Table Of ContentM o d e rn E l e c t r o n ic
M a i n t e n a n ce
P r i n c i p l es
by D.J. Garland and F.W. Stainer
Pergamon Press Limited
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Printed in Great Britain
First Edition 1970
Copyright © 1970 Pergamon Press Limited
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Library of Congress Catalog Card No. 74-101488
08 014188 9
THE A U T H O RS
F.W. Stainer was, until 1 967, a mennber of the Research Branch, Headquarters Technical Training
Command, Royal Air Force. He was responsible for most of the RAF research on the subject matter of this
book, including working out the new methods which are advocated, and carrying out experiments on
technicians under training in order to test the methods he evolved. In 1 967 he was loaned by the Ministry
of Aviation to the European Space Research Organization, and is now manager of the ESRO satellite
telemetry station in the Falkland Islands, where he is in charge of the operation and maintenance of the
station. He is an Associate Member of the Institution of Electrical Engineers.
D.J. Garland, B.Sc, C.Eng., MIEE, MIERE, at present a Group Captain in the Royal Air Force, was
associated for some years with the work at Headquarters Technical Training Command in his capacity as
a staff officer responsible for the training of all electronic and guided missile maintenance technicians of
the Royal Air Force. He was connected closely with the introduction of the new maintenance training
methods described in this book, and has been concerned, as a training officer at various RAF radio
schools, with the implementation of these methods. He is at present a staff officer at the RAF Group
Headquarters which is responsible for the efficiency of all technician training in the Royal Air Force.
A C K N O W L E D G E M E N TS
The authors would like to thank Mr. AJ. Kenward for helpful suggestions, and the British Motor
Corporation, Ltd., for permission to reproduce Figure 8.1 (a).
Figures 8.1 b, 8.2a, 8.2b and 9.5 are reproduced by permission of the Controller of Her Majesty's Stationery
Office.
VI
THE A U T H O RS
F.W. Stainer was, until 1 967, a mennber of the Research Branch, Headquarters Technical Training
Command, Royal Air Force. He was responsible for most of the RAF research on the subject matter of this
book, including working out the new methods which are advocated, and carrying out experiments on
technicians under training in order to test the methods he evolved. In 1 967 he was loaned by the Ministry
of Aviation to the European Space Research Organization, and is now manager of the ESRO satellite
telemetry station in the Falkland Islands, where he is in charge of the operation and maintenance of the
station. He is an Associate Member of the Institution of Electrical Engineers.
D.J. Garland, B.Sc, C.Eng., MIEE, MIERE, at present a Group Captain in the Royal Air Force, was
associated for some years with the work at Headquarters Technical Training Command in his capacity as
a staff officer responsible for the training of all electronic and guided missile maintenance technicians of
the Royal Air Force. He was connected closely with the introduction of the new maintenance training
methods described in this book, and has been concerned, as a training officer at various RAF radio
schools, with the implementation of these methods. He is at present a staff officer at the RAF Group
Headquarters which is responsible for the efficiency of all technician training in the Royal Air Force.
A C K N O W L E D G E M E N TS
The authors would like to thank Mr. AJ. Kenward for helpful suggestions, and the British Motor
Corporation, Ltd., for permission to reproduce Figure 8.1 (a).
Figures 8.1 b, 8.2a, 8.2b and 9.5 are reproduced by permission of the Controller of Her Majesty's Stationery
Office.
VI
P R E F A CE
The ever-increasing complexity of electronic equipment continuously increases the demands made
on the maintenance technician. Until recently reliance has been placed on theoretical training, leaving
the techniques of maintenance to be gained by experience. This approach to training for maintenance
results in the need to obtain technicians of high ability and to provide long and expensive training courses.
It is unfortunately true that most courses for technicians contain little or no instruction in the
principles of maintenance or in systematic, logical procedures supported by efficient and instructive aids
to maintenance. The reason for this omission undoubtedly lies in the lack of suitable information.
The problem of training technicians to maintain modern, complex electronic equipment has faced
the Royal Air Force acutely in recent years, and research investigations and experiments were conducted
in order to find an economical and logical method of training for maintenance. These efforts resulted in
the adoption of new methods of training which are based upon the principles described in this book.
To the best of the authors' knowledge this is the first book dealing with the logical bases of
electronic maintenance which is suitable for technicians, but it should also interest engineers,
technologists and supervisors who have an interest in, or responsibility for, maintenance or the training
of maintenance personnel. The bias of the book is electronic, but the principles described are applicable
to other complex systems such as instrumentation, hydraulics and electric power.
This is not a servicing handbook, but deals with the fundamentals of maintenance. It covers, with
examples, the basic principles of maintenance using logical methods which have been developed
recently. It describes new aids to maintenance which can assist the technician in his maintenance task.
Unfamiliar subjects such as the half-split method of fault location, functional diagrams and fault finding
guides are explained.
Because reliability is so closely connected with maintenance, and is so important to the maintenance
technician, this subject is explained in some detail. The few mathematical proofs are confined to the
Appendices to Chapters 5 and 6, and can be omitted. Otherwise, there is very little mathematics in the
book, although an exponential expression appears in connection with reliability.
Use is made of the modern graphical symbols that were introduced in British Standard BS 3939,
and the names of quantities and units conform to the SI system of units which is soon to be introduced
generally into British industry. The note on page ix gives further details.
The book should interest technicians pursuing Ordinary and Higher National Certificate courses and
City and Guilds technicians' courses; students following these courses are expected to know the
fundamentals of logical fault-finding. It should also interest engineers, training staffs and maintenance
technicians everywhere.
VII
A B B R E V I A T I O NS A ND S Y M B O LS
ABBREVIATIONS
This country's iinpending change from its present system of units to the metric system coincides
with the introduction of a set of internationally recognized units known as Systeme International (SI)
units. As far as electrical units are concerned, SI units are almost the same as Metre-Kilogram-Second
(MKS) units, and common units such as volts, amperes, farads, