Tektronix 7A18 Service manual

INSTRUCTION

7A18

IAi8/tA"%H

DUAL

TRACE

AMPLIFIER

7A18

TABLE

7A18

SECTION 1 SPECIFICATION Page OPTION INFORMATION

OF

CONTENTS

lntroduction
Electrical Characteristics
7A18 And Mainframe Frequency

Response 1-3
Environmental Characteristics 1-3
Physical Characteristics 1-3

SECTION

SECTION

SECTION 4 MAINTENANCE Accessories

2

OPERATING INSTRUCTIONS

Installation

Front Panel Controls and Connectors 2-1
General Operating Information
Basic Applications 2-4

3

CIRCUIT DESCRiPTlON

Introduction
Block Diagram Description

Detailed Circuit Description 3-1 Index

Preventive Maintenance
Troubleshooting
Replacement Parts
Component Replacement

I-

I

1-1 SECTION 6 ELECTRICAL PARTS LIST

Abbreviations and Symbols
Parts Ordering lnformation

SECTION 7

2-

1

2-2

SEC'TION 8 MECHANICAL PAR1'S LIST

3-1
3-1 Mechanical Parts List

4-3

4-4

CHANGE INFORMATION

DIAGRAMS AND CIRCUIT BOARD

ILLUSTRATIONS

Symbols and Reference Designators
Voltage and Waveform Conditions

of

Mechanical Parts Illustrations

Mechanical Parts List

Information

SECTION 5 CALIBRATION

Recalibration Interval 5-1 or taken directly from
'Test Equipment Required 5-1

I

-.-

Part
Part

Performance Check 5-3 the electronics industry. Change information, if any,

I

I

-

Adjustment

5-7

REV.

Abbreviations and symbols used in this manual are based on

IEEE Standard

Symbols for Units", MIL-STD-12B and other standards of.

located at the rear of this manual.

JUNE

1974

260

"Standard

is

7A18

Change information, if any, affecting this section will be found at the rear of the manual.

7A18

t

ntroduction

The 7A18 and 7A18N Dual Trace Amplifier plug-in

units are designed for use with
Oscilloscopes. The
tical except that readout encoding capabilities and an
"IDENTIFY" function are provided only in the
references made to the
unless othewise noted. The 7A18 is a dual-channel,
medium-bandwidth amplifier. Internal gain and compensa­tion circuits are automatically switched to correspond to

7A18 and 7A18N are electrically iden-

7A18 apply equally to the 7A18N

l-ektronix 7000-Series

7A18. All

TABLE 1-1

ELECTRICAL

i

wrViDiv

to

5

VIDIV;

the setting of the
inverted for differential measurements. The
operated in any plug-in
Oscilloscopes.

The following electrical characteristics are

stated environmental range for instruments calibrated at an

ambient temperature of
minute

tcrt

warmup unless otherwise noted.

step5

111

6

VOL-I"S/DIV

i-20"~ to +30°c, and after a five

I,

switch. Channel 2 can be

7A18 can be

compartment of the 7000-Series

valid over the

Deflectran

Uncalibrated

(VARIABLE)

GAlN

Frequency

Syslorn

(8

Dependent

dtv

refc~ence

Upper

DC

{Direct)

ower

Factor

Respar7se

stgnal)

Bandwrritlr

CuuplecJ

ljandwiclth

Accuracy

Vrrhrri

0

mV/Div,

:ontinuously
teps;

extends

East

12.5

-

296

ViDiv

with

GAlN

vat

iablc

deflection

adjusted

between

factor

at

cal~brart

tn

at

Pern-rits

for

cdlrbrdted

set

rcs

uscrltuzcopes,

adjustrrrcnt

or>erdtiot~

of

tic:flect~nn

wit17

all

factor

7000-

-

--

Specif ication-7A1817AI 8N

7A18

Characteristic

Maximum Input Voltage

DC Coupled

AC Coupled

Channel Isolation

Input R and

---

C

Resistance

Capacitance

RC

Product

Displayed

No~se

TABLE

""

Performance

1-1

(contj

Requiremen_t-.

-

---

--

-

-

--

Supplemental

Information

!50 volts, (DC + Peak AC); AC com­Ionent 500 volts peak-to-peak maxi­num. one kilohertz or less.

j00 volts, (DC + Peak AC); AC compo­lent 500 volts peak-to-peak maxi­num, one kilohertz or

less.

50: 1 display ratio up to 50

megahertz.

---

--

-----

Approximately 20.0 pf

---*-----~"."

Nithin

:k

.."."-"".----,-....-*-

1%

between

all

deflection

(Tangentially Measured)

".

"-

,"

......

Overdrive Recovery Time

Common Mode Rejection

Ratio

DC Drift

Dr~ft with Time (ambient
temperature and line voltage

constant)

Dr~fr w~th Temperature

(line voltage constant)

Time Delay between Channels

---

----

-

--

-*

----

-

Display Modes

--

---

-----

.,"-'"--

At least 10:

1

up to 59 megahertz.

-

Zhannel 1 only.
Dual-trace, alternate between channels.

Added

algebraically.
Dual-trace chapped between channels.
Shannel 2 only.

$00 microvolts or

less

at

5

mV/Div in

7000-Series Oscilloscope.

1.1 ms or

less

to recover to within one
livision after the removal of an over­jrive signal of up to -1-75 divisions or

-75 divisions regardless of overdrive

;ignal duration.

"

.,..----..-..-..

1.02 divisron

fter one hour

-

--

- - --

do

more than 0.01 d~vis~on per degree C.

'00 picoseconds or

~

or

less

warmup.

--

..-"

in

any one minute,

--

"-

less.

---

TABLE

7A18

7A78

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FRONT

AND

PAMEL

CONTROLS

CONNECTORS

r"lIS17

ION

Con

trcS5

llrrnrnrj

Wlsirlay

POSSTXUN

CIS.

Mode

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iihc

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Operating

7A18

Instructions--7A18/7A18N

VOLTSIDIV Selects calibrated deflection factors

from

5

mV/Div to 5 VIDiv; ten

steps in a

VARIABLE Provides continuously variable

(VOLTSiDI V) calibrated settings between cali-

brated steps. Extends the deflection
factor range to 12.5
or more.

GAlN Adjustment When the VARIABLE control is

pushed in,
screw-driver adjustment for cali­bration of deflection factor.

DISPLAY MODE Selects one of the following modes

of operation:
CH

ALT--A dual-trace display of the

ADD-Algebraically adds the signals

CHOP---A dual-trace display of the

CH single-trace display of the

TRIGGER SOURCE

Selects source of the trigger signal.
The trigger signals provide internal
triggering for the oscilloscope

base units.

CH

MODE-.-,I nternal trigger signal auto-

1-2-5 sequence.

un-

voltsidivision

it

becomes a front-panel

I.--A single-trace display of the

signal applied to Channel

signal applied to both channels.
'The channels are alternately dis­played, and switching occurs at
the end of each time-base
sweep.

applied to the CH 1 and CH 2

input connectors, and the
algebraic sum is displayed on
the CRT. The CH 2 POLARITY
switch allows the display to be

+

CH 1
Position of the trace in this dis-

play mode is controlled by CH 1

POSITION control only.

signals applied to both channels.

The two channels time-share the

sweep as determined by the in-

dicator oscilloscope.

signal applied to CH 2.

obtained from signal applied to

CH

matically follows DISPLAY

MODE selection. In ADD or

CH 2 or CH

1

-

i nternal triggering signal

1.

1

1.

-

CH 2.

time-

CHOP display modes, the trigger
signal is the algebraic sum of CH

1

and CH 2 trigger.

CH 2---Internal trigger signal

obtained from signal applied to
CH 2.

CH 2 POLARITY

Provides

-WP--.A positive-going signal at the

INVERT--.A positive-going signal at

means of inverting the CH

2 display.

CH

2

input connector deflects

the

CRI

display upward.

2

the CH
flects the CRT display down­ward.

input connector de-

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

1 or CH 2 and connecting the signal to be observed to the

appropriate input. In the discussions to follow, single-trace

1

operations using CH

only apply equally to CH 2 only.

Signal Connections

In general, probes offer the most convenient means of
connecting a signal to the input of the
uator probe offers a high input impedance and allows the
circuit under test to perform very close to normal operating
conditions.

The Tektronix
ring, was designed specifically for use with 'Tektronix 7A­series amplifier units equipped with readout. The readout

coding ring on the probe connects to
fier 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 without readout, see the Tektronix, Inc. catalog.

P6053A probe, with its readout coding

7A18. A 10X atten-

a

circuit in the ampli-

Vertical Gain Check and Adjustment

TO check the gain of either channel, set the VOLTSlDfV
switch to 10 mV and connect
oscilloscope calibrator to the input connector 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

is

engaged by pressing in the GAIN control knob and turn-

ing the knob with a narrow-blade screwdriver (see Front

40

mV, 1 kHz signal from the

GAlN adjustment

Operating

7A18

Instructions--~7A'18/7A18N

Panel Controls and Connectors), Turn the knob clockwise,
then counterclockwise, until the GAIN control is engaged.
When the
will change as the knob is turned.

knob with the screwdriver until the deflection is set to

exactly four divisions, then remove

GAIN control is engaged, the vertical deflection

Turn the GAIN control
the screwdriver.

Input Coupling

The Channel 1 and Channel 2 coupling (AC-GND-DC)
switches allow a choice of input coupling methods. The
type of display desired and the applied signal will determine
the coupling to use.

DC coupling position must be used to display the

The

It

DC component of the signal,

AC signals below about

probe) and square waves with low-frequency components as
these signals are attenuated in the AC position.

In the AC coupling position, the DC component of the

is

signal
coupling position provides the

DC component much larger than the AC components. The

precharge feature should be used with large DC inputs.
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.

input of the amplifier
input connectors. However, the signals connected to the
inputs are not grounded, and the same DC load is presented
to the

blocked by a capacitor in the input circuit. Ihe AC

The GND position provides a ground reference at the

signa! source.

must also be used to display

30

hertz (ten hertz with a

best display of signals with a

without externally grounding the

10X

70

VOLTSIDIV and VARIABLE Controls

The amount of vertical deflection produced by a signal is

determined by the signal amplitude, the attenuation factor

of the probe, the setting of the

the setting of the

tion factors indicated by the settings of the
witch 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
switch. With the VARIABLE control fully counterclock-

wise and the

brated vertical deflection factor

volts/division. By applying a calibrated voltage source to
the input connector, any specific deflection factor can be
set within the range of the VARIABLE control,

VARlABLE control. Calibration deflec-

VOtTSlDlV set to 5 voltsldiv the uncali-

VOLTSlDlV switch, and

VOLTS/DIV

VOLTS/DIV

is

extended to at least 12.5

CH

2

POLARITY

2

The CH
displayed waveform of the signal applied to the CH
This

is

when differential measurements are to be made. The CH
POLARITY switch has two positions, +UP and INVER1. In

+UP position, the displayed waveform will have the

the
same polarity as the applied signal and a positive
will move the CRT trace up. In the INVERT position,
positive-going waveform at the CH 2 input will be displayed
on the CRT in inverted form and a positive
move the trace down.

POLARITY switch may be used to invert the

particularly useful in added operation of the 7A18

Switch

2

input.

2

DC voltage

a

DC voltage will

DISPLAY MODE Switch

For single-trace operation, apply tire signal either to the

1

inpttt or the CH 2 input and set the DISPLAY MODE

CH

f

or CH

switch to the corresponding position: CH

To display a signal in one channel independently when a
signal is also applied to the other channel, simply select the
desired channel by setting the DISPLAY MODE switch to

1

the appropriate CH

Alternate Mode. The

MODE switch produces a display which alternates between

channel 1 and channel

Although the ALT mode can be used at all sweep rates, the
CHOP mode provides a more satisfactory display at sweep
rates below about
rates alternate mode switching becomes visually per-

ceptible,

or CH 2 position.

ALT

position of the DISPLAY

2

with each sweep on the CRT.

0.2

millisecond/division. At slow sweep

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, or for DC offset (applying a DC voltage to one
channel to offset the DC component of a signal on the
other channel). The overall deflection factor in the ADD

mode with both

position is the deflection factor indicated by either
VOLTS/DIV switch. However, if the CH 1 and CH

VOLTSlDIV 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.

the ADD mode, positioning of the trace is contro:led by the

1

channel

Chop Mode.
MODE switch produces a display which
switched between channels at approximately a
hertr rate (controlled by mainframe). In general the CHOP
mode provides the best display at sweep rates slower than

POSITION control only.

VOLTSlDIV switches set to the same

The CHOP position of the DISPLAY

2.

is

electronically

500

2

In

kilo-

REV.

APR

1974

2-3

Operating

7A18

about
repetitive phenomena is to be displayed.

instructions~--7Al8/7A18N

0.2

miilisecondIdivision or whenever dual-trace, non-

TRIGGER SOURCE Switcl~

CH

1.

'The CH 1 position of the 1-RIGGER SOURCE

switch provides a trigger signal obtained from the signal

1

applied to the CH
display of the signal applied to the CH

CH

2.

The CH 2 position of the TRIGGER SOURCE
switch provides a trigger signal obtained from the signal
applied to the CH
display of the signal applied to the CH

MODE. In this position of the TRIGGER SOURCE
switch, the trigger signal for
on the setting of the DISPLAY MODE switch. The trigger
source for each position of the DISPLAY MODE switch is
as follows:

MODE TRIGGER

CH

1

CH

2

ADD Algebraic sum of channel 1 and channel
CHOP Algebraic sum of channel 1 and channel
A LT Alternates between channel 1 and channel

inp~~t connector. This provides a stable

1

input connector.

2

input connector. This provides a stable

2

input connector.

the time-base unit is dependent

Channel
Channel

SIGNAL

1

2

SOURCE

2

2

2

2.

Set the DISPLAY MODE and TRIGGER SOURCE

switches to display the channel used.

3.

Set the co~lpling switch to AC.

For low-

the

4.

divisions of the waveform vertically.

5.

play. Set the time-base unit to a sweep rate which displays
several cycles of the waveform.

6.

tion of the waveform coincides with one of the

lines below the center hori~ontal 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.

7.

peak. Check that the VARIABLE

in the CAL IN position.

frequenc y signals helo w about

DC

position to prevent attenuation of the signal.

Set the VOL'T'SIDIV switch to display about five

Set the time-base Triggering controls for a stable dis-

Turn the 7Af8 POSITION control so the lower por-

Measure the divisions of vertical deflection peak-to-

(VOLTSIDIV) control is

30

hertz use

2-2).

graticuie

'Trace Identification (7A18 only)

When the IDENTIFY button is pressed, the trace

0.2

flected about
feature is particularly useful when multiple traces are dis­played.
tion factor readout with the word "IDFNTIFY".

In instruments with readout, also replaces deflec-

division to identify the 7A18 trace. This

is

de-

BASIC APPLICA'TIONS

General

The following information describes the procedures and
techniques for making basic
the associated Tektronix oscilloscope and time-base.
applications are not described in detail since each applica­tion must be adapted to the requirements of the individual
measurements. This instrument can
applications not described in this manual. Contact your

'l-ektronix Field Office or representative for assistance

local
in making specific measurements

meastlrerrlents with a 7A18 and

These

also be used for many

with this instrument.

Peak-to-Peak Voltage Measurements (AC)

To

make peak-to-peak voltage measurements,

following procedure:

I.

Apply the signal to either input connector

use the

NOTE

This techrtique can also be used to make measore-

ments between two points on the wavefom, rather

titan peak to peak.

Ftg.

2-2.

Measurtng the peak-to-peak voltage of a waveform.

8.

7A18

Multiply the deflection measured in step 7 by the
VOLTSiDlV switch setting. Include the attenl~ation factor
of the probe if used.

Operating

7.

Measure the distance in divisions between the

reference line

DC

level

measurement is between the reference line and point A.

anrf the point on the waveform at which the

is

to be measured. For example, in Fig.

Instrtictions--7Al8/7A18N

2-3

the

EXAMPLE: Assume that the

is

flection
uator probe, and the VO LTSIDIV switch is set to 1 V.

Peak to Peak

Substituting the given values:

The peak-to-peak voltage is 45 volts.

Instantaneous Voltage Measurements (DC)

Po measure the

proceed as follows:

1.

2.

switches to display the channel used.

3.

divisions of

4.5 divisions (see Fig.

Volts

Connect the signal to either- input connector.

Set the DISPLAY MODE and TRIGGER SOURCE

Set the VOLTSIDIV switch to display about five

vertical probe

-

deflection X
(divisions) factor

Volts Peak-to-Peak

DC

level at a given point on a waveform,

the waveform.

peak to peak vertical de-

2-2)

'OL

=

4.5 X

rS'DIV

setting

using a

1

X

x

10

10X

attenuation

atten-

8.

Establish the polarity of the waveform. With the
POLARITY switch in the +UP position, any point above
the reference line is ~ositive.

9.

Multiply the distance measured in step 7 by the

VOLTS/DIV setting. Include the attentlation factor of the

if

probe,

divisions (see Fig.
reference line usinq a
of 0.5 V.

Using the formula:

Instan- vertical VOLTS! probe
taneous
Voltage (divisions) setting factor

Substitllting

used.

EXAMPLE: Assume

2-3)

=

distance X polarity

given

Instantaneous

Voltage

the vertical distance measured is

and the waveform is above the

10X probe wit11 a VOLTSIDIV setting

X

DIV X attenuation

values:

3.6

X i 1

X

0.5

V

X

CH

3.6

10

2

4. Set the couplinq switch to GND and
trace to
the voltage
trace to the top
control after this reference line has been establ~shed.

line can be checked at any time by
position.

display. Set the time-base sweep rate for an
display of the waveform.

thc lnottom graticule line or other reference line. If

is

ncqative with respect to ground, position the

graticitle line. Do not move the POSl PION

NOPE

Po

n~easrire a voltage Icvel with respect to a voltage

orher ti~afi grout?d,

step

4.

Set tl?~ COLI/?IIII~ switch to DC ar~d

reference voltage to the inpiit connector. Then

position the trace to the refere17ce ltne.

5.

Set the coupling switch to DC. The ground reference

6.

Set the time-base

make

the followirig c/)at?ges

sw~tclr~nq to the GND

'I

r~ggering controls for a stable

pos~tion the

apply

optinium

to

tlte

The instantaneous voltage

Fig.

2-3.

ence.

Measuring instantaneous voltage

1s

18

volts.

with

respect

to

some

refer-

Operating Instructions-7Al8/7A18N

7A18

Comparison Measurements

In some applications it may be desirable to establish

arbitrary units of measurement other than those indicated

VOL-ISiDIV switch. This is particularly useful when

by the

a

comparing unknown signals to
use for the comparison-measurement
facilitate calibration of equipment where the desired
arnplitude does not produce an exact number of
of deflection. The adjustment will be easier and more
accurate if arbitrary units of measurement are established
so that the correct
number of divisions of deflection. The following procedure

describes

conlparison measurcmc?nts.

To estatrtlish an arbitrary vertical deflection factor- based

upon a specific refert?ncc amplitude, proceed as follows:

1. Connect the reference signal to the input connector.

Set the rime-base

tile signal.

of

2.

control to produce a display
vertical
VARIABLE control

3. To

the amplitude of an unknown signal can he measured
accurately at any setting of the
ai-nl,litude of the t-eierence signal must be icnown. If it is
not known, it
VOLTS'DIV control is set in step

how to establish arbitrary units of measure for

Set tile VOLTS'DIV switch and the VARIABLE

div~sions in atnplitude. Do not cf.tange the

estat,lish an arbitrary vertical deflection factor so

adjiistment is indicated by an exact

unit sweep rate to display several cycles

after obta~nrng the desiretl cfeflect~on.

can

be

measured before the VARIABLE

reference arnpiitcide. One

technitlue is to

which

is

an exact number of

VOLT-S/DIV switch, the

2.

tlivisions

Signal VOLT'SIDIV vertical vertical

Amplitude

EXAMPLE: Assume

volts, a

control
divisions. Substituting these values in
conversion factor formula (step

]-hen with a VOLTS/DIV setting of 2 V, the
peak to-peak amplitude of an unknown signal which
produces a vertical deflection of five divisions can be
determined by using

-

setting X conversion X deflection

factor (divisions)

a

reference signal atnplitude of 30

VOLTSIDIV setting of 5 V and the VARIABLE

adjusted to provide a vertical deflection of four

tlie vertical

4)

:

2

-

V X

30 V

4 X 5

1.5

V = 1.5

X

5

=

15

6):

volts

Vertical

Conversion

Factor

Signal

Amnlitude

tlie signal amplitut.le formilla (step

=

Dual-Trace Phase Difference Measurements

Phase conlparison between two signals of the same fre­quency can be made
7A18. 'Th~s method of phase dilference measurement can
be used
system. To make the comparison, use the following pro­cedure:

position, clepending on the type of

up to the frequency limit ot the oscilloscope

1.

Set the CH 1 and CH 2 coi~pling switches to the same

using the dual-trace feature of the

cot~plir~g desired.

4.

Divide the atnplitude of the reference signal (volts) by

the product of tho vertical deflection (divisions) established

2

in step
the vertical conversion factor. TRIGGER SOURCE to CH 1.

disconnect the
signal to the input connector. Set the
to a setting that provides sufficient vertical cleflection to
make an accurate measurement. Do not readjust the
VARIABLE control.

6.

calculate the amplitude of the unknown signal using the controls of the two channels so the displays are equal and

following

and the settirig of the VOLTS/DIV switch. 1-his is ALT is more suitable for high frequencies. Set the

Vertical reference signal

Conversion amplitude (volts)

Factor vert~cal VOLTSIDIV

5.

To measure the amplitude of an unknown signal,

Measure the vertical deflection in divisions and 5. Set the VOLTS/DIV switches and the VARIABLE

formula. about five divisions in amplitude.

--

-

.-

--

--

-

deflection
(divisions) setting

reference signal and connect the unknown

X

-

--

switch

VOLTSI'DIV switch

2.

Set the DISPLAY MODE to ALT or CHOP. In

general, CHOP is more suitable for low frequencies and

3.

Connect the reference signal to the CH 1 input and

2

the comparison signal to the CH
or probes which have similar time delay characteristics to
connect the signals to

4.

If the signals are of opposite polarity, set the CH

POLARITY switch to invert the channel 2 display.

(Signals may be of opposite polarity due to

difference; if so, take this into account in
culation.)

the input connectors.

input. Use coaxiat cables

180' phase

the

final cal-

2

Operating

7A18

Instructions--7A18/7A18N

6. Set the time-base unit to a sweep rate which displays

about one cycle of the waveforms. Set the Triggering

controls for a stable display.

7. Center the waveforms on

POSFTION controls.

the graticule with the 7A18

Substitiitinq the given values:

Phase Difference

The phase difference is 13.5".

-

0.3 X 45"

High Resolution Phase Measure~nents

8. Adjust the time-base Variable TitneiDiv 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.
graticuie represents
45"idivision). The sweep rate can now be stated in terms of
degrees as

sponding points on the waveform.

45Oidivision to obtain the exact amount of phase dif- zontal difference of 3 divisions, tile phase difference is.
ference.

sion with a sweep rate

45'/division.

