Tektronix 7A14 User manual

INSTRUCTION
MANUAL
Tek tr onix, Inc.
Seriol Nu mber ____ _
TYPE 7AJ~
CURRENT PROBE
AMPLIfIER
S.W . Millikan Way . P. O. Box 500 • Beoverlo n, Oregon 97005 • Phone 644 ·0 1 61 • Cab l es ! Tektronix
070 ·0 979 -00
Scans by Oulso u, ce- Op tions == > 869

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Type 7A14
TABLE OF CONTENTS
SECTION 1 SPECIFICATION
Introduction
Electrical Characteristics
Environmental Characteristics
Physical Characteristics
SECTION 2 OPERATING INSTRUCTIONS
General
Installation
Front Panel Controls and Connectors
First-Time Operation
General Operating Information
Measurement Techniques
SECTION 3 CIRCUIT DESCRIPTION
Introduction
Block Diagram
Input Connector
Input Attenuators
Input Amplifier
Low-Frequen'Cy Compensation
First Cascode Stage
Second Cascode Stage
Third Cascode Stage
Output Amplifier
Trigger Pickoff
Readout Block
SECTION 4 MAINTENANCE
®
Introduction
Preventive Maintenance
Troubleshooting
Introduction
Troubleshooting Aids
Troubleshooting Equipment
Troubleshooting Techniques
Corrective
Maintenance
Obtaining Replacement Parts
Soldering Techniques
Component Replacement
Recalibration After Repair
Page
1-1
1-1
1-1
1-1
2-1
2-1
2-1
2-1
2-3
2-5
3-1
3-1
3-1
3-1
3-1
3-2
3-2
3-3
3-3
3-4
3-4
3-4
4-1
4-1
4-1
4-1
4-2
4-2
4-4
4-5
4-5
4-5
4-6
4-7
SECTION 5 PERFORMANCE CHECKI
CALIBRATION
Introduction
Test Equipment Required
Short-Form Procedure
General
5-1
5-1
5-2
5-3
Preliminary Procedure for Complete Calibration 5-4
Adjust DC Balance 5-4
Adjust Internal Gain Ratio 5-5
Adjust 5 mA Gain 5-5
Adjust P6022 Current Probe Gain
Check/Adjust Front-Panel GAIN
Check Deflection Accuracy
Adjust High-Frequency Compensation
Check/Adjust Low-Frequency Compensation
Check VAR
CURRENT /DIV
Check Trigger Out Gain
Check Trace IDENTIFY
Check LF -3 dB Point
Check
20 MHz Bandwidth
Check HF -3 dB Point
Check
Overall Noise Level Tangentially
SECTION 6 ELECTRICAL PARTS LIST
Abbreviations and Symbols
Parts Ordering Information
Index of Electrical Parts. List
Electrical Parts List
5-5
5-5
5-6
5-7
5-8
5-9
5-9
5-9
5-9
5-10
5-10
5-12
SECTION 7 DIAGRAMS AND MECHANICAL
PARTS ILLUSTRATIONS
Diagrams
Mechanical Parts Illustrations
SECTION 8 MECHANICAL PARTS LIST
Mechanical Parts List Information
Index of Mechanical Parts Illustrations
Mechanical Parts List
Accessories
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7A14
POSITION
LF COMP
POLARITY
•• UP
• INVERT
BANDWIDTH
II FULL
.20MH •
.... _, ..
YA~'AIIU CA L IN
CURRENT/ DIV
CURRENT PROB! INPUT
- :URRENT PROBE AMPLIfiER
7A14
Scllm bv OIlI$Ollrc~-Oplio"s ->
®
ArtekMedia
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DN: cn=ArtekMedia, o=ArtekMedia.com,
ou, [email protected], c=US
Date: 2008.11.16 17:26:19 -06'00'

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Type 7A14
SECTION J
SPECIfICATION
Change information, if any, affecting this section will be found at the
rear of the manual.
Introduction
The Type 7A14 Current Amplifier vertical plug-in unit is
designed for use with Tektronix P6021 and P6022 Current
Probes or at reduced performance with P60191 and P6020
Current Probes. The Type 7A14
is part of a Tektronix 7000-
series Oscilloscope System. The Type 7A14 provides cali-
brated deflection factors from 1 mA/DIV to 1 A/DIV.
In-
ternal gain and compensation circuits are automatically
switched to correspond with the requirements of the current
probe being used, when the current probe is connected to
the amplifier
INPUT connector. A polarity switch permits
ELECTRICAL
Characteristics Performance
Deflection Factor 1 mA/Div to 1A/Div
in a 1-2-5
Calibrated Ranges sequence
Gain Ratio Accuracy Within 2% of GAIN adjusted
at
10 mA/div
Uncalibrated Variable between steps, extends
(Variable) deflection factor to
at least 2.5A/
div
Gain Range
GAIN (front-panel) Permits adjustment of deflection
factor
at 10 mAldiv for all indi-
cator oscilloscopes
Frequency Response
Upper
-3 dB Band- With P6021 With P6022
width (system de-
pendent)
7500-series Indicator 45 megahertz 75 megahertz
Oscilloscope
7700-series Indicator 50 megahertz 105 megahertz
Oscilloscope
Lower -3 dB Band- 30 hertz
or 250 hertz or less
width less, 00 to
+200i C 25
hertz
or less,
+20
0 C to
+75
0 C
Noise (tangentially 0.2 div
or less (square wave val-
measured)
ue) throughout calibrated deflec-
tion factor
range when amplifier
is operated in a 7000-series oscill-
oscope.
Maximum
CW Current
With P6021
15 A P-P to 5 MHz decreasing to
1 A
P-P at 100 MHz
With P6022 6 A
P-P to 10 MHz decreasing to
2 A
P-P at 150 MHz
inversion of the input current signal within the amplifier.
A bandwidth limiting switch allows low-frequency, low-level
signals to be viewed with reduced interference from signals
above 20 megahertz.
The electrical characteristics described
in this section are
valid over the stated environmental range for instruments
calibrated
at an ambient temperature of +20
0 C to
+30
0 C
and after a five minute warmup unless otherwise noted.
