Tektronix 7A26 User manual

Tektronix, Inc.
P.0. Box 500
Beaverton, Oregon 97005
TEKTRONIX®
7A26
DUAL TRACE
AMPLIFIER
ZOO /VI Ha.
INSTRUCTION MANUAL
Serial Number
070-1484-00 1072

') r’:: ft,
WARRANTY
All TEKTRONIX instruments are warranted against
defective materials and workmanship for one year. Any
questions with respect to the warranty should be taken up
with your TEKTRONIX Field Engineer or representative.
All requests for repairs and replacement parts should be
directed to the TEKTRONIX Field Office or representative
in your area. This will assure you the fastest possible
service. Please include the instrument Type Number or Part
Number and Serial Number with all requests for parts or
service.
Specifications and price change privileges reserved
Copyright ©1972 by Tektronix, Inc., Beaverton, Oregon.
Printed in the United States of America. All rights reserved.
Contents of this publication may not be reproduced in any
form without permission of Tektronix, Inc.
U.S.A. and foreign TEKTRONIX products covered by U.S
and foreign patents and/or patents pending.
TEKTRONIX is aregistered trademark of Tektronix, Inc

TABLE OF CONTENTS
Page
SECTION 1OPERATING 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 2SPECIFICATION
Introduction 2-1
Electrical Characteristics 2-1
Environmental Characteristics 2-2
Physical Characteristics 2-2
SECTION 3THEORY OF OPERATION
Introduction 3_1
Block Diagram 3_1
Detailed Circuit Description 3_1
SECTION 4MAINTENANCE
Introduction 4_1
Preventive Maintenance 4_1
Troubleshooting 4_1
Corrective Maintenance 4.3
SECTION 5CALIBRATION
Introduction 5_1
Performance Check 5-1
Test Equipment Required 5_1
Calibration Procedure 5.3
Index To Calibration Procedure 5.3
SECTION 6ELECTRICAL PARTS LIST
Parts Ordering Information
Abbreviations and Symbols
SECTION 7DIAGRAMS AND CIRCUIT BOARD ILLUSTRATIONS
Symbols and Reference Designators
SECTION 8MECHANICAL PARTS LIST
Mechanical Parts List Information
Index of Mechanical Parts Illustrations
Mechanical Parts List

linn ®push
VARIABLE (CAL INI
VOLTS/PIV
DISPLAY
MODE
CHI
TRIGGER
SOURCE
CH 2
'POLARITY
+UP FULL
20MHz
INVERT
POSITION rnn tj push
VARIABLE (CAL IN
VOLTS/PIV
TEKTRONIX® DUAL TRACE AMPLIFIER
Fig. 1-1. 7A26 Dual Trace Amplifier.

Section 1—7A26
OPERATING INSTRUCTIONS
7A26 Features
The 7A26 Dual Trace Amplifier plug-in unit is designed
for use with TEKTRONIX 7000-Series Oscilloscopes. The
7A26 is adual-channel wide-bandwidth amplifier. Internal
gain and compensation circuits are automatically switched
to correspond to the setting of the VOLTS/DIV switch.
Channel 2can be inverted for differential measurements.
PRELIMINARY INFORMATION
Installation
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
Introduction
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
1or CH 2and connecting the signal to be observed to the
appropriate input. In the discussions to follow, single-trace
operations using CH 1only apply equally to CH 2only.
Signal Connections
In general, probes offer the most convenient means of
connecting asignal to the input of the 7A26. A10X
attenuation probe offers ahigh input impedance and allows
the circuit under test to perform very close to normal
operating conditions.
Any TEKTRON IX probe, with areadout coding ring can
be used with TEKTRONIX 7A-series amplifier units
equipped with readout. The readout coding ring on the
probe connects to acircuit 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
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, 1kilohertz
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 anarrow-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 1and Channel 2coupling (AC-GND-DC)
switches allow achoice 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 a10X
probe) and square waves with low-frequency components as
these signals are attenuated in the AC position.
Fig. 1-2. Release latch.
RELEASE
LATCH
1-1