9.

Measure the horizontal difference behveen corre-

10. Multiply the measured distance (in divisions) by Fig.

EXAMPLE: Assume a horizontal difference of 0.3 divi- Phase Difference

45O

of

of

2-4).

Each division of the

the cycle (360°

45°/division as shown in

+

8

divisions

=

Fig.

2-4. (divisions) (cfegreesi'division)

More accurate dual-trace phase
made by increasing the sweep rate
Variable

crease the sweep rate is
Set the Magnifier to

sweep rate
by the amount of sweep magnification.

the Magnifier, the magnified sweep rate is

-

TirneIDiv control). One of the easiest ways to in-

with the time-base Magnifier switch.

XI0

and determine the magnified

t~y dividing the sweep rate obtained previously

EXAMPLE. If the sweep rate is increased 10 times by

4.5°/division. Fig. 2-5 shows the same signals as used in
2-4 but with the Magnifier set to X10. With a iiori-

horizontal magnified

-

difference

measuremenis can be

(witho~lt changing the

45Oidivision : 10

X

sweep rate

Using the formula:

Phase Difference

Substiti~tintj the given values:

hori7ontal sweep rate Phase Difference - 3 X 4.5"

-

difference X (degrees/division)

(divisions) The phase difference is 13.5".

Fig.

2-4.

Measuring phase difference between two signals.

Fig.

f

ier.

2-5.

High resolution phase measurement using time-base

magrii-

2-7

Operating

7A18

Instructions--7A18/7A18N

Common Mode Rejection

The ADD feature of the 7A18 can

signals

which contain undesirable components. These un­desirable components can be eliminated through common­mode rejection. The procedure is as follows:

1.

Set the DISPLAY MODE s~zlitcli to ALT or CHOP

and the TRIGGER SOURCE switch to MODE,

be

used to display

2. Connect the signal containing

undesired information to the CH

3.

Connect a signal similar to the unwanted portion of

both the desired and

1

input connector.

tlie CH 1 signal to the CH 2 input connector. For example,

2-6

a

in Fig.

line-freclcrency signal is connectetl to Channel

to cancel out the line-frequency component of the Channel

1

signal.

4.

Set botii coupling switches to the same setting, DC or

depericling on the applied signal.

AC,

5.

Set the VOLTS/DIV switches so the signals are about

equal in amplit~~de.

6.

Set tlie DISPLAY MODE switch to ADD. Set the CH

2

POLAR1 rY switch to INVERT so the common-mode

sign;ils are

7,

VAR

of

opposite polarity.

Adjust the Channel

l

ABLE

control for maxiinurn cancellation of the

2

VOLTSiDlV switch and

cor~7mon-i~lode signal. The signal which remains shocrld be

tlie

only

desired portion of the Channel 1 signal.

2

EXAMPLE: An example of this mode of operation is

shown in Fig.

2-6. The signal applied to Charinel 1 contains
unwanted line frequency components (Fig. 2-6A). A corre­sponding line frequency signal is connected to Channel
(Fig. 2-GB). Fig. 2-6C shows the desired portion of tlie
signal as displayed when common-mode rejectioli

The

above procedure can also be used for examining a

siiperimposed on some DC level when DC coupling is

signal

is

used.

usecf. A DC voltage of the Proper polarity
Channel 2 can be used to cancel out the DC portion of the
signal applied to Channel

1.

2

Fig.

Channel
frequency component.

(Cf

~estrltant

2-6.

Using the

3

CRT

ADD

mode for common-mode rejection.

contains desired information along with line-

(B)

Channel 2 contains line frequency only.

display using common-mode rejection.

(A)

Change information, if any, affecting this section will

7A18

be

found at the rear of this manual.

l

ntroduction

This section of the manual contains a description of the

circuitry used in the

tion begins with
block diagram shown in the Diagrams section. 'Then, each

circuit is described in detail using block diagrams 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.

7A18 dual-trace amplifier. The descrip-

a

discussion of the instrument using the

BLOCK DIAGRAM

The following discussion
standing the overall concept of the 7A18 before the indivi­dual circuits are discussed in detail. Only
connections between the individual blocks are shown on
the block diagram (see Diagrams section). Each block
represents a major circuit within the instrument. The
number on each block refers to the schematic on

is

complete circuit

The signal to be displayed on the CRT is applied to the

input connector. The signal passes through the input
coupling switch, where the appropriate coupling
to the attenuators. The
correct amount of attenuation and the signal
the input amplifier.

The Channel

setting, variable gain control, and trace positioning. The

2

Channel
in addition to gain setting, variable gain control, and trace
positioning. The outputs of these circuits are applied push­pull to the Signal and Trigger Channel Switches.

The Channel Switches select the proper signal and trigger

as determined by the DISPLAY MODE and TRIGGER

Input Amplifier provides signal polarity inversion

found.

1

Input Amplifier circuit provides gain

is

provided to aid in nnder-

the basic inter-

which the

is

selected,

VOLTSIDIV switch selects the

is

passed to

SOURCE switches. The signal and trigger outputs are
provided to the oscilloscope via the Interface Connector.

The Readout Encoding circuit
readout logic for the oscilloscope readout system. Data
supplied to the mainframe readout system identifying the
polarity, deflection factor, the uncalibrated symbol (when
the VARIABLE control is in the outward position), and the
plug-in mode. When the
trace is deflected about
factor readout is replaced by the word

IDEN-SIFY button

0.3

(7A18 only) provides

IS

pressed, the

division and the deflection

"IDEN1-IFY".

DETAILED CIRCUIT' DESCRIP'TION

ATTENUATOR

General

The Attenuator circuit determines the input coupling

and the 7A18 deflection factor.

NOTE

The

Cff

I

and

Cff

2

Attentlator circuits are identicai.

To minimize duplication, only

detail

throi~ghout tt~is discussion.

CH

7

is

described in

AC-GND-DC Switch

Input signals connected to the input connector can be
AC-coupled, DC-coupled, or internally disconnected.
SlOOA is a cam-type switch; a contact-closure chart

is

showing the operation
this chart indicate when the associated contacts are
position
switch
directly to the input Attenuator stage. In the AC position,
the input signal passes through capacitor
acitor prevents the DC component of the signal from
passing to the amplifier. The
path and connects the input circuit of the amplifier to
ground. This provides a ground reference without the need
to disconnect the applied signal from the input connector.
Resistor

allows
trace remains on screen when switching to the AC position

if the applied signal has a high DC level.

shown (open or closed). When the AC-GND-DC

is

in the DC position, the input signal

R

102, connected across the AC-GND-DC switch,

C10 to be precharged in the GND position so the

given on Diagram

GND position opens the signal

1.

The dots on

III

the

is

cot~pled

C10. This cap-

IS

Circuit Description--7A18/7Al8N

7A18

lnput Attenuator

The ef.fective overall deflection factor of the 7A18 is
determined by the setting of the
SIOOB. The basic deflection factor
sion of CRT deflection. To increase the basic deflection
factor to the values indicated on the front panel, precision
attenuators are switched into the circuit. These attenuators

are hybrid devices which contain the necessary resistances
and capacitors. Each attenuator is

SIOOB is a cam-type switch and the dots on the
closure chart (see Diagram 1) indicate when the associated
contacts are in the position shown (open or closed). In the

5

mV/Div position, input attenuation

signal is connected directly to the input amplifier.

For switch positions above five millivolts, the
uators are switched
produce the deflection factor indicated on the front panel.
These a
dividers. For DC and low-frequency signals, the attenuators
are primarily resistance dividers and the voltage attenuation
is determined by the resistance ratio in
reactance of the capacitors in the circuit
frequencies that their effect
higher frequencies, the reactance of the capacitors decreases

and the attenuator becomes primarily a capacitance divider.

frequencies within

input attenuators are designed to maintain the same input
RC characteristics (one megohm

of the
adjustable series capacitor to provide correct attenuation at
high frequencies and an adjustable shunt capacitor to pro-

vide correct input capacitance.

ttenuators are frequency-compensated voltage

In addition to providing constant

VOLTS/DIV switch. Each attenuator contains an

into the circuit singly or in pairs to

the bandwidth of the instrument, the

CHANNEL

VOLTSIDIV switch,

is

five millivolts per divi-

replaceahie as a unit.

is

not used; the input

the circuit. The

is

is

negligible. However, at

atLenuation at all

X

20 pF) for each setting

contact-

so high at low

1

atten-

INPUT AMPLIFIER

General

The Channel 1 lnput Amplifier converts the single-ended
signal applied to the Channel 1 input connector to a dif­ferential (push-pull) output. Fig. 3-1 shows a detailed block

diagram of the Channel 1 lnput Amplifier. A schematic of

this circuit is shown on Diagram 2 in the Diagrams section.

lnput Source Follower

The Input Source Follower Q210A provides a high input
impedance with a low impedance drive for the following

stage.

R210 limits the current drive to the gate of Q210A.

Dual-diode CR210 provides circuit protection by limiting

the voltage swing at the gate of
or negative) 15 volts.

Q210B provides a constant current

0210A to about 4 (positive

source for
the same case so that
circuit.

0210A. 0210A and Q210B are encapsulated in

0210B temperature-compensates the

Paraphase Cascode Amplifier

Paraphase amplifier 0220-0320, in conjunction with
0225-0325, forms a
the single-ended input signal to a differential output signal.
Diodes
0220 close to ground to limit the voltage swing to about

40.6

isolation between the paraphase amplifier and the

VARIABLE controls. 'Phe gain of the Channel 1 lnput
Amplifier is set in this stage by front-panel

R237A with the CAL IN switch pressed in. When the CAL
IN switch is in the outward (uncalibrated) position and

turned fully
gain of the amplifier is reduced by a factor of at least 2.5.
Adjustment 1 R321 varies the base level of Q320 to provide
the same voltage levels at the collectors of
This prevents a
position when varying the

CR220-CR221 hold the voltage level at the base of

volt. Common-base connected 0225-Q325 provide

co~~nterclockwise to minimum resistance, the

cascode amplifier. Q220-Q320 convert

GAIN-

GAlN control

(2225 and 0325.

rerouolt reference trace from changing

GAlN or VARIABLE controls.

Second Cascode Amplifier

The Second Cascode Amptifier stage provides a signal
gain of
TION control and, in the

circuitry. The emitters of common-base connected
(2250-0350 provide a low-impedance point for injection of

the POSITION control and

Position of the trace is determined by the setting of the
POSITION control, R 11. This control changes the current
drive to

impedance point in the circuit, there
change at these points. However, the change in current
from the
voltage difference at the collectors to change the position
of the trace. Trace identification is accomplished by

inserting resistor R357 from ground through
junction of
the emitter current of Q250 to cause the trace to move.
This aids in identifying the channel 1 trace when multiple
traces are displayed.

vides high frequency compensation.
work provide high-frequency response adjustment for this
stage.

approximateiy two. This stage includes the POSI-

7A18 only, trace IDENTIFY

IDENrlFY switch currents.

Q250-0350. Since the emitters are a very low-

is

negligible voltage

POSI1"ION control produces a resultant DC

CR357 to the

R11-R256. This results in a slight increase in

The network

C246-C345-C245-R246-R345-R245

R245-C245 in this net-

pro-

Emitter Follower

Emitter Follower stage 0260-(2360 provides a low out-

put impedance to drive the Signal and Trigger Channel
Switches,

between the Second Cascode Amplifier and

U270-U470. This stage also provides isolation

U270-U470.

Circuit Description-7Al8/7A18N

7A18

Fig.

3-1.

Channel 1 lnput Amplifier detailed block diagram.

CHANNEL 2 INPOUAMPLIFIER

General

The Channel 2 lnput Amplifier circuit

1

same as the Channel

lnput Amplifier circuit. Only the
differences between the two circuits are described here.
Portions of this circuit not described in the following des­cription operate in the same manner as for the Channel

lnput Amplifier circuit (corresponding circuit numbers

assigned in the

400 599

block diagram of the Channel

schematic of this circuit

range). Fig.

2

lnput Amplifier circuit. A

is

shown on Diagram 3 in the

Diagrams section.

is

basically the

3-2

shows a detailed

Paraphase Cascode Amplifier

The Paraphase Cascode Amplifier for Channel 2 consists

of

Q420, Q520, (3425, Q525,0426,
to the functions described under Channel
fier, the Channel
provides a means of inverting the displayed signal. With the

1

CH

2

POLARI1-Y switch set to

biased on and the signal

Amplifier stage as for the Channel

CH

2

the

Q525

POLARITY switch set to

are biased off and

provide signal inversion.

CHANNEL

-+

and

Q526.

2

Paraphase Cascode Amplifier stage

+UP,

0425

is

passed to the Second Cascode

1

lnput Amplifier. With

INVERT,

Q426-Q526

are turned on to

SWITCH

U470

In addition

1

lnput Ampli-

and

0525

Q425

are

and

FROM

INPUT

ATTEN

REV.

D, DEC.

1976

PARAPHASE

Fig.

FROM

S23

17A18

ontv)

3-2.

Channel 2 lnput Amplifier detailed block diagram.

TO

SIGNAL

GMANNEL

SWITCH

U270

CHANNEL

SWiTCW

U470

Circuit Description-7Al8/7A18N

7A18

Second Cascode Amplifier

Ttie Second Cascode Amplifier for Channel 2 consists of
0440, 0540,
by the POSITION control, R21 or by network R455-H555
as determined by the DISPLAY MODE switch. In any DIS­PLAY MODE switch position other than ADD,
applied to the center arm of the POSl I^ION control through
R32. The POSITION control varies the current drive to the
emitters of
impedance point in the circuit, there is negligible voltage
change at these points. However, the change in current
from the POSITION control produces a resultant DC
voltagt-: difference at the collectors to change the position
of the trace. When the DISPLAY MODE switch is in the
ADD position, +50 volts
tors

R455-R555 through R32 to balance the current drive

to the emitters of

volts (approximately) difference between the collectors.

Since +50 volts is not applied to the
the ADD position of the DISPLAY MODE switch, the

control setting has no

Q450, and Q550. Position of the trace

Q450-0550. Since the emitters are a very low-

is

applied to the junction of resis-

(2450-0550. This results in a fixed zero

POSII-ION control in

effect on the circuit operation.

i

50 volts

is

set

is

CHANNEL SWITCHES

General

Ttie Channel Switches circuit provides Signal and Trigger
outputs to the oscilloscope via the
determined by the DISPLAY MODE and
SOURCE switches. A schematic of this circuit is given on
Diagram 4 in the Diagrams section.

Interface Connector as

THIC<GER

Signal Channel Switch

The Signal Channel Switch stage consists of integrated

circuit

U270 and its external components. This staqe selects

one, or mixes two input analog signals in response to inputs

from tlie DISPLAY MODE switch. The Signal Cliannel

Switch stage determines which input (CH 1 or CH 2) pro­vides

the signal to the oscilloscope as controlled by the
DISPLAY MODE switch setting. Resistors
R376-R377 set the current gain for each channel. Networks

C274-R274-C215-R275 and C374-R374-C375-R375 pro- vide the DC level shifting necessary to drive the
vide high-frequency compensation for each channel. C275
and

C315 in these networks are high-frequency compen.

sation adjustments.

R276-R277 and

DISPLAY MODE

SELECTED

*Level is switched between the HI-level and LO-level

0.5

mate
**Level is switched between the Hl-level and LO-level at a rate

determined by the setting

megahertz rate.

of

the time-base unit sweep rate.

mm

I

Fig.

3-3.

U270

input combinaliotis for DISPLAY MODE selection

'Trigger Channel Switching

The Trigger Channel Switch U470
Signal Channel Switch. This stage determines which
(CH 1 or CH 2) provides the trigger stgnal for internal
triggering of the time-base
signal

is

controlled by inputs from the TRIGGER SOURCE
switch. Resistors
rent gain for each channel. Networks
C474-R474-C475-R475 and C574-R574-C575-R575 pro-

vide high-frequency compensation for each channel.

An

inputloutput table for this stage is shown in Fig. 3-4.

Wlien the level at pin 14 is LO, the output of U470 is

determined

the level at pin 4 LO, the channel 1 and channel 2

and
triggers are added algebraically.

Signal

Output stage,
of a pair of common-base connected transistors which pro-

and

'Phe Signal Output stage, Q280-Q380, and the Trigger

circuits.

R476-R477 and R576-R577 set the cur-

by

the level at pin 4. With the level at pin 14 HI

0480-0580, are similar. Each stage consists

unit. 7 he selection of the trigger

Output

is

identical to the

DISPLAY MODE AND

---

at

an approxi-

input

mainframe

I

Fig. 3-3 shows the U270 input combinations for each
position of the DISPLAY MODE switch. When the
pin 14 is LO the output of U270 is determined by
at pin

4.

With the level at pin 14 HI and the level

LO, the signals from both channel 1 and channel 2 are

passed to the Signal Output stage. This condition occurs

only when the DISPLAY MODE switch is set to ADD. In

is

this operating mode the signal output

of channel 1 and channel

determines the mainframe deflection.

2

signals and the resultant signal

the algebraic sum

level at

the level

at

pin 4

TRIGGER SWI"T'CHING

General

The Display Mode and Trigger Switching circuit deter-

mines

which input signal (Channel 1 or Channel
the Signal and Trigger outputs to the mainframe as selected
by the DISPLAY MODE and TRIGGER SOURCE switches.

This circuit also provides plug-in mode information to the

mainframe chop blanking circuit, and readout control infor­mation for proper

CRT display.

2)

providcs

Circuit Description--7A18/7A18N

7A18

DISPLAY MODE

'The DISPLAY MODE switch provides logic level outputs
to the Signal Channel Switch stage
Switches diagram 4). A table of the outputs for each
position of the DISPLAY MODE switch is shown in Fig.
3-3.

'TRIGGER SOURCE

The TRIGGER SOURCE switch provides logic level out-

puts to the Trigger Channel Switch

Switches diagram 4).

position is shown in Fig.

ALT

1

I

Fig.

'TRIGGER

-G6-D-E

3-4.

Input/Output combinations for

SOURCE

Switch

(U270, Channel

Switch

(U470, Channel

A

table of the outputs for each switch

3-4.

Trigger S~gnaI

--"---------

..f-"T--i5

switch selections.

*f-.

-cG"-t""Ei

;-me..----

ernates between

and

CH

2

+-*

-"------

CY"?

-...

cw_

GM

-"

?.

----

1

and

DISPLAY

Source

------

--*

-----"

CW

2

MODE

--

-- - -

GW

.--*-

----

-"

----

added

---

1

-

and

CONNECTORS AND READOUT

General

The Connectors and Readout circuit consists of the
power supply and signal distribution from the lnterface
Connector and the Readout Encoding circuit. A schematic

is

of this circuit
section.

shown on Diagram 6 in the Diagrams

Connectors

All the connections made to the mainframe by the 7A18
are shown on the Connectors portion of Diagram 6. Also
shown are the power

sopply decoupling components.

Readout Encoding (7A18 only)

'The Readout Encoding circuit consists of switching

resistors and probe sensing stage

2,

the Channel 1 and
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 0620.

R647-CR647 are switched between time-slot three

(TS-3) and Column output line when the CAL

uncal position. This results in the symbol > (greater

in the
than) being displayed preceding the deflection factor read­out. R648 (Channel
the Column output line when the

is in the INVERT position.

(inverted) being displayed preceding the deflection factor

readout.

Switching resistors are used to indicate the setting of the

VOLTSiDIV switch to the mainframe readout system. The
VOLTSIDIV switch is a cam-type switch. The dots on the
contact-closure chart (see Diagram 6) indicate when the
associated contacts are closed.
select the number 1,
combination that
prefix and R639 selects the symbol V (volts) in the 5 mV
through
switch. R638 selects the symbol V in the
positions.
stage (0620) select the decimal point (number of zeroes)
again depending on the resistor combination switched in by
the

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.
input connector provides the input to the probe sensing
stage from the probe coding resistance (coded probes
only; see
used for the
voltage divider with R621 through
supply.
determines, along with emitter resistor
current. When the -15 volt time-slot pulse is applied to

Interface Connector B33, 0620 is interrogated and
collector current
through Interface Connector A37.

.5

V (500 mV) positions of the VOLTSiDIV

R630, R631, and the output of the probe sensing

VOLTSiDIV switch.

Probe sensing stage Q620 identifies the attenuation

0pera;ing Instructions). The third contact is also

IDENTIF-Y input. The coding resistor forms a

The resultant voltage sets the bias on Q620 and

Row and Column output lines for

2

only) is switched between TS-2 and

2,

is

switched in. R637 selects the m (milli-)

is

added to the column current output

Q620. This circuit encodes

IN switch is

CH

2 POLARITY switch

This results in the symbol

R633, R634, and R635

or 5 depending on the resistor

1,

2, and 5 V

'The third contact of the

CR621 to the

R622,

--I5

the collector

its

1

V

REV.

D,

JAN.

1975

Circuit Description--7A1817A18N

7A18

With a
connector, 0620 is turned off. The deflection factor read­out

10X probe connected, the bias on 0620 will allow 100

a
microamperes of collector current to flow.
the deflection factor readout by a factor of 10.

The IDENTIFY button (S13 or 523 on Diagram
two things when pressed

I.

channel of the
discussion on the Channel 1 or Channel 2

1X

probe (or no probe) connected to the input

is

determined by the VOLTS!DIV switch position. With

1-his increases

I)

:

It

causes the trace representing the appropriate

7A18 to move about 0.3 division (see the

Input Amplifier).

does

2.

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

when multiple traces are displayed. When the IDENTIFY

is

button
position are restored.

For further information on the operation of

system, see the oscilloscope instruction manual.

released, the deflection factor readout and trace

7A18 trace

the readout

Change information, if any, affecting this section will be found at the rear of this manual.

7A18

l

ntroduction Lubrication

This section of the manual contains maintenance infor-

mation for use in preventive maintenance, corrective main- connecting plug contacts, and switch contacts. Lubricate

tenance, and troubleshooting of the

Further maintenance information relating to component

color codes and soldering techniques can be found in the

instruction manuals for the 7000-series oscilioscopes.

7A18. switch detents with a heavier grease. A lubrication kit

PREVEN'TIVE MAINTENANCE

Use a cleaning-type lubricant on shaft bushings, inter-

containing the necessary lubricating materials and instruc-

is

tions

Tektronix Part Number 003-0342-01.

available through any Tektronix Field Office. Order

Recalibration

To ensure accurate measurements, the 7A18 should be
checked after each
months if used infrequently. A complete performance
check procedure

1000 hours of operation or every six

is

given in Part I for Section

5.

General

Preventive maintenance, consisting of cleaning, visual
inspection, lubrication, etc., performed on a regular basis,
will improve the reliability of this instrument. Periodic
checks on the semiconductor devices used in the unit are
not recommended as a preventive maintenance measure.
See semiconductor-checking information given under
Troubleshooting.

The performance check procedure can be helpful in

isolating

troubles not apparent during regular operation may be

revealed

major

and

corrected.

troubles

in

the

unit.

Moreover,

minor

TROUBLESHOOTING

Cleaning

Avoid the use of chemical cleaning agents which

might damage the plastics in this instrument. Avoid

chemicals containing benzene, toluene, xylene, ace-

tone, or similar solvents.