'A special coding ring is required when using the P6019 Current
Probe.
Characteristics Performance
Maximum Pulse Current
With P6021 25 A Peak, not to exceed an amp-
second product of 2 A-msi not to
exceed 5.3
Arms
With P6022 25 A Peak, not to exceed
an amp-
second product of 30
A-P.Si not to
exceed
2.1 Arms
Maximum Voltage
With
P6021 or P6022 600 V
(DC + Peak AC)
DC Saturation
With
P6021
0.5 A
With P6022
0.2A
Insertion Impedance
With P6021
0.03 n or less at 1 MHz, increasing
to 1 n
or less at 60 MHz
With P6022 0.03 n or less at 1 MHz, increasing
to 0.7 n
or less at 175 MHz
ENVIRONMENTAL
Altitude
Non-operati
ng
Transportation
Finish
Dimensions
Weight
To 50,000 feet and _55
0 C
Qualified under National Safe
Transit Committee test procedure
1
A, Category II
PHYSICAL
Front panel is anodized aluminum
141f4 inches Lx 2% inches W x 5
inches H overall
:::::1% Ibs
1-1

NOTES
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Type 7A14
SECTION 2
OPERATING INSTRUCTIONS
Change information, if any, affecting this section will be found at the
rear of the manual.
General
The Type 7A14 vertical plug-in unit operates with a Tek-
tronix 7000-series indicator oscilloscope, a 7B-series time
base unit, and Tektronix P6021 or P6022 Current Probes to
form a complete current measuring system. To effectively
use the Type 7A14, the operation
and capabilities of the
instrument should
be known. This section describes the op-
eration of the front-panel controls, gives first-time operation
and general operating information, and lists some measure-
ment techniques using the current probes.
Installation
The Type 7A14 is designed to operate in a vertical plug-
in compartment of Tektronix 7000-series oscilloscopes. For
X-Y operation, the plug-in unit may also be installed in a
horizontal plug-in compartment. (Refer to the indicator os-
cilloscope instruction manual for information concerning
X-Y
operation.)
To install the Type 7A14 into a plug-in compartment, push
it in until it is seated against the front panel of the indicator
oscilloscope.
To remove, pull the release latch to disengage
the plug-in unit from the indicator oscilloscope. Continue to
pull the release latch to remove the unit from the oscilloscope.
FRONT-PANEL CONTROLS AND CONNECTORS
All controls and the connector required for operation of
the Type 7A14
are located on the front panel of the unit (see
Fig. 2-1). A brief description of the function or operation of
the front-panel controls
and the connector follows. More
detailed information
is given under General Operating
Information.
CURRENT IDIV Selects vertical deflection factor from 1
mAIDiv to 1 AMP/DIV in 1-2-5 sequence
(VAR control must be in the calibrated
position for indicated deflection factor).
VAR (CAL IN)
POSITION
IDENTIFY
POLARITY
Provides variable uncalibrated deflection
factors to
at least 2.5 times the setting of
the
CURRENT IDIV switch. Push knob in
and release to activate; the knob moves
outward from the
CURRENT IDIV control
when activated.
Controls vertical position of trace.
Moves trace upward
about 0.25 divisions
for identification. Also identifies the read-
out corresponding to the Type 7A14.
Provides means of inverting the display.
BANDWIDTH
LF COMP
GAIN
NORM:
INVERT:
Conventional current flowing
in the direction of the arrow
on the probe (see Fig. 2-2) pro-
duces a positive deflection
on
the CRT.
Conventional current flowing
in the direction of the arrow
on the
probe produces a neg-
ative deflection on the
CRT.
Provides limiting of upper bandwidth.
FULL:
20 MHz:
Normal operation with
full
bandwidth capabilities.
Amplifier bandwidth limited
to 20 MHz to provide a re-
duction in displayed high-fre-
quency noise
or interference.
Screwdriver adjustment to compensate low-
frequency response.
Screwdriver adjustment to set the gain of
the amplifier. The 10
mA position is color
shaded to indicate that the GAIN should
be adjusted in this position of the switch.
CURRENT PROBE Signal input connector. Similar to standard
INPUT BNC connector, but with additional con-
ductor.
When the P6021 probe is con-
nected to the input, the additional con-
ductor
is shorted to ground by the probe
connector, actuating relays
in the Type
7A14 to set the gain
and peaking circuits
to correspond with the
P6021 probe.
NOTE
P6021 probes with regular BNC connector must be
modified with a special coding ring to be compat-
ible with the 7 -series plug-in units. Contact your
local Tektronix Field Office or Representative for
assistance in procuring the coding ring.
FIRST-TIME OPERATION
General
When shipped from the factory, the Type 7A14 has been
calibrated to meet the specifications listed
in Section 1 and
is ready to be used with an indicator oscilloscope.
The following steps demonstrate the basic operation of
the controls
and connectors of the Type 7A14. It is recom-
mended
that this procedure be followed completely for
familiarization with the instrument.
Operation of the indi-
cator oscilloscope and time base unit is described in the
instruction manuals for those units.
2-1

Operating Instructions-Type 7A14
.""",
POSmoN
LF COMP
POLARITY
•
+UP
•
INVERT
~DW1DTtI
•
FUU
•
20MHz
~.
A
VARIABLE l CALIN
CURRENT/OIY
CURRENT PROBE INPlIT
7A14 CURRENT PROBE AMPUFIER
Fig. 2- 1. Loco'ion of "onl-pon el ~onl , oh and cU"e nt prob e conneclor .
2-2
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.....
-
Fig. 2-2. Curre nt flow I.. a co nd uctor.
Setup Information
1. Insert the Type 7A14 into Q vertical plug,in comportment
of 0 7000-series indicator oscilloscope.
2. Insert 0 7B-series time-bose unit inlo 0 horizontal plug-
in comportment.
3. Sel the controls as follows:
Type 7AI4
CURRENT IDIV lOrnA
VA'
Calibrated
(CAL IN)
POSITION Midrange
BANDWIDTH
FULL
Time Bose
Time/Div
0.5 msee
Variable TimelDiv Calibrated
MODE
p.p AUTO
Slope +
Coupling
AC
Source
,,'
Position
Midrange
Indicotor Oscilloscope
Cali brator
1 kHz
Intensily Counterclockwise
Focus Midrange
Astigmatism Midrange
Scale Ilium
As desired
4. Connect the oscilloscope to 0 power source which meets
the voltage and frequency requirements of the oscilloscope
power supply.
5. Turn the oscillosc ope power on and allow about five
minutes
warmup time.
6. Connect either a Tektronix P6021 or P6022 Current Probe
to the INPUT connector.
7. Open the probe slider, pl ace the probe around the
oscilloscope calibrator 40 mA current loop and lock the
slider.
Op e rating Instructions.----Type 7A14
Fig. 2-3 . 40 mA Calibrator . quor . -wa .. . .
8. Turn the Intensity control clockwise and adjust trig-
gering controls to obtain a tr iggered display. (Adiust hori-
lontal and vertical POSITION controls as necessary.)
9. Adjust Focus
and Astigmatism controls for on optimum
display of the square wove.
10 . Check the display for four divisions of vertical deflec-
tion (see
Fig . 2·3).
II. Press the
IDENTIFY button to identify the readout. The
Type 7A14
readout wi ll be replaced by the word "I DENTIFY".
When the IDENTIFY bullon is pressed and released, the dis-
play moves up and down about 0.25 division. This movement
identifies the Type 7A 14 waveform when more than one dis-
play
is on the CRT .
12. Press the
INVERT button to display the opposite polarity
of the
square wove . (When observing a symmetrical wave-
form, such as the calibrator square wave, inverting the vertical
signal makes no
apparent change in the display. This is due
to the time- bose unit triggering on the indicated slope of the
displayed wa veform.)
This completes the basic
operating procedure for the Type
7A14. Instrument operations not explained here, or operations
which need further explanation,
are discussed under General
Operoting Information.
GENERAL OPERATING INFORMATION
Current Probe Selection
The two current probes recommended for use with the Type
7A14 are the Tektronix P6021 and P6022. Generally , the
P6021 shou ld be used when measuring current waveforms in
the low
to medium·frequency range, while the P6022 should
be used for medium or high-frequency measurements. The
P6022 is con si derably smaller in sile than the P6021.
The P6021 ond P6022 probes require different gain and
peaking in the amp li fier to provide on accurate represen-
tation of the CUffent signal. To accomplish these changes
automatically, a speci al BNC connector with a third con-
ductor is used for the INPUT. When the P6021 is connected
2-3
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Opera ting Instruc ti ons---Type 7A14
to the INPU T, this additional conductor is shorted to ground,
relay-switching the necessory internal circuits to correspond
with the probe being used. No contact
is mode to this con-
ductor by the P6022 probe connector.
Signal Connection
The current signal is connected to the amplifier by sliding
the thumb-controlled portion 01 the probe bock, placing the
probe around the current-carrying conductor, and moving the
slider forward, closing the probe transformer core around the
conductor.
Signal Polarity
Conventional current (plus to minus) flowing in the direction
of the arrow on the probe produces a positive deflection on
the
CRT when the POLARITY switch + UP button is pressed.
When the
INVERT button is pressed, conventional current
flowing
in the direction of the arrow produces a negative
dellection an the CRT .
Deflection Factor
The amount of vertical deflection produced by a signal is
determined by the signal amplitude, the setting of the CUR-
RENT/DIV switch, and the setting of the VAR CURRENT / DIV
control. The calibrated deflection factors indicated by the
CURRENT/D IV switch apply only when the VAR control is
in the calibrated (pushed in) position.
The
VAR CURRENT/DIV control provides uncalibrated
vertical deflection factors between the settings of the
CUR-
RENT/DIV switch. The VAR control extends the maximum
vertical deflection,factor of the Type 7A14 to at least 2.5
amperes/division with the
CURENT/D IV switch at 1 A/Div.
Bandwidth Limit er
The bandwidth switch provides a method of reducing in-
terference from unwonted high-frequency signals when view-
in g low-frequency signals. With the fULL bullon pressed,
the
full bandwidth capabilities of the amplifier are available.
When the 20 MHz bullon is pressed, the upper -3 dB band-
width point of the amplifier is limited to about twenty mega-
hertz. The unwanted high-frequency signals (such as tel e-
vision broadcast radiation interference) are reduced in the
displayed waveform.
Fig. 2-4 illustrates the use of this fea·
ture. The waveform in Fig . 2-4A is the display produced
when a low-level, low-frequency signal
is viewed in the pres-
ence of a strong SO-megahertz radiation (FULL bulton pres-
sed). Fig. 2-4B shows the resultant CRT display when the high.
frequency interference
is reduced by pressing the 20 mega-
hertz button.
Low-Frequency Compensation
The low-frequency response of the probe and amplifier
varies when changing fr
om one probe to another, even
among probes of the some type. Therefore, a low-frequency
compensation adjustment
is provided on the front panel to
match the amplifier response to the response of the probe
being used.
To compensate the amplifier, proce(!d as follows:
1. Connect the P602! probe from the Type 7A14. to the
indicator oscilloscope
40 rnA current loop and set the cali-
brator rate to 1 kHz. Set the TIME/DIV to .2 ms.
2-4
F la . 2-4. (A I C RT dI s pl ay showl..g hlgh-' .. q ... . n'Y In l. rI ." ..c. wh. n
att.mp tl..g to yl. w 1 0w-l.y. l, !o w- lr.q u, ..'Y , Ig not, IBI "su ltant dis_
play whe.. the 20 MH
~ bullon I, p .. ,,, d.
2. Set the CURRENT/DIV to lOrnA and the VA R to the
CAL IN position. Adjust the triggering controls for a stable
display.
3. Adjust the LF CaMP (a front-panel adjust) for 0.2 div i-
sion of tilt in the first 500 p.s of the square-wave. See F ig .
2-5A.
4. Reset the CURRENT/DIV to 5mA and adjust LF COMP
for 0.4 division of lilt in the first 500 }J-S. See Fig . 2-56.
5. Replace the P6021 probe with the P6022 probe. Reset
the CURRENT /DIV to 10 rnA and the TlME/ D IV to .5 ms.
6. Adjust LF COMP for best flat top on waveform (see
Fig .2- 5C) .
7. Reset the CUREN T/DIV to 5 rnA and adjust LF COMP for
best flat top.
NO TE
T he a ppr op r iate steps in the above proced ure
should be checked e ach time wh en ch anging
probes.
Sc llns bp Oll tsoIlT Cf!..() pti ons ::>

fig . 2. 5. low fr e quen cy Com pe nl otlon odju ltme nl . A. P60 2 1 ot
10 m
A/ OI V. 8. P6021 01 5 mA / DIV. C. P6 0n 01 10 m A/ O IV .
Amplifier Gain
The Type 7A14 amplifier contains two gain-setting adjust-
ments. The GAIN control, a front·panel screwdriver adjust-
ment, affects the gain of the amplifier when using either of
the current probes.
The P6021/P6022 Gain Ratio adjustment
®
Op erating Instructions-Type 7AI4
!RI5S), located internally affects the gain of the amplifier
only when using the P6022
probe .
When only one probe type
is being used, the gain may be
set with th e front-panel GAIN control. However, to provide
accurate gain when both the P 6021 and P6022 probes are
being used, proceed as fallows:
1. Connect the P6021 probe from the Type 7Al 4 IN PUT
to the oscilloscope calibrator 40 mA current loop.
2. Wi
th the CURRENT / DIV switch set to 10mA and the
VAR control in the calibrated position, obtain a display on
the
CR T.
3. Adjust the front- pane l GAIN control for four divisions
of vertical deflection.
4. Remove the P 6021 probe and connect the P6022 probe
from the Type
7Al4 INPUT to the 40 mA curre nt loop.
S. Check the display fo r four divisions of vertical deflection.
If adjustment is necessary, refer to the Calibration section
for
P6021 /P6022 Gain Ratio adjustment procedure.
MEASUREMENT TECHNIQUES
Increasing the Sensitivity
The current sensitivity of the probe can be increased by
increasing the number of turns passing
th rough the core of
the probe. F
or example, if the conductor is looped through
the probe two times, a two·turn primary winding is formed,
in creasing the secondary current by a factor of two. (The
ratio of current in a transformer is inversely proportional to
the turns ratio.) With the Type
7Al4 CURE NT/ DIV switch
s
et 10 I mA, Ihe deflection fa clor would actually be changed
by a factor of two to 0.5 mA/division.
Remember, however, that the impedance reflected into
th e
primary
!c ircuit being measured) from the secondary !prabe
winding) varies as the square of the primary turns. When
observing high-frequency current wo
... eforms or fosl-rise
pulses, the
in ductance added to the primary ci rcuit by the
additional turns may be significant.
Simultaneous Current and Voltage
Meas urement s
Simultaneous current and voltage measurements can be
made using the Type 7A14 in one ... ertical plug-in compart-
ment (of a two-vertical indicat or oscilloscope), and a ... altage
amplifier in the other vert ic al plug-in compartment.
1. Insert the Type 7A14 into one of the ... ertical plug- in
comportments of the indicator oscilloscope.
2. Connect the current probe to the Type 7A14 IN PU T
connector.
3. Insert the
... ollage amplifier unit in the other ... erticol
plug·in compartment.
4. Connect a ... ohage probe to the voltage amplifier input
connector.
2-5
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Operating Instructions-Type 7A 14
2.5
Frequency in HERTZ
Fig. 2-6. Type P6021/7A14 low frequency response vs. peak-to-peak current. At the lower frequencies, sine wave distortion occurs as a result of
core saturation.
5. Connect the current probe around the conductor at the
point where the current is to be measured. (Connect the
probe ground to the chassis of the instrument under test.)
6. Connect the voltage probe tip to the point where the
voltage is to be measured. (Connect the probe ground to
the instrument chassis.)
7. Set the
appropriate controls on the Type 7A14, the
voltage amplifier unit, and the indicator oscilloscope to dis-
play both the current and voltage waveforms.
2-6
8. The shape and time duration of the two waveforms may
now
be compared. If desired, the polarity of the current
waveform may be inverted for better comparison.
X- Y Operation
In some applications it is desirable to display one signal
versus another (X-Y) rather than against time (internal sweep).
The Type 7A14 will operate in the horizontal (time-base)
compartment.
®
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-Ji
~

Operating Instructions-Type 7A 14
3
2
10
9
8
7
6A
6
5
P6022/7A
distortion al
4
3
2
12% total
._, .~ u,~.v' .. VII
1.0
,9
.8
.7
.6
,6
,4
.3
.2
.11 2 3 4 6 6 7 8
910
2 3 4 6 6 7 8
910 2 3 4 6 6 7 8
910
Frequency in HERTZ
Fig. 2-7. Type P6022/7A14 low frequency response vs. peak-to-peak current. At the lower frequencies, sine wave distortion occurs as a result of
core saturation.
To make X- Y measurements, insert the Type 7A14 into the
horizontal plug-in compartment. Insert
another Type 7A14
(or
any other vertical unit) into the vertical plug-in compart-
ment. The
calibrated X-axis deflection is indicated by the
CURRENT/DIY setting of the Type 7A14 in the horizontal
compartment; the
calibrated Y-axis deflection is indicated
by the deflection factor set
on the vertical compartment plug-
In.
Refer to the indicator oscilloscope manual for the various
applications of the X-V mode of operation.
2-7
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NOTES
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Type 7A14
SECTION 3
CIRCUIT DESCRIPTION
Change information, if any, affecting this section will be found at the
rear of the manual.
Introduction
This section of the manual contains a description of the
circuitry used
in the Type 7A14 amplifier. The description
begins with a discussion of the instrument using the block
diagram shown in the Diagrams section. Each circuit is then
described
in detail using the block diagram to show intercon-
nections between the circuits and the relationship of the
front-panel controls to the individual stages.
Complete schematics of each circuit are given in the Dia-
grams section. Refer to these diagrams for electrical values
and relationship throughout the following circuit description.
Block Diagram
The following discussion is provided to aid in understanding
the overall concept of the Type
7A14 before the individual
circuits are discussed in detail. Each block on the block
diagram, located in the Diagrams section, represents an
individual circuit within the instrument. The number on each
block refers to the diagram on which the complete circuit
is found.
The signal to be displayed on the CRT is applied to the
INPUT connector and directly to the Input Attenuators. The
CURRENT IDlY switch provides the appropriate attenuation
and applies the signal to the Input Amplifier Stage.
Low-
Frequency Compensation is included in the collector circuit
of the input stage.
This compensation is applied to the side
of the amplifier which
is receiving the input signal. The first
cascode stage, Q140-Q240-Q149-Q249, contains the gain-
switching relays which, with the input attenuators and the
input amplifier, provide the appropriate overall gain for
each setting of the
CURRENT IDlY switch This stage also
provides gain-switching to correspond with the requirements
of the probe being used. C141-C143-C145-C245 provide
high-frequency compensation.
The second cascode stage, Q157-Q257-Q168-Q268, con-
tains a peaking network and the
VAR CURRENT IDlY control.
The AC-coupled outputs are connected through the POLARITY
switch to the third cascode stage, Q307-Q407-Q323-Q423.
This stage contains another peaking network and the front-
panel GAIN control. The output of this stage is connected
through the bandwidth limiting circuit to the Output
Ampli-
fier where vertical positioning is provided. In the Trigger
Pickoff circuit, a portion of the output signal is taken from
the emitters of Q345 and Q445 and amplified by Q360 and
Q460 to the same level as the output signal.
In addition to the functions of the amplifier just explained,
the Type 7A14 also provides knob readout logic for the
indicator oscilloscope.
Logic identifying the de.flection factor,
whether calibrated or uncalibrated, and polarity
is supplied.
When the IDENTIFY button is pressed, this readout is re-
®
placed by the word "identify". At the same time some
additional current is pulled through Q323 to deflect the trace
upward approximately 0.25 division.
Input Connector
The input connector is similar to a standard BNC, but has
an additional conductor. When the additional conductor,
the outside
ring on the connector, is grounded by the P6021
probe connector, three relays, K155, K160 and K235, are
activated. These three relays switch
in the peaking and
gain circuits appropriate for the
P6021 probe. When a P6022
probe is used instead of a P6021 , the outer ring is not
grounded, leaving
K155, K160 and K235 as shown on the
Input Amplifier schematic. The center conductor of the input
connector
is connected to the CURRENT IDlY switch.
Input Attenuators
The portion of the CURRENT IDlY switch shown on the In-
put Attenuator Switching diagram provides proper termi-
nation and attenuation of the input signal.
In the 1 mA, 2 mA,
and 5 mA positions of the switch, the signal is connected
directly through the switch (-;- 1), then AC-coupled through
C101 to the emitter of Q106. In the seven remaining posi-
tions of the
CURRENT IDlY switch, the signal is passed through
one of
six attenuators and DC-coupled to the base of Q206.
The positions of the CURRENT IDlY switch and the corre-
sponding attenuators are as follows:
1
mA
50 mA
2
2
mA
.1 AMP 4
5
mA .2 AMP
8
10 mA
--;-
.5 AMP
--;-
20
20
mA 2 1 AMP
--;-
40
Input Amplifier
In the 1 mA, 2 mA, and 5 mA positions of the CURRENT I
DIY switch, the input signal is applied through C101 and a
terminating network to the emitter of Q106, the grounded-
base input transistor.
At high frequencies the impedance of
the terminating network,
R1 00-C1 00-R1 O1-L 1 01-R1 02-R1 03-
Q106 is 62.5 ohms. This matches the impedance of the 62.5
ohm probe cable. At low frequencies the input impedance
decreases to
10 ohms for improved low-frequency response.
A positive signal
at the input decreases the conduction of
Q106. The AC signal path is through C120 and the low-
frequency compensation network. The signal at the base
of Q140
is in-phase with the input signal.
In the 10 mA through 1 A positions of the CURRENT IDlY
switch, the input signal is applied to the base of Q206. A
3-1
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Circuit Description-Type 7A14
positive signal at the input increases the conduction of Q206.
The AC signal path is through C220 and the low-frequency
compensating network. The signal at the base of Q240 is
inverted from the input signal.
R11 0 is used to DC balance the first cascode stage. R111
sets the quiescent current through this stage and is adjusted
for +4.5 volts to ground at TPl15 and TP215.
As previously noted under Input Attenuators, there is no
attenuation of the signal applied to the emitter of Q106. The
sensitivity
at the collector of Q106 is approximately 100
millivolt/milliamp
at the INPUT connector. In the 10 mA
position there is no attenuation, but the signal is applied to
Q206, which has a gain one-fifth that of Q106. The sensi-
tivity at the collector of Q206 is approximately 20 millivolts
per 1 milliamp at the input connector. R202, with R203-C202-
C203, provides a
gain adjustment to set this one-to-five ratio
between the two input transistors.
In the event that Q106 or Q206 are removed, zener diodes
YR107 and YR208 prevent the common-emitter circuit from
reaching
-50 volts. This provides protection for C101, C207,
and C203. The two zener diodes, which appear capacitive
to the circuit, are isolated from the input signals by R107 and
R207. CR120 and CR220 prevent C120, C220, and the com-
ponents in the low-frequency compensation circuit from being
damaged by the +50-volt supply should the input transistors
be removed. C11 0 and C118 provide additional ripple fil-
tering for the supply voltages.
Low-Frequency Compensation
Because the current probe is an AC-coupled device, low-
frequency signals have an exponential decay as the field in
the probe transformer collapses. The low-frequency com-
pensation circuit has the effect of connecting a variable
capacitor to the junction of R114 and R116 (or R214 and R216,
depending upon the side of the amplifier being used). The
impedance of the circuit (X c) increases at low frequencies,
causing a low-frequency
boost of the signal. The apparent
value of capacitance is adjusted by the LF COMP, a front-
panel adjustment, which sets the gain of the circuit.
With the CURRENT/DIY switch set to 1 mA, 2 mA, or
5 mA, K50 is connected as shown in the schematic. That is,
the signal from Q106 is applied through K50A to the base
of Q222, which provides high input impedance, to the base
of Q225. At the collector of Q225 the inverted signal is
connected through two emitter followers, Q231 and Q233,
which provide low output impedance. In the remaining posi-
tions of the CURRENT/DIY switch, the signal from Q206 is
applied through K50B to the base of Q222. The compensation
requirements of the P6021 and P6022 probes differ, therefore,
when the P6021 probe is connected, C237 is paralleled with
C238 by K235A. The output of the low-frequency compen-
sation amplifier is connected through K50 to the same side
of the amplifier as the input signal.
First Cascode Stage
Q140 and Q240 form a para phase amplifier. When one
base is driven with the input signal, the opposite base is at
signal ground. The signal divides through R141, producing
push-pull signals
at the collectors of Q140 and Q240. In all
positions of the CURRENT/DIY switch, the signal at the col-
lector of Q140 is inverted from the signal at the INPUT
connector. The collector of Q240 is in phase with the input.
R143 and C143 with C141 provide peaking for the stage.
The first cascode stage, Q140-Q240-Q149-Q249, contains
the gain-switching relays,
K51 A, K52A, K53A, and K54A.
These relays, with the input attenuators and the factor of
five difference between the two inputs to the amplifier, com-
bine to provide ten deflection factors of the CURRENT/DIY
switch. R245 is a calibration adjustment for the -;- 5 gain
switching. C145-C245 provide high-frequency peaking. Fig.
3-1 illustrates the three configurations of the gain-switching
relays. The gain of this stage, with the four gain-switching
relays and K155A connected as shown in the schematic, is
approximately 1.6 per side or 3.2 push-pull. K155A opens
when the P6021 probe is connected to the input and increases
the
gain by a factor of 2.5 to a maximum of 8 push-pull. The
gain ratio of 2.5 to 1 is adjusted by R155. R152 is the DC
balance adjustment for the second cascode stage. Table
3-1 shows the input attenuator used for each position of the
CURRENT /DIY switch and the sensitivities of the amplifier
through the first cascode stage. The third column indicates
the
voltage at the collector of either Q106 or Q206 per 1 mA
at the INPUT connector. The fourth and fifth columns indicate
the peak to peak voltage at the collectors of Q149-Q249 per
1 mA at the INPUT connector. The sixth column gives the
overall sensitivity
of the probe and amplifier from the probe
primary (circuit under test) to the collectors of Q149-Q249.
TABLE 3-1
CURRENT IDlY Input Sensitivity from
Sensitivity from INPUT to
Sensitivity from
Switch Setting
Attenuator INPUT to collector
collectors of Q149-Q249
probe primary to
of Ql06
1
or Q206
2
P6022 gain
P6021
gain
collectors of
circuit circuit
Q149-Q249
1 mA 1 100
mY/mN
320 mY/mA 800
mY/mA 6.4 mY /mA
2 mA 1 100
mY/mN
160 mY/mA 400
mY/mA 3.2 mY/mA
5 mA 1 100
mY/mN 64 mY/mA 160 mY/mA 1.28 mY/mA
10
mA
1 20
mY/mN
32 mY/mA 80
mY/mA
0.64 mY/mA
20
mA
2 10 mY/mN 16 mY/mA 40 mY/mA 0.32
mY/mA
50 mA
2 10
mY/mN
6.4 mY /mA 16 mY/mA 128 /J-Y/mA
.1 AMP
-;-
4 5 mY/mA2 3.2 mY/mA 8 mY/mA
64 /J-Y /mA
.2 AMP
8 2.5 mY/mN
1.6 mY/mA 4 mY/mA 32
/J-Y /mA
.5 AMP
20 1
mY/mN
0.64 mY/mA 1.6 mY/mA 12.8 /J-Y /mA
1 AMP 40 0.5
mY/mN
0.32 mY/mA 0.8 mY/mA 6.4 /J-Y /mA
3-2 ®
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Circuit Description-Type 7A14
(A) 1 rnA position
R147
R247
(B) 2 rnA,
10 rnA, and 20 rnA positions
R146
0140
R242
0240
R246
R247
(e) 5 rnA, 50 rnA, .1 AMP, .5 AMP and 1 AMP positions
R147
0140 0149
R242 R244
0240
, _____ , 0249
R246 R247
Fig. 3-1. Three configurations of gain-switching relays showing corresponding positions of CURRENT IDIV switch.
Second Cascode Stage
The second cascode stage, Q157-Q257-Q168-Q268, con-
tains peaking circuits and the VAR control. C259 and C170
provide high frequency peaking for both P6021 and P6022
probes, while C160-R160-C161-R161 are relay-switched into
the circuit by K160A only when the P6021 probe is con-
nected. C162-R162
and C262-R262 provide thermal com-
pensation. With the V
AR control switched off, as shown in
the schematic, the gain of the stage is set by R260, R171, R271
for approximately two. With the VARIABLE control in the
outward position,
K62A closes, reducing the gain of the
stage by a factor of up to 2.5, depending upon the position
of the
VAR control. With the control fully counterclockwise
the resistance of R165 is zero, which allows maximum current
to flow through
R164 and R264. Under this condition, the
gain of the
stage is minimum, or about 0.8. The DC level at
the collectors of Q168 and Q268 is set by voltage divider,
VR173-R173
and the load resistors, R171-R271-Rl72.
The
output of the second cascode stage is AC-coupled
through C176-C178
and C276-C278 to the following stage.
With the POLARITY switch + UP button pressed, K280 is
energized and K180 is not energized, connecting the signals
to the
bases of Q307 and Q407 as shown in the schematic.
With the INVERT button pressed, K180 is energized and K280
is not energized, reversing the inputs to Q307 and Q407. This
inverts the
output signal. Common-mode transformer, T180,
attenuates high-frequency common-mode signals.
Third Cascode Stage
The third cascode stage, Q307-Q323 and Q407-Q423,
provides a gain of approximately 2.4, adjustable to +25%
by the gain control (R318). The emitter resistor, R310, and
the load resistors, R335-R435, with the gain-adjusting network,
R317-R318-R417, determine the gain of the stage. With the
GAIN control fully counterclockwise the resistance of R318
3-3
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Circuit Description-Type 7A 14
is zero, which allows maximum current to flow through R317
and R417. Under this condition, the gain of the stage is mini-
mum,
or about 1.8.
The
DC level at the bases of Q323 and Q423 is set to
-10.8 volts by R326 and R327. The current source for the
emitters of Q307 and Q407 is through R313, with about 15 mA
per side through R312 and R412. C309 and R309, with C31O,
provide high frequency peaking for the stage. C415-R415-
C315-R315 provide thermal compensation. DC Common
Mode adjustment R414, adjusts the output DC level at All
and B11 to zero with no signal in. R321 , in the emitter circuit
of Q323, is connected to ground when the trace IDENTIFY
button is pressed. This adds a small amount to the collector
current of Q323 causing the trace to deflect upward about
0.25 division.
With the BANDWIDTH switch FULL button pressed, the
output of the third cascode stage is connected directly
through K330 to the output amplifier. In the 20 MHz position,
the
bandwidth limiting circuit, C331-L331-L330-L430-C333, is
switched in by K330. This low-pass filter limits the upper
frequency limit of the amplifier to approximately 20 MHz.
Common-mode transformer (T344) attenuates high-frequency
common-mode signals.
Output Amplifier
Under quiescent conditions, and with the POSITION control
set to midrange, the current through the third cascode stage
sets the DC level at the bases of Q345 and Q445 to -7.5
volts. The center taps of R342 also being -7.5 volts, there is
no current through R340 and R440. With the POSITION con-
trol set to midrange, the Position Bal adjustment (R338) bal-
ances the two sides of the amplifier. The output DC levels
are set to zero volts by R345 and R445. A clockwise rotation
of the POSITION control moves the
base of Q345 more neg-
ative and the base of Q445 toward ground. This raises the
level at B11 and lowers the level at All, moving the display
upward.
The collectors of Q345 and Q445 are terminated in 50 ohms
in the indicator unit. The gain of the output stage, determined
by the 50 ohms and the emitter load, is approximately 1.6.
The deflection
factor at the output is 0.5 mA (through the
3-4
50 ohms) per division or 25 mV (per side) per division. R346-
C346 and R446-C446 provide thermal compensation. C347
and R347 provide peaking for the output amplifier.
Trigger Pickoff
The divider network in the emitter circuit of Q345 and
Q445, R350-R354-R450-R454-R353, isolates the trigger pickoff
circuit from the
output amplifier to avoid loading. The +
and -signals from the divider network are amplified by
Q360 and Q460 to the same level as the output signals. The
+ and - trigger signals are terminated in 50 ohms in the
indicator unit. R360 and R460 set the output DC levels to
zero volts. The Trigger Bal adjustment (R368) balances the
levels between the two trigger outputs. R361-C361 and R461-
C461 provide thermal compensation. C363 and R363 pro-
vide
peaking, bringing the risetime of the trigger pickoff
circuit to about 3.5 nanoseconds.
Readout Block
An analog coding system is used in the Type 7A14 to con-
vey information from the plug-in to the indicator oscilloscope
readout section. Each symbol used by the Type 7A14 requires
two currents to define it. This readout information is en-
coded on two output lines, interface connectors A37 and
B37, by the resistors between output lines and interface
connectors A30, A32, A33, B29, B32, B33.
The resistor combinations and switch closures used for each
symbol are as follows: The symbol "A", for ampere, is se-
lected by R77 and R78; the prefix to the unit of measurement
"m", for milli, is selected by R70 and R67 when readout
switch contact 7 is closed; the numeral 1, 2, or 5 is selected,
in the above order, by R59 for 1, by R59//R58 and switch
contact 3 for 2, and by R59/ /R60 and switch contact 4 for
5; the uncalibrated symbol ">", is selected by R75, R62-
CR62 when the VAR CURRENT IDIV knob is in the out posi-
tion;
the invert symbol" ", is selected by R73, R72 when
the POLARITY switch is in the INVERT position; the number
of zeros,
if any, following the numeral 1, 2, or 5 is selected
by R69, R64, R65 and switch contacts 5 and 6.
Refer to the indicator oscilloscope Manual for a complete
discussion on the readout system operation.
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Type 7A14
SECTION 4
MAINTENANCE
Change information, if any, affecting this section will be found at the
rear of the manual.
Introduction
This section of the manual contains maintenance infor-
mation for use
in preventive maintenance, corrective main-
tenance
and troubleshooting of the Type 7A14.
PREVENTIVE MAINTENANCE
General
Preventive maintenance consists of cleaning, visual inspec-
tion, lubrication, etc. Preventative maintenance performed
on a regular basis may prevent instrument breakdown
and
will improve reliability of this instrument. The severity of
the environment to which the Type 7A14
is subjected deter-
mines the frequency of maintenance. A convenient time to
perform preventative maintenance
is preceding recalibration
of the instrument.
Cleaning
The Type 7A14 should be cleaned as often as operating
conditions require. Accumulation of dirt
in the instrument
can cause overheating and component breakdown. Dirt on
components acts
as an insulating blanket and prevents ef-
ficient
heat dissipation. It also provides an electrical con-
duction path.
The covers
of' the indicator oscilloscope minimize the
amount of dust which reaches the interior of the Type 7A
14.
Operation of the system without the indicator oscilloscope
covers
in place necessitates more frequent cleaning. When
the Type 7A14 is not in use, it should be stored in a pro-
tected location such
as a dust"tight cabinet.
CAUTION
Avoid the use of chemical cleaning agents which
might damage the plastics used in this instrument.
Avoid chemicals which contain benzene, toluene,
xylene, acetone or similar solvents.
Exterior. Loose dust accumulated on the outside of the
Type 7A14
can be removed with a soft cloth or small paint
brush. The paint brush
is particularly useful for dislodging
dirt on
and around the front-panel controls. Dirt which re-
mains can be removed with a soft cloth dampened in a mild
detergent and water solution. Abrasive cleaners should not
be used.
Interior. Dust in the interior of the instrument should be
removed occasionally
due to its electrical conductivity under
high-humidity conditions. The best
way to clean the interior
is to blow off the accumulated dust with dry, low-velocity air.
®
Remove any dirt which remains with a soft paint brush or a
cloth
dampened with a mild detergent and water solution.
A cotton-tipper
applicator is useful for cleaning in narrow
spaces.
Visual Inspection
The Type 7A14 should be inspected occasionally for such
defects
as broken connections, broken or damaged circuit
boards, improperly
seated transistors or relays, and heat-
damaged parts.
The corrective procedure for most visible defects
is obvious;
however, particular
care must be taken if heat-damaged
components are found. Overheating usually indicates other
trouble in the instrument; therefore, it is important that the
cause of overheating
be corrected to prevent a recurrence
of the
damage.
Transistor Checks
Periodic checks of the transistors in the Type 7A14 are
not recommended. The best check of transistor performance
is its actual operation in the instrument. More details on
checking transistor operation
is given under Troubleshooting.
Recalibration
To ensure accurate measurements, check the calibration
of this instrument
after each 1000 hours of operation or every
six months
if used infrequently. In addition, replacement of
components may necessitate recalibration of the affected
circuits. Complete calibration instructions
are given in Sec-
tion 5.
The Performance Check/Calibration Procedure can also
be helpful in localizing certain troubles in the instrument.
In some cases, minor troubles may be revealed and/or cor-
rected by calibration.
TROUBLESHOOTING
Introduction
The following information is provided to facilitate trouble-
shooting of the Type 7A14. Information contained in other
sections of this manual should
be used along with the fol-
lowing information to
aid in locating the defective compo-
nent.
An understanding of the circuit operation is very help-
ful in locating troubles. See the Circuit Description section
for complete information.
4-1

Maintenance-Type 7A14
Composition
Metal-Film Resistors:
Ceramic Capacitors:
00 and 0 -1 st, 2nd and 3rd significant figures;
® -multiplier; 0 -tolerance;
Color
Silver I
Gold I
Resistor
Signifi-
cant
Figures
and Capacitor Color Code
Multiplier Tolerance
Resis-
I ca
paci-l Resis-
Capaci-
tors tors tors tors
10-2 I
+10%
10-1
+5%
1
+20% or
2 pF*
10 10 +1% +1% or
~! __ ~0 ___ ~~ __ ~~ __ ~~~~~~7.- ___
Brown I
0.1 pF*
10
2 102
+2%
I
+2%
~-~I ~2--·~~-+~~+--=~-i-~~--1
Orange I 3 103 103 I
+3%
I
+3%
10
4 104
1_~4% I
+100%
-0%
Yellow I 4
105 105
+0.5% +5% or
0.5 pF*
Green I 5
Blue 6 106 106
Violet I 7 I 10-2
+80%
-20%
or 0.25 pF*
10-1
+10% or
1 pF*
Gray I 8 I
White M
+20% +10% or
1 pF*
(none) I I
*For capacitance of 10 pF or less.
@ -temperature coefficient. NOTE: CD and/or ® color code for capacitors depends upon
manufacturer
and capacitor type. May not be present In some ca.es.
Fig. 4-1. Color-code for resistors and ceramic capacitors.
Troubleshooting Aids
Diagrams. Circuit diagrams are given on foldout pages
in Section 8. The component number and electrical value of
each component
in this instrument are shown on the diagrams.
Important voltages and waveforms are also shown on the
diagrams.
Switch Cam Identification. Switch cam numbers shown
on the diagrams indicate the position of the cam
in the
complete switch assembly. The cams
are numbered from the
front,
or mounting end of the switch, toward the rear.
Resistor
Color-Code. In addition to the brown compo-
sition resistors, some metal-film resistors
and some wire-
wound resistors
are used in the Type 7A 14. The resistance
values of wire-wound resistors are printed on the body
of the component. The resistance values of composition
resistors
and meta I-film resistors are color-coded on the
components (some metal-film resistors may have the value
printed on the body) with ErA color-code. The color-code
is read starting with the stripe nearest the end of the re-
sistor. Composition resistors have four stripes which con-
sist of two significant figures, a multiplier
and a tolerance
value .(see Fig. 4-1). Metal-film resistors have five stripes
consisting of three significant figures, a multiplier
and a
tolerance value.
Capacitor Marking. The capacitance values of common
disc capacitors
and small electrolytics are marked in micro-
4-2
farads on the side of the component body. The white ceramic
capacitors used
in the Type 7A14 are color coded in pico-
farads using a modified
ErA code (see Fig. 4-1).
Diode Color Code. The cathode end of each glass-
encased diode is indicated by a stripe, a series of stripes or
a dot. For most silicon
or germanium diodes with a series
of stripes, the color-code also indicates the type of diode
or identifies the Tektronix Part Number using the resistor
color-code system (e.g., a diode color-coded blue
or pink-
brown-gray-green indicates Tektronix Part Number 152-0185-
00). The cathode and anode end of metal-encased diodes
can
be identified by the diode symbol marked on the body.
Troubleshooting Equipment
The following equipment is useful for troubleshooting the
Type 7A14.
1. Transistor Tester
Description: Tektronix Type 576 Transistor-Curve Tracer
or equivalent.
Purpose:
To test the semiconductors used in this instru-
ment.
2. Volt-ohmmeter
Description: 20,000 ohms/volt. 0-500 volts DC. Accurate
within
3%. Test probes must be well insulated.
®
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