -75 divisions regardless of over-
drive signal duration .
200 picoseconds or less .
REV B
NOV 1979

WARNiNG

Introduction
This section of the manual contains
adescription of the
circuitry used in the 7A26 amplifier . The
7A26
description
begins with a discussion of the instrument using the
block
diagram shown in the Diagrams section . Then, each circuit
is described in detail
using the
block diagram to
show the
interconnections between stages in each major circuit and
the relationship of the front-panel controls to the individual
stages .
Complete schematics of each circuit are given in
the
Diagrams section . Refer to these schematics throughout the
following circuit description for electrical values and
relationship .
BLOCK
DIAGRAM
The Channel 1 Amplifier circuit provides gain setting,
variable gain control, and trace positioning . The Channel 2
Amplifier provides signal polarity inversion in addition to
gain setting, variable gain
control, and trace positioning .
The signal to be displayed on the CRT is applied to the
CH 1 or CH 2 INPUT connector . The signal passes through
the input coupling switch, where the appropriate
coupling
is selected, to the attenuators . The VOLTS/DIV switch
selects the correct amount of attenuation and
the signal is
passed to the Input Source
Follower .
When the VOLTS/DIV switch is set to the 5 mV and
10 mV positions, the signal connected to the INPUT
connector is passed through the attenuators without attenu-
ation . When the VOLTS/DIV switch is set in the 5 mV
position, the 2X Gain Amplifier operates at full gain . In all
other positions of the VOLTS/DIV switch, the 2X Gain
Amplifier's gain is reduced by two . Internal gain and
balance adjustments are included in the 2X
Amplifier
.
Overall GAIN and VARIABLE gain is adjusted in the
Gain Amplifier . Variable Balance and high frequency
adjustments are also controlled in the Gain Amplifier . The
output of the Gain Amplifier is connected to the Posi-
tioning circuitry where the POSITION and
IDENTIFY
functions are controlled . Channel 2 is identical to Channel
1, with the exception of the polarity inversion function in
Channel 2 .
REV
.
B,
MAR . 1975
THEORY OF OPERATION
Section 3-7A26
The Display and Trigger Channel switch amplifiers
provide differential signal outputs for the signal and trigger
lines, from each channel, to a common display and trigger
output .
These stages also contain a bandwidth limiter that
limits the upper frequency response to 20 megahertz .
The output of the Display and Trigger Channel switch
Amplifier is connected to the oscilloscope mainframe via
the interface connector .
Readout encoding circuitry used in the 7A26 is standard
to the 7000-series .
DETAILED CIRCUIT DESCRIPTION
AC-GND-DC Switch
Input Attenuator
NOTE
The CH 1 and CH 2 amplifier circuits are identical
with the exception of the CH 2 GAIN stage 02450,
which includes a POLARITY inverting circuit . Only
CH 1 is described in detail throughout this discussion .
Input signals connected to the INPUT connectors can be
AC-coupled, DC-coupled,
or internally disconnected .
S100A is a cam-type switch ; a contact-closure chart
showing the operation is given on the schematic diagrams .
When the AC-GND-DC switch is in the DC position, the
INPUT signal is connected directly to the attenuators . In
the
AC
position, the
INPUT
signal passes through capacitor
C10 .
The
capacitor
prevents the
DC component of the
signal from passing to the amplifier . The GND position
opens the signal path and connects the input circuit of the
amplifier to ground . This provides a ground reference
without
the
need to disconnect the applied signal from
the
INPUT connector . Resistor R102, connected
across
the
AC-GND-DC switch, allows C10
to
be pre-charged in the
GND
position
.
The effective overall deflection factor of the 7A26 is
determined by the setting of the VOLTS/DIV switch,
S100B . The basic deflection factor is 5 millivolts per
division of CRT deflection . To increase the basic deflection

Theory of Operation--7A26
factor to the values indicated on the front panel, precision
attenuators are switched into the circuit . S10013 is a
cam-type switch and the dots on the contact-closure chart
(see
Diagram 1) indicate when the associated contacts are in
the position shown (open or closed) . In the 5 mV/Div and
10 mV/Div positions, the attenuators are not used ; the
input
signal is connected directly to the Input Source
Follower . The
10 mV/Div position decreases the gain of the
2X Gain Amplifier . For switch positions
above 10
mV/Div,
the attenuators are switched into the circuit
singly or
stacked to produce the deflection
factor indicated on the
front panel . These hybrid attenuators are frequency-
compensated voltage dividers . For DC and low-frequency
signals, the
attenuators act as resistance dividers ; at high
frequencies
the attenuator acts as a capacitive divider .