Operating Instructions—7A26
CH 1POSITION
Controls position of the CH 1
trace and "ADD” display mode
trace.
CH 1Input Connector
Provides signal connection to CH 1
CH 2POLARITY
Provides means of inverting the
CH 2display.
BANDWIDTH Switch
Provides ameans of limiting the
upper bandwidth.
CH 2POSITION
Controls position of the CH 2
trace only (disabled in ADD).
IDENTIFY (Same as CH 1)
Deflects trace about 0.2 division
for trace identification. In instru-
ments with Readout, also replaces
readout with the word
"IDENTIFY".
TRIGGER SOURCE
Selects source of the trigger
signal.
DISPLAY MODE
Selects the mode of operation.
VARIABLE VOLTS/DIV (same
asCH 1)
Provides continuously variable
uncalibrated settings between
calibrated steps.
GAIN Adjustment
When the VARIABLE control is
pushed in, it becomes afront
panel screwdriver adjustment for
calibration of deflection factor.
VOLTS/DIV (same as Ch 1)
Selects calibrated deflection
factors from 5mV/Div to 5
V/Div; ten steps in a1-2-5
sequence.
CH 2Input Connector AC-GND-DC (same as CH 1)
Provides signal connection to Ch 2. Selects signal input coupling
mode.
Fig. 1-3. 7A26 front-panel controls and connectors.

Operating Instructions— 7A26
In the AC coupling position, the DC component of the
signal is blocked by acapacitor 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 aground 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 produced by asignal 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 5volts/division the
uncalibrated vertical deflection factor is extended to at
least 12.5 volts/division. By applying acalibrated voltage
source to the input connector, any specific deflection
factor can be set within the range of the VARIABLE
control.
CH 2POLARITY Switch
The CH 2POLARITY switch may be used to invert the
displayed waveform of the signal applied to the CH 2input.
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 apositive DC voltage
will move the CRT trace up. In the INVERT position, a
positive-going waveform at the CH 2input will be displayed
on the CRT in inverted form and apositive DC voltage will
move the trace down.
DISPLAY MODE Switch
For single-trace operation, apply the signal either to the
CH 1input or the CH 2 input and set the DISPLAY MODE
switch to the corresponding position: CH 1or CH 2.
To display asignal 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 1or CH 2position.
Alternate Mode. The ALT position of the DISPLAY
MODE switch produces adisplay which alternates between
channel 1and channel 2with each sweep on the CRT.
Although the ALT mode can be used at all sweep rates, the
CHOP mode provides amore 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 1and CH 2VOLTS/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 adisplay which is electronically
switched between channels at approximately a500 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 1position of the TRIGGER SOURCE
switch provides atrigger signal obtained from the signal
applied to the CH 1input connector. This provides astable
display of the signal applied to the CH 1input connector.
CH 2. The CH 2position of the TRIGGER SOURCE
switch provides atrigger signal obtained from the signal
applied to the CH 2input connector. This provides astable
display of the signal applied to the CH 2input 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
REV. B, MAR. 1975 1-3

Operating Instructions—7A26
source for each position of the DISPLAY MODE switch is
as follows:
MODE TRIGGER SIGNAL SOURCE
CH 1Channel 1
CH 2Channel 2
ADD Algebraic sum of channel 1and channel 2
CHOP Algebraic sum of channel 1and channel 2
ALT Alternates between channel 1and channel 2
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 ameans 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.
BASIC APPLICATIONS
General
The following information describes the procedures and
techniques for making basic measurements with a7A26 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:
1.Apply the signal to either input connector.
2. Set the DISPLAY MODE and TRIGGER SOURCE
switches to display the channel used.
3. Set the coupling switch to AC.
NOTE
For low-frequency signals below about 30 hertz use
the DC position to prevent attenuation of the signal.
4.
Set the VOLTS/DIV switch to display about five
divisions of the waveform vertically.
5.
Set the time-base Triggering controls for astable
display. Set the time-base unit to asweep 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 7by the
VOLTS/DIV switch setting. Include the attenuation factor
of the probe if used.
Fig. 1-4. Measuring the peak-to-peak voltage of awaveform.
1-4

Operating Instructions—7A26
EXAMPLE: Assume that the peak to peak vertical
deflection is 4.5 divisions (see Fig. 1-4) using a10X
attenuator probe, and the VOLTS/D IV switch is set to 1V.
Volts
Peak to Peak
vertical
deflection X
(divisions)
VOLTS/D IV
setting
probe
Xattenuation
factor
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
2POLARITY switch in the +UP position, any point above
the reference line is positive.
Substituting the given values:
Volts Peak-to-Peak =4.5 X1X10
9. Multiply the distance measured in step 7 by the
VOLTS/DIV setting. Include the attenuation factor of the
probe, if used.
The peak-to-peak voltage is 45 volts.
Instantaneous Voltage Measurements (DC)
To measure the DC level at agiven point on awaveform,
proceed as follows:
EXAMPLE: Assume the vertical distance measured is
3.6 divisions (see Fig. 1-5) and the waveform is above the
reference line using a10X probe with aVOLTS/DIV setting
of 0.5 V.
Using the formula:
1. Connect the signal to either input connector.
2. Set the DISPLAY MODE and TRIGGER SOURCE
switches to display the channel used.
3. Set the VOLTS/DIV switch to display about five
divisions of the waveform.
Instan- vertical VOLTS/ probe
taneous =distance Xpolarity XDIV Xattenuation
Voltage (divisions) setting factor
Substituting the given values:
Instantaneous 3.6 X+1 X0.5 VX10
Voltage
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 avoltage level with respect to avoltage
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. The ground reference
line can be checked at any time by switching to the GND
position.
6.
Set the time-base Triggering controls for astable
display. Set the time-base sweep rate for an optimum
display of the waveform.
The instantaneous voltage is 18 volts.
1
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Fig. 1-5. Measuring instantaneous voltage with respect to some
reference.
1-5

Operating Instructions—7A26
Comparison Measurements
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 areference 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.
6. Measure the vertical deflection in divisions and
calculate the amplitude of the unknown signal using the
following formula.
Signal =VOLTS/DIV
Amplitude setting
vertical vertical
Xconversion Xdeflection
factor (divisions)
EXAMPLE: Assume areference signal amplitude of 30
volts, aVOLTS/DIV setting of 5Vand the VARIABLE
control adjusted to provide avertical deflection of four
divisions. Substituting these values in the vertical conver-
sion factor formula (step 4):
To establish an arbitrary vertical deflection factor based
upon aspecific 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 adisplay which is an exact number of
vertical divisions in amplitude. Do not change the
VARIABLE control after obtaining the desired deflection.
Vertical
Conversion
Factor
30 V
4X5V1.5
Then with aVOLTS/DIV setting of 2V, 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
Amplitude 2VX1.5 X5=15 volts
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 2and the setting of the VOLTS/DIV
switch. This is the vertical conversion factor.
reference signal
Vertical amplitude (volts)
Conversion =vertical VOLTS/DIV
Factor deflection Xswitch
(divisions) setting
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 asetting that provides sufficient vertical deflection to
make an accurate measurement. Do not readjust the
VARIABLE control.
Dual-Trace Phase 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 1and CH 2coupling 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 1input and
the comparison signal to the CH 2input. 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 2display. (Signals
may be of opposite polarity due to 180° phase difference;
if so, take this into account in the final calculation.)
®
1-6

Operating Instructions—7A26
5.
Set the VOLTS/D IV 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 asweep rate which displays
about one cycle of the waveforms. Set the Triggering
controls for astable 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° T8divisions =
45 /division). The sweep rate can now be stated in terms of
degrees as 45° /division.
Using the formula:
horizontal
Phase Difference =difference Xsweep rate
..... ,(degrees/division)
(divisions)
Substituting the given values:
Phase Difference =0.3 X45°
The phase difference is 13.5°.
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 XI 0and determine the
magnified sweep rate by dividing the sweep rate obtained
previously by the amount of sweep magnification.
9. Measure the horizontal difference between corre-
sponding points on the waveform.
10. Multiply the measured distance (in divisions) by
45°/division to obtain the exact amount of phase
difference.
EXAMPLE: Assume ahorizontal difference of 0.3
division with asweep rate of 45° /division as shown in Fig
1-6.
EXAMPLE: If the sweep rate is increased 10 times by
the Magnifier, the magnified sweep rate is 45° /division -M0
=4.5°/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 3divisions, the phase difference is:
horizontal magnified
Phase Difference =difference Xsweep rate
(divisions) (degrees/division)
Substituting the given values:
Phase Difference =3X4.5°
The phase difference is 13.5°.
Fig. 1-6. Measuring phase difference between two signals.
1-7

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 theTRIGGER SOU RCE switch to MODE
.
2.
Connect the signal containing both the desired and
undesired information to the CH 1input connector.
3.
Connect asignal similar to the unwanted portion of
the CH 1signal to the CH 2input connector. For example,
in Fig. 1-8 aline-frequency signal is connected to Channel 2
to cancel out the line-frequency component of the Channel
1signal.
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
2POLARITY switch to INVERT so the common-mode
signals are of opposite polarity.
7.
Adjust the Channel 2VOLTS/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 1signal.
EXAMPLE: An example of this mode of operation is
shown in Fig. 1-8. The signal applied to Channel 1contains
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 on some DC level when DC coupling is
used. ADC voltage of the proper polarity applied to
Channel 2can be used to cancel out the DC portion of the
signal applied to Channel 1.
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s<sIs /
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(A) Channel 1Signal
(C) Resultant Display
Fig. 1-8. Using the ADD mode for common-mode rejection. (A)
Channel 1signal contains desired information along with line-
frequency component. (B) Channel 2 contains line frequency only.
(C) Resultant CRT display using common-mode rejection.
1-8

Section 2—7A26
SPECIFICATION
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 afive-minute warmup unless otherwise noted.
TABLE 2-1
Electrical
Characteristic Performance Requirement Supplemental Information
Deflection Factor
Calibrated Range 5mV/Div to 5V/Div; ten steps in a1,2,5
sequence.
Aberrations; 5mV and 10 mV. +4.0%, —4.0%,
total not to exceed 6% P-P; 20 mV to 5 V, not
to exceed 8%.
Deflection Factor Accuracy Within 2% with GAIN adjusted at 10 mV/Div.
Uncalibrated (VARIABLE) Continuously variable between calibrated steps;
extends deflection factor to at least 12.5 volts
per division.
GAIN Range Permits adjustment of deflection factor for
calibrated operation with all 7000-series oscillo-
scopes.
Frequency Response >t:C(#.»/<" .1, J.>> tCI" <9
Upper Bandwidth Depends upon oscilloscope used. See the oscilloscope mainframe specifications
or the current Tektronix, Inc. catalog.
Lower Bandwidth (AC coupled) 10 hertz or less.
20 MHz Bandwidth 20 MHz, ±3 MHz.
Risetime 21 ns maximum.
Maximum Input Voltage
DC Coupled 250 volts, (DC +Peak AC); AC component 500
volts peak-to-peak maximum, one kilohertz or
less.
AC Coupled 500 volts, (DC +Peak AC); AC component 500
volts peak-to-peak maximum, one kilohertz or
less.
Channel isolation 50:1 display ratio up to 200 megahertz.
Input Rand C
Resistance One megohm within 2%.
Capacitance Approximately 20 picofarads.
Overdrive Recovery Time 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.
REV. B, MAR. 1975 2-1

Specification—7A26
TABLE 2-1 (cont)
Characteristic Performance Requirement Supplemental Information
Delay Time Difference Between Channels 200 picoseconds or less.
Common Mode Rejection Ratio At least 10:1, DC to 50 MHz.
Display Modes Channel 1only.
Dual-trace, alternate between channels.
Added algebraically.
Dual-trace chopped between channels.
Channel 2trace only.
Trigger Source Selection Channel 1only.
Follows DISPLAY MODE selection.
Channel 2only.
TABLE 2-2
Environmental Characteristic
Refer to the specification for the associated mainframe.
TABLE 2-3
Physical
Size Fits all 7000-series plug-in compartments.
Weight 2Pounds 9Ounces (1 .2 kilograms).
2-2

Section 3—7A26
THEORY OF OPERATION
Introduction
This section of the manual contains adescription of the
circuitry used in the 7A26 amplifier. The 7A26 description
begins with adiscussion 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 1Amplifier 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 1or CH 2INPUT 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 5mV 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 5mV
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 2is identical to Channel
1, with the exception of the polarity inversion function in
Channel 2.
The Display and Trigger Channel switch amplifiers
provide differential signal outputs for the signal and trigger
lines, from each channel, to acommon display and trigger
output. These stages also contain abandwidth 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
NOTE
The CH 1and CH 2amplifier circuits are identical
with the exception of the CH 2GAIN stage U2450,
which includes aPOLARITY inverting circuit. Only
CH 1is described in detail throughout this discussion.
AC-GND-DC Switch
Input signals connected to the INPUT connectors can be
AC-coupled, DC-coupled, or internally disconnected.
S100A is acam-type switch; acontact-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
CIO. 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 aground reference
without the need to disconnect the applied signal from the
INPUT connector. Resistor R102, connected across the
AC-GND-DC switch, allows CIO to be pre-charged in the
GND position.
Input Attenuator
The effective overall deflection factor of the 7A26 is
determined by the setting of the VOLTS/DIV switch,
S100B. The basic deflection factor is 5millivolts per
division of CRT deflection. To increase the basic deflection
REV. B, MAR. 1975 3-1

Theory of Operation—7A26
factor to the values indicated on the front panel, precision
attenuators are switched into the circuit. S100B 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 5mV/Div and
lOmV/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 X20 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 SIM B080000. Q1 50A and Q1 40 form acascode
amplifier with Q1 50B providing constant current. R132
limits the current drive to the gate of Q150A. Dual-diode
CR130 provides circuit protection by limiting the voltage
swing at the gate of Q1 50A to about ±9 volts. R1 34, Cl 30,
and the capacitance of R130 provide low frequency com-
pensation. Input capacitance for the 5mV and 10 mV
positions is set by Cl 34. The output of the 2X Gain Amplifier
(U 1350) is from the source of Q1 50A and high frequencies
from the collector of Q140. R160 is used to balance the
input to the 2X Gain Amplifier between the 5mV and
10mV positions.
SN B080000 Up. Q150A is asource follower with
Q150B providing aconstant current. R132 limits the
current drive to the gate of Q150A. Dual-diode CR130
provides circuit protection by limiting the voltage swing at
the gate of Q1 50A to about ±1 0volts. Cl 30, Cl 34, and the
capacitance of R1 30 provides low frequency compensation.
Input capacitance for the 5mV and 10mV positions is set by
Cl 30. The output of the 2X Gain Amplifier (U 1350) is from
the source of Q1 50A. Cl 34 and R134 form anegative
resistance network for Q150A.
2X Gain Amplifier
T1301 is abalun transformer which provides differential
drive to U1350 at high frequencies. U1350 is aparaphase
type amplifier with dual differential output capabilities.
In the 5mV position, full drive is provided from pins 5
and 9of U1350 to the U1450 load resistors R1401 and
R1403. 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 6and 8of U1350
and is dissipated in dummy load resistors R1343 and
R134 1
.
CR1319 and R1319 maintain proper collector voltage
while switching between the 5mV and 10 mV positions.
C1331, R1331, C1332 and R1332 are thermal compensa-
tions. Cl 334 and RT1334 provide high-frequency tempera-
ture compensation. R1336, C1336, C1345, L1345 and
R1345 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 U1350.
Gain Amplifier
U1450 is avariable-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 U1450. The base-current ratio
determines the shared collector output levels betweens pins
5, 6and 8, 9.
R1436 provides adjustable low frequency compensation.
Fixed components R1434, C1434, C1436, R1431, and
C1431 are thermal compensations. R1435 and C1435 are
adjustable high frequency compensations. U1450 Input
T-coil peaking inductors and capacitors are part of the
etched circuit board. DC balance over the variable range is
adjusted by R1353.
Position Circuit
Positioning current is added to the signal current of
U1450 output from the current sources Q1470 and Q1490.
R1465 controls the voltage at the bases of the current
sources, which in turn determines the amount of posi-
tioning current added. R1467, R1466, and CR1465 provide
trace shift current for the IDENTITY function.
Display Channel Switch Amplifier
The third cascode amplifier, U1550 is used for con-
trolling the channel 1display modes. When the DISPLAY
CH 1ON Level at pin 12 is HI, the channel 1signal passes
through the transistor pair with outputs at pins 5and 9to
the level shifters. At the same time the DISPLAY
CH 1OFF Level at pin 11 is LO, turning off the second
transistor pair collectors, pins 6and 8. When pin 12 is HI,
channel 1is displayed and when pin 11 is HI channel 1is
not displayed. Pins 1 1 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 U1550
to the trigger amplifier switch, U1750, which is also
T-coiled. C1531 and R1531 are high frequency adjust-
ments.
3-2 REV. C, MAR. 1975