Front Panel. Loose dust may be removed with a soft

cloth or a dry brush. Water and mild detergent may be

used; however, abrasive cleaners should not be used.

Interior. Cleaning the interior of the unit should pre-

cede calibration, since the cleaning process could alter the

settings of the calibration adjustments. Use low-velocity
compressed air to blow
dirt can be removed with a soft, dry brush, cotton-tipped

swab, or cloth dampened with a mild detergent and water

solution.

off the accumulated dust. Hardened

General

The following
contained in other sections of this manual when trouble­shooting the 7A 18. The schematic diagrams, Circuit
Description, and Calibration sections should be used to full
advantage.
information on circuit behavior and output requirements.

The Circuit Description section gives detailed

is

provided to augment information

Troubleshooting Aids

Diagrams. Circuit diagrams are given on foldout pages in
Section 7.
component in this instrument are shown on the diagrams.
important voltages are also shown.

Circuit Board. The circuit board used in the
outlined on the schematic diagrams, and a photograph of
the board
board-mounted electrical component
photograph by

The circuit number and electrical value of each

7A18

is

shown on the back of Diagram

is

identified on the

its

circuit number.

1.

Each

is

2.

7A18

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

7A18 are color coded to facilitate tracing a wire from one

point to another in the unit.

4-1

Semiconductor Lead Configuration. Fig.

lead configuration of the semiconductor devices used in this
instrument.

shows the

DC Voltmeter and Ohmmeter -A voltmeter for
checking voltages within the circuit and an ohmmeter for
checking resistors and diodes are required.

3.

Test Oscilloscope-.A test oscilloscope is required to

view waveforms at different points in the circuit.

A Tektronix 7000-series Oscilloscope equipped with a
readout system,
Time-Base unit, and a 7A-series Amplifier unit with a
probe will meet the needs for items 2 and

7D13 Digital Multimeter unit, 76-series

10X

3.

Troubleshooting Procedure

This troubleshooting procedure is arranged in an order

Troubleshooting Equipment

The following equipment is useful for troubleshooting

the 7A 18.

1. Semiconductor Tester--Some means of testing the
transistors, diodes, and
helpful. A transistor-curve tracer such as the Tektronix tion of a control or front-panel connector, see the

Type 576 will give the most complete information.

FET's used in this instrument is

which checks the simple trouble possibilities before pro­ceeding with extensive troubleshooting.

1. Check Control Setting. An incorrect selting of the
7A18 controls can indicate a trouble that does not exist. If
there is any question about the correct function or opera-

Operating Instructions section.

Fig.

4-1.

Electrode configuration for semiconductors used in this instrument.

2.

7A18

Check Associated Equipment. Before proceeding
with troubleshooting of the
ment used with this instrument is operating correctly. If
possible, substitute an amplifier unit known to be operating

correctly into the indicator unit and

persists. Check that the inputs 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 cali­bration of the unit, or the affected circuit by performing

I

--

Part
trouble may only be a result of misadjusrment and may be
corrected by calibration. Complete calibration instructions
are given in Part

5.
component or stage can be located by checking for the
correct voltage or waveform in the circuit. Typical voltages
and waveforms are given on the diagrams; however, these
are not absolute and may vary slightly between instru­ments. To obtain operating conditions similar to those used
to take these readings, see the instructions in the Diagrams
section.

6. Check
methods are provided for checking the individual compo-

nents in the
are best checked by disconnecting one end to isolate the
measurement from the effects of surrounding circuitry.

A. 'TRANSISTORS AND INTEGRATED CIRCUITS.
The best check of transistor and integrated circuit
operation is actual performance under operating conditions.

If a transistor or integrated circuit is suspected of being

defective,

ponent known to be good; however, be sure that circuit
conditions are not
damaged. If substitute transistors are not available, use a
dynamic 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. Fig.
of semiconductor devices. Be sure the power is off before
attempting to remove or replace any transistor or integrated
circuit.

Performance Check of Section

II

of Section

Check Voltages and Waveforms. Often the defective

l

ndiv idual Components. The following

7A18. Components which are soldered in place

it

can best be checked by substituting a com-

sl.ich that a replacement might also be

4-1

shows base pin and socket arrangements

7A18, check that the equip-

see if the problem

5.

The apparent

5.

Integrated circuits can be checked with a voltmeter, test
oscilloscope, or by direct substitution. A good under­standing of the circuit description is essential to trouble­shooting circuits using integrated circuits. Use care when
checking voltages and waveforms around the integrated
circuits so that adjacent leads are not shorted together. An
integrated-circuit test clip provides a convenient

14-

clipping a test probe to the
cuits. Phis device also doubles as an integrated-circuit
extraction tool.

A

B. DIODES.

a

short circuit by measuring the resistance between termi­nals with an ohmmeter set to the R
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.

Do not use an ohmmeter scale that has a high 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. A leaky 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 a capacitance meter, or
by checking whether the capacitor passes AC signals.

7.

Repair and Readjust the Circuit. Special techniques
required to replace components in this unit are given under
Component Replacement. Be sure to check the perfor­mance of any circuit that has been repaired or that has had
any electrical components replaced. Recalibration of the

affected circuit may be necessary.

diode can be checked for an open or for

and 16-pin integrated cir-

X I k scale. The diode

rneans of

REPLACEMEN-T PARTS

Standard Parts

Ail electrical and mechanical part replacements for the

7A18

can be obtained through your local Tektronix Field

Office or representative. However, many of the standard
electronic components can be obtained locally in less time
than is required to order them from Tektronix, Inc. Before
purchasing or ordering replacement parts, check the parts

lists for value, tolerance, rating, and description.

REV.

D,

MAY,

1975

COMPONENT REPLACEMENT

7A18

When selecting replacement parts, it is important to
remember that the physical size and shape of the
component may affect its performance in the instru­ment. All replacement parts should be direct replace­ments unless it is known that a different component

will not adversely affect the instrument performance.

Special Parts

Some parts are manufactured or selected by Tektronix

to satisfy particular requirements, or are manufactured for
Tektronix to our specifications. These special parts are
indicated in the parts list by an asterisk preceding the part
number. Most of the mechanical parts used in this instru­ment have been manufactured by Tektronix. Order all
special parts directly from your local Tektronix Field
Office or representative.

Ordering Parts

When ordering replacement parts from Tektronix, Inc.,

refer to the Parts Ordering Information and Special Notes
and Symbols on the page immediately preceding the Elec­trical Parts List section. Include the following information:

1. lnstrument type

2.

Instrument Serial Number

3.

A description of the part (if electrical, include the

circuit number)

4.

Tektronix Part Number

(7A18)

General

The exploded-view drawing associated with the Mechan­ical Parts List may be helpful when disassembling or re­assembling individual components or sub-assemblies.

Circuit Board Removal

In general, the circuit boards used in the 7A18 need

never be removed unless they must be replaced. Electrical

connections to the boards are made by soldered con-

it

is

nections. If

assembly, use the following procedures.

A.

READOUT CIRCUIT BOARD REMOVAL (7A18 only)

1. Disconnect the wires connected to the outside of the

board.

2.

Remove the seven screws holding the board to the

mounting surface.

3.

Disconnect the wires connected to the inside of the

board.

4.

Remove the board from the unit.

5.

To replace the board, reverse the order of removal.

B. ATTENUATOR CIRCUIT BOARD REMOVAL

1. Remove the readout board as outlined in the previous

procedure.

necessary to replace a circuit board

Soldering Techniques

Attenuator Circuit Boards. The Attenuator circuit
boards are made from polyphenylene oxide because of
excellent electrical characteristics. Use more than normal
care when cleaning or soldering this material. The following
rules should be observed when removing or replacing parts:

1.

Use a very small soldering iron (not over 15 watts)

2.

Do not apply more heat, or apply heat for a longer

time, than is absolutely necessary.

3.

Use a vacuum-type desoldering tool to remove the

excess solder from the circuit board.

4.

Do not apply any solvent containing ketones, esters,

or halogenated hydrocarbons.

5. To clean, use only water-soluble detergents, ethyl,

methyl, or isopropyl alcohol.

its

2.

Disconnect the resistor/capacitor connected to the

rear of the board.

3.

Loosen the front set screw on the VARIABLEIGAIN

control shaft coupling (use a 0.050-inch hex-key wrench).

4.

Retnove the red VARIABLE control knob and rod

from the control shaft.

5. Remove the remaining front-panel knobs using a

111 6-inch hex-key wrench.

6. Remove the front panel from the instrument.

7.

Remove the attenuator shields.

8. Disconnect the wires and resistor from the input BNC

connector.

9.

Remove the input BNC connector.

10. Remove the POSITION control using a 5116-inch

7A18

nut driver.

11. Remove the attenuator board with cam switch from

the instrument.

12. To replace the board, reverse the order of removal.

C. AMPLIFIER CIRCUIT BOARD REMOVAL

1. Remove the Readout circuit boards as given pre-

viously.

2.

Remove the plastic plug-in guide from the rear of the

instrument.

3.

Disconnect the wires connected to the board from

the front-panel controls.

B. ROTARY SWITCHES

Single wafers on the DISPLAY MODE and TRIGGER
SOURCE switches are not normally replaced. If any part of
these switches
should be replaced. A new switch can be ordered through
your Tektronix Field Office.

When disconnecting or connecting leads to a wafer-

type rotary switch, do not let solder flow around and

beyond the rivet on the switch terminal. Excessive
solder can destroy the spring tension of the contact.

is

defective, the entire switch assembly

Transistor and Integrated Circuit Replacement

4.

Loosen the front hex-socket screw in the front

coupling of the VARIABLE control shaft using a

0.050-inch hex-key wrench. Pull the VARIABLE knob and
shaft from the front of the instrument.

5. Loosen the front hex-socket screw in the coupling

between the DISPLAY MODE and 'TRIGGER SOUCE

switch sections. Pull the T'RIGGER SOURCE knob and

long shaft from the front of the instrument.

6. Loosen the front hex-socket screw in the coupling of

the DISPLAY MODE switch shaft using a 5116-inch

key wrench. Pull the DISPLAY MODE knob and long shaft

from the front of the instrument.

7.

D i scon nect the resistor-capacitor combinations

connected to the ceramic strips at the front of the board.

8.

Remove the screws and nuts securing the board to

the chassis or other mounting surface.

9.

Remove the board from the instrument.

10. To replace, reverse the order of removal.

hex-

Switch Replacement

Several types of switches are used in the 7A18. The
following special maintenance information is provided for
the cam-type switches and rotary switches.

A. CAM-TYPE SWITCHES

CAUTION

m

Repair of cam-type switches should be undertaken

only by experienced maintenance personnel. Switch
alignment and spring tension of the contacts must be
carefully maintained for proper operation of the
switch. For assistance in maintenance of the cam-type
switches, contact your local Tektronix Field Office or

resen ta tive.

rep

Transistors and IC's should not be replaced unless
they are actually defective. If removed from their sockets
during routine maintenance, return them to their original
sockets. Special care must be given to integrated circuit
leads, because they can easily be damaged in removal
from sockets. Unnecessary replacement or switching of
components may affect the calibration of the instrument.
When a transistor
part of the instrument that may be affected.

is

replaced, check the operation of that

Recalibration After Repair

After any electrical component has been replaced, the
calibration of that particular circuit should be checked, as
well as the calibration of other closely related circuits.
Performance Check instructions given in Part I of Section
provide a quick and convenient means of checking the
instrument operation. The Calibration Procedure in Part
of Section 5 can then be used to adjust the operation to
meet the Performance Requirements listed in Section 1.

The

5

II

Repackaging for Shipment

If the 1-ektronix instrument is to be shipped to a 1-ektronix
Service Center for service or repair, attach a tag showing:
owner (with address) and the name of an individual
firm that can be contacted, complete instrument serial
number and a description of the service required.

Save and re-use the package in which your instrument was
shipped. If the original packaging
available, repackage the instrument as follows:

Surround the instrument with polyethylene sheeting to
protect the finish of the instrument. Obtain a
corrugated cardboard of the correct carton strength and
having inside dimensions of no
than the instrument dimensions. Cushion the instrument
by tightly packing three inches of dunnage or urethane
foam between carton and instrument, on all sides. Seal
carton with shipping tape or industrial stapler

The carton

test

strength for your instrument is 200 pounds.

is

unfit for use or not

less

than six inches more

at

your

caiton of

REV.

E

DEC

1976

4-5

Change information, if any, affecting this sec

7A18

,tion will be found at the rear of the manual.

Recalibration l nterval

To assure instrument accuracy, check the calibration of

7A18 every 1000 hours of operation, or every six

the

months if used infrequently. Before complete calibration,
thoroughly clean and inspect this instrument as outlined in
the Maintenance section.

Tektronix Field Service

Tektronix, Inc. provides complete instrument repair and

recalibration at local Field Service Centers and the Factory
Service Center. Contact your local Tektronix Field Office

or representative for further information.

Using 'This Procedure

General. This section provides several features to facili

tate checking or adjusting the 7A18. These are:

Index. To aid in locating a step in the Performance

Check or Adjustment procedure, an index is given pre-

----

ceding Part I

ment procedure.

Performance Check and Part II

--

Adjust-

instrument is both correctly adjusted and performing

within all Performance Requirements as given in Section

TEST'

EQUIPMEN'T REQUIRED

General

The following test equipment and accessories, or its
equivalent, is required for complete calibration of the
7A18. Specifications given for the test equipment are the
minimum necessary for accurate calibration. Therefore,
some of the specifications listed here may be somewhat less
precise than the actual performance capabilities of the test
equipment. All test equipment is assumed to be correctly
calibrated and operating within the listed specifications.

The Performance Check and Adjustment procedures are
based on this recommended equipment. If other equipment
is substituted, control settings or calibration
need to be altered to meet
ment used. Detailed operating instructions for the test
equipment are not given in this procedure. Refer to the
instruction manual for the test equipment if more infor­mation is needed.

tlie requirements of the equip-

setup may

1.

Performance Check. The performance of this instru­ment can be checked without removing the side shields or
making internal adjustments by performing only Part I
Performance Check. This procedure checks the instrument

against the tolerances listed in the Performance Requirement
column of Section 1. In addition, a cross-reference is pro-

II

---

vided to the step in Part
the instrument to correct calibration. In most cases, the
adjustment step can be performed without changing control
settings or equipment connections.

Adjustment Procedure. To return this instrument to

correct calibration with the minimum number of steps,
perform only Part
cedure gives the recommended calibration procedure for all
circuits in this instrument.

Complete Performance

pletely check and adjust all parts of this instrument, per-

form both Parts I and I I. Start the complete procedure by
performing the Adjustment procedure and follow this with
the Performance Check. 'i'his method will assure that the

II

Adjustment which will return

--

Adjustment. The Adjustment pro-

Check/Adjustment. To com-

Calibration Equipment Alternatives

All of the test equipment is required to completely
check and adjust this instrument. However, some of the
items used only for the Performance Check can be deleted
without compromising
capabilities. For example, the low-frequency
amplitude signal generator is used on1 y in the Performance
Check and may be deleted if the user does not desire to

check the lower frequency response or
operation. Equipment used only for the Performance
Check procedure is indicated by note
only for
note

tlie Adjustment procedure are indicated by

2.

the instrument's measurement

constant-

triggel- source

1

;

items required

Test Equipment

1.

7000-series oscilloscope, referred to
Oscilloscope in this procedure. Tektronix 7403 recom­mended.

2.

T'ime-Base plug-in unit, Tektronix 7B50.

as

the Indicator