In addition to providing constant
attenuation at all
frequencies within the bandwidth of the instrument, the
input attenuators are designed to maintain the same input
RC characteristics (one megohm X 20 pF) . Each attenuator
contains an adjustable series capacitor to provide correct
attenuation at high frequencies, and an adjustable shunt
capacitor to provide correct input capacitance .
Input Source Follower
Below SN B080000
. Q1 50A
and
Q140
form
a cascode
amplifier with
Q150B
providing
constant current . 8132
limits the current drive to the gate of Q1 50A . Dual-diode
CR130 provides circuit protection by limiting the voltage
swing at the gate of 0150A to about 4 :9 volts . 8134,C130,
and
the capacitance of R130 provide low frequency com-
pensation . Input capacitance for the 5 mV and
10
mV
positions is set by
C134
. The
output of the 2X Gain Amplifier
(01350) is from the source of Q1 50A and high frequencies
from the collector of
Q140 . 8160 is used to balance the
input to the 2X Gain Amplifier between the 5 mV and
10 mV positions .
SN B080000-Up
.
Q150A is a source follower with
01508 providing a constant current
. 8132 limits the
current drive to the gate of Q150A . Dual-diode
CR130
provides circuit protection by limiting the voltage swing at
the
gate of Q150A to about ± :10 volts . C130, C134, and the
capacitance of R130 provides low frequency
compensation .
Input capacitance for the 5 mV and 10 mV positions is set by
C130 . The output of the 2X Gain Amplifier (01350)
is
from
the source of Q150A . C134 and R134
form a negative
resistance network for Q1 50A .
2X Gain Amplifier
T1301 is a balun transformer which provides differential
drive to U1350 at high frequencies . 01350 is a paraphase
type amplifier with dual differential output capabilities .
In the 5 mV position, full drive is provided from
pins 5
and 9 of U1350 to the U1450 load resistors
81401 and
3-2
81403 . In all other attenuator
positions the signal path
drive current through the load resistors is divided in half .
The other half is diverted through pins 6 and 8 of 01350
and is dissipated in dummy load resistors 81343 and
81341 .
CR1319 and 81319
maintain proper collector voltage
while switching between the 5 mV and 10 mV positions .
C1331,81331, C1332 and 81332 are thermal compensa-
tions . C1334 and RT1334 provide high-frequency tempera-
ture compensation . 81336, C1336, C1345, L1345 and
81345 are high-frequency adjustments
.
Fixed length inductors and
capacitors are part of the
Amplifier etched circuit
board and provide T-coil peaking
at
the input
of 01350 .
Gain Amplifier
01450 is a variable-gain cascode amplifier which sets the
overall channel
gain . The
GAIN
(R1423A) and VARIABLE
(R1423B)
controls determine the ratio of base currents
through pins
11 and 12 of 01450 . The base-current ratio
determines the shared collector
output levels betweens pins
5,6and8,9 .
81436 provides adjustable low frequency
compensation .
Fixed components 81434, C1434, C1436, 81431,and
C1431 are thermal compensations . 81435 and C1435 are
adjustable high frequency compensations . 01450 Input
T-coil
peaking inductors and capacitors are part of the
etched
circuit board . DC balance over the variable range is
adjusted by 81353
.
Position Circuit
Positioning current is added to the signal current
of
01450 outputfrom the current sources
01470
and
01490 .
81465 controls the voltage at the bases of the
current
sources, which in turn determines the amount of posi-
tioning current
added . 81467, 81466, and CR1465 provide
trace shift current for the
IDENTITY function .
Display Channel Switch Amplifier
The third cascode amplifier, 01550 is used for con-
trolling the channel
1 display modes . When the DISPLAY
CH 1 ON Level at pin 12 is H I, the channel
1
signal
passes
through the transistor pair with outputs at pins 5 and 9 to
the level shifters . At the same time the DISPLAY
CH 1 OFF Level at pin 11 is LO, turning off the second
transistor pair collectors,
pins
6
and 8 . When pin 12 is HI,
channel 1 is displayed and when pin 11 is HI channel 1 is
not displayed . Pins 11 and 12 are always in opposite states,
the levels being selected by the DISPLAY MODE switch
S30A . The signal is routed through T-coiled bases of 01550
to the trigger amplifier switch, 01750, which is also
T-coiled . C1531 and 81531 are high frequency adjust-
ments .