Theory of Operation—7A26
Trigger Channel Switch Amplifier
U1750 is acascode 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 1 1 and 12of U1 750 for
trigger selection.
Display and Trigger Common Base Level Shifters
The Display Out Common Base Level Shifters, Q820,
Q840, 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 Q860, Q880 are used
at 20 megahertz. The level shifters also serve as acurrent
summing point for CH 1and CH 2selection.
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 Q920,
Q940, Q960, and Q980 operation is similar to the Display
Output Shifters just discussed.
Channel 2Gain-Polarity Amplifier
CH 2operation is the same as CH 1.For circuit number
reference the prefix number for CH 1is 1and CH 2is 2.
For instance, U2350 functions in CH 2the same as does
U1350 in CH 1. In CH 2aPolarity 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
Aschematic of the Translator circuit is shown on
Diagram 4in the Diagrams section. The translator, Q1050
and Q1070, increase the CHOP and ALT control logic DC
levels from the mainframe to ausable 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 1and 2, Row and Column output lines for
readout of deflection factor, uncalibrated deflection factor
(VARIABLE) information, and signal inversion (Channel 2
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 2only) is switched between TS-2
and the Column output line when the CH 2POLARITY
switch is in the INVERT position. This results in the
symbol J(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 5depending on the combination that is
switched in. R368 and R642 select the m(milli) prefix in the
5mV through 0.5 V(500 mV) positions of the VOLTS/DIV
swtich. R639 and R643 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 combination switched 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 aIX 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 a10X probe connected, the bias on Q620 will allow
100 microamperes of collector current to flow. This
increases the deflection factor readout by afactor of 10.
REV. B, DEC. 1974 3-3

Theory of Operation—7A26
The IDENTIFY button (S1465 on Diagram 2or 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 2isolate readout
circuitry from the position circuitry. For further informa-
tion on the operation of the readout system, see the
oscilloscope instruction manual.
3-4

Section 4—7A26
MAINTENANCE
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.
PREVENTIVE MAINTENANCE
General
Preventive maintenance, consisting of cleaning, visual
inspection, etc., performed on aregular basis, will improve
the reliability of this instrument. Periodic checks of the
semiconductor devices used in the unit are not recom-
mended as apreventive maintenance measure.
Cleaning
SCAUTION <
vy- vy- yyyy
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 dean, use only water
soluble detergents, ethyl, methyl or isopropyl
alcohol.
Front Panel. Loose dust may be removed with asoft
cloth or adry 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 asoft dry brush, cotton-tipped
swab, or cloth dampened with amild detergent and water
solution.
Lubrication
Use acleaning-type lubricant on shaft bushings, inter-
connecting plug contacts, and switch contacts. Lubricate
switch detents with aheavier grease. Alubrication kit
containing the necessary lubricating materials and instruc-
tions is available through any TEKTRONIX Field Office.
Order TEKTRONIX Part Number 003-0342-01.
TROUBLESHOOTING
General
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.
Troubleshooting Aids
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 photograph by 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.
Troubleshooting Equipment
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. Atransistor-curve tracer such as the TEKTRONIX
Type 576 will give the most complete information.
REV. MAY 1974 4-1

Maintenance—7A26
2.
DC Voltmeter and Ohmmeter—Avoltmeter is
required for checking voltages within the circuits, and an
ohmmeter for checking resistors and diodes.
3.
Test Oscilloscope—Atest oscilloscope is required to
view waveforms at different points in the circuit. A
TEKTRONIX 7000-series mainframe equippped with a
readout system. 7D13 Digital Multimeter unit, 7B-series
Time-Base unit, and a7A-series amplifier unit with a10X
probe will meet the needs of both items 2and 3.
4.
Plug-in Extender—Afixture that permits operation of
the unit outside of the plug-in compartment for better
accessibility during troubleshooting. Order TEKTRONIX
Part Number 067-0589-00.
Troubleshooting Procedure
This troubleshooting procedure is arranged in an order
which checks the simple trouble possibilities before pro-
ceeding with extensive troubleshooting.
1. Check Control Settings. An incorrect setting of the
7A26 controls can indicate atrouble that does not exist. If
there is any question about the correct function or
operation of acontrol or front-panel connector, see the
Operating Instructions section.
2. Check Associated Equipment. Before proceeding
with troubleshooting of the 7A26 check that the equip-
ment used with this instrument is operating correctly. If
possible, substitute an amplifier unit known to be operating
correctly into the indicator unit and see if the problem
persists. Check that the input signals are properly
connected and that the interconnecting cables are not
defective.
3. Visual Check. Visually check the portion of the
instrument in which the trouble is suspected. Many troubles
can be located by visual indications, such as unsoldered
connections, broken wires, damaged circuit boards,
damaged components, etc.
4. Check Instrument Performance. Check the calibra-
tion of the unit or the affected circuit, by performing
Performance Check of Section 5. The apparent trouble may
only be aresult of mis-adjustment, and may be corrected
by calibration. Complete calibration instructions are given
in Section 5.
5.
Check Voltages. Often the defective component or
stage can be located by checking the voltage in the circuit.
6.
Check Individual Components. The following
methods are provided for checking the individual com-
ponents. Components which are soldered in place are best
checked by disconnecting one end to isolate the measure-
ment from the effects of surrounding circuitry.
NOTE
To locate intermittent or temperature sensitive com-
ponents mounted on the attenuator board, Quik
Freeze (Miller Stephenson, MS-240, TEKTRONIX
Part Number 006-0173-01) is recommended. Dry ice
or dichlordi-fluorremethane (Freon 12, Dupont or
Can-O-Gas) may also be used. Other types of circuit
coolant may damage the polyphenylene oxide boards.
A. TRANSISTORS. The best check of transistor opera-
tion is actual performance under operating conditions. If a
transistor is suspected of being defective, it can best be
checked by substituting acomponent known to be good;
however, be sure that circuit conditions are not such that a
replacement might also be damaged. If substitute transistors
are not available, use adynamic tester (such as
TEKTRONIX Type 576). Static-type testers may be used,
but since they do not check operation under simulated
operating conditions, some defects may go unnoticed. Be
sure the power is off before attempting to remove or
replace any transistor.
B. DIODES. Adiode can be checked for an open or for
ashort circuit by measuring the resistance between termi-
nals with an ohmmeter set to the RXIk scale. The diode
resistance should be very high in one direction and very low
when the meter leads are reversed. Do not check tunnel
diodes or back diodes with an ohmmeter.
>'"s
1CAUTION (
w
Do not use an ohmmeter scale that has ahigh internal
current. High currents may damage the diodes.
C. RESISTORS. Check resistors with an ohmmeter.
Resistor tolerance is given in the Electrical Parts List.
Resistors normally do not need to be replaced unless the
measured value varies widely from the specified value.
D. CAPACITORS. Aleaky or shorted capacitor can be
detected by checking resistance with an ohmmeter on the
highest scale. Use an ohmmeter which will not exceed the
voltage rating of the capacitor. The resistance reading
should be high after initial charge of the capacitor. An open
capacitor can best be detected with acapacitance meter, or
by checking whether the capacitor passes AC signals.
4-2 REV. B, MAR. 1975