REV . C, MAR . 1975

Trigger Channel Switch Amplifier
U1750 is a cascode amplifier used as the trigger switch,
and operates similarly to the Display Channel Switch
Amplifier,
U1550
. The TRIGGER SOURCE switch, S30B
determines the base levels on pins 11 and 12 of U1750 for
trigger selection .
Display and Trigger Common Base Level Shifters
The Display Out Common Base Level Shifters, Q820,
0840,Q860, and Q880 are used to return the DC signal
level to zero volts at the
plug-in
interface for display
output .
Bandwidth selection is obtained by controlling base
currents with the BW switch, S32, Q820 and Q840 are
shifters at FULL BANDWIDTH and 0860, Q880 are used
at 20 megahertz . The level shifters also serve as a current
summing point for CH 1 and CH 2 selection .
The pi filter is used in
the collectors of
Q860
and Q880 .
The pi filter is isolated
from the
output by
CR860 and
CR880 when the BW switch is
in FULL .
The
Trigger Output
Common Base Level Shifters 0920,
0940, Q960, and Q980 operation is similar to the Display
Output
Shifters just discussed .
Channel 2 Gain-Polarity Amplifier
CH 2 operation is the same as CH 1 . For circuit number
reference the prefix number for CH 1 is 1 and CH 2 is
2 .
For instance, U2350 functions in CH 2 the same as does
U1350 in CH 1 . In CH 2 a Polarity feature is
included in
the second
cascode amplifier U2450 . S22A allows base
drives to be reversed to
U2450 . Polarity Gain R2411,
matches the gain in both polarity positions .
Translator
A schematic of the Translator circuit is shown on
Diagram 4 in the Diagrams section . The translator, Q1050
and Q1070, increase the CHOP and ALT control logic DC
levels from the mainframe to a usable level
in the
7A26 .
CR1060 and CR1062 keep Q1050 and Q1070
from going
into saturation .
Readout
Encoding
The Readout
Encoding circuit consists of switching
resistors
and probe sensing stage Q620 . This circuit encodes
the Channel 1 and 2, Row and Column output lines for
readout of deflection factor, uncalibrated deflection factor
(VARIABLE) information, and signal inversion (Channel 2
REV .
B,
DEC
. 1974
Theory of Operation--7A26
only) . Data is encoded on these output lines by switching
resistors between them and the time-slot input lines, or by
adding current through Q620 .
R647-CR647 are switched between time-slot three
(TS-3) and Column output line when the CAL IN switch is
in the uncal position . This results in the symbol
> (greater
than) being displayed preceding
the
deflection factor
readout . R648 (Channel 2 only) is switched between TS-2
and the Column output line when the CH 2
POLARITY
switch is in the
INVERT
position .
This results in the
symbol
I (inverted) being displayed preceding the deflec-
tion factor readout .
Switching resistors are used to indicate the setting of
the
VOLTS/DIV switch to the mainframe readout system
. The
dots on the contact-closure chart (see Diagram
Section)
indicate when the associated contacts on the
VOLTS/DIV
cam switch are
closed .
R633,R634,
and R635 select the
number 1, 2, or 5 depending on the combination that is
switched in . R368 and R642 select the m (milli) prefix in the
5 mV through 0 .5 V (500 mV) positions of the VOLTS/DIV
swtich . R639 andR643 select the V (volts)
symbol in all
ranges . R630,
R631, and the output
of
the probe sensing
stage (Q620) select the decimal point (number of zeroes),
again depending on the resistor combinationswitched in by
the VOLTS/DIV switch .
Probe sensing stage
Q620 identifies the attenuation
factor of the
probe connected to the input connector by
sensing the amount of current flowing from the current
sink through the probe coding resistance . The output of
this circuit corrects the mainframe readout system to
include the probe attenuation factor .
The third contact of
the input connector
provides the input to the probe sensing
stage
from the probe coding resistance (coded probes only ;
see Operating
Instructions)
.
The third contact is also used
for the
IDENTIFY input . The coding resistor forms a
voltage divider with R621 through CR621 to the -15 V
supply . The resultant voltage sets the bias on Q620 and
determines, along with emitter resistor R622, the collector
current . When the -15 volt time-slot pulse is applied to
Interface Connector B33, Q620 is interrogated and its
collector current is added to the column current output
through Interface Connector A37 .
With a 1 X probe (or no probe)
connected
to
the input
connector, Q620 is
turned off . The deflection factor
readout is
determined
by the
VOLTS/DIV switch position .
With a 10X probe
connected, the bias on Q620 will allow
100 microamperes of collector current to
flow
.
This
increases the deflection factor readout by a factor of 10 .
3-3

Theory of Operation--7A26
The IDENTIFY button (S1465 on Diagram 2 or S2465
on Diagram
3)
does two things when pressed :
1 . It causes the trace
representing the
appropriate
channel
of
the 7A26 to move about 0 .3 division (see the
front
panel
controls
and
connectors, Fig . 1-3) .
2 . It forward biases CR621
and Q620 to result in a
sufficient amount of collector current
which, when added
to the column current output, replaces
the deflection factor
readout with the word "IDENTIFY" .
These two actions aid in identifying the
7A26
trace
when multiple traces are displayed . When the IDENTIFY
button is released, the deflection factor readout and trace
position are restored .
CR1465 in CH 1, and CR2465 in CH 2 isolate readout
circuitry from the position circuitry . For further informa-
tion on the operation of the readout system,
see
the
oscilloscope instruction manual .

Introduction
This section of the manual contains maintenance infor-
mation for use in preventive maintenance, corrective
maintenance, and troubleshooting of the 7A26 .
Further maintenance information relating to general
maintenance can be found in the instruction manuals for
the 7000-series oscilloscopes .
General
Preventive maintenance,
consisting of cleaning, visual
inspection, etc ., performed on a regular basis, will improve
the reliability
of this
instrument
. Periodic checks of the
semiconductor devices used in the unit are not recom-
mended
as a
preventive maintenance measure .
Cleaning
PREVENTIVE MAINTENANCE
Avoid the use
of chemical
cleaning agents which
might
damage the plastics used in this instrument,
Special care should be taken when
cleaning the
Polyphenylene Oxide attenuator boards
.
Do not
apply any solvent containing ketones, esters or
halogenated hydrocarbons . To clean, use only water
soluble detergents, ethyl, methyl or isopropyl
alcohol .
Front Panel . Loose dust may be removed with a soft
cloth or a dry brush . Water and mild detergent may be
used ; however, abrasive cleaners should not be used .
Interior . Cleaning the interior
of the
unit
should
precede calibration, since the cleaning process
could alter
the settings
of the
calibration
adjustments . Use low-velocity
compressed air to blow off the accumulated
dust . Hardened
dirt can be removed with
a soft
dry
brush, cotton-tipped
swab, or cloth dampened with a mild detergent
and water
solution .
Lubrication
Use a cleaning-type lubricant on shaft bushings, inter-
connecting plug contacts, and switch contacts . Lubricate
switch detents with a heavier grease . A lubrication kit
REV OCT 1982
MAINTENANCE
General
Troubleshooting Aids
TROUBLESHOOTING
Troubleshooting Equipment
Section
4-7A26
containing the necessary lubricating materials
and instruc-
tions is available through any TEKTRONIX Field Office
.
Order TEKTRONIX Part Number 003-0342-02 .
The following
is provided to augment information
contained
in
other sections of this manual when trouble-
shooting the 7A26 .
The
schematic diagrams, circuit descrip-
tion,
and
calibration
sections should be used to full
advantage . The theory
of operation section gives detailed
information
on circuit behavior and output requirements .
Diagrams . Circuit diagrams are given
on foldout pages in
Section 7 . The circuit number
and electrical value of each
component in
this
instrument
are shown on the diagrams .
Circuit Boards
.
The
circuit boards used in the 7A26 are
outlined
on the schematic diagrams, and photographs of the
boards are shown on the backs of the schematic diagrams .
Each board-mounted electrical component
is
identified
on
the photographby its circuit number .
Component and Wiring Color Code . Colored stripes or
dots
on
resistors
and capacitors
signify
electrical values,
tolerances, etc ., according
to the
EIA
standard color code .
Components not color coded usually have the value printed
on the body .
The insulated wires used for interconnection in the
7A26 are color coded to facilitate tracing wires from one
point to another in the unit .
Semiconductor Lead Configuration . The lead configura-
tions of the semiconductor devices used in this instrument
are shown on the foldout following the schematic diagrams .
The following equipment is useful for troubleshooting
the 7A26 .
1 . Semiconductor Tester--Some means of testing the
transistors, diodes, and FET's used in this instrument is
helpful . A transistor-curve tracer such as the TEKTRONIX
Type576 will give the most complete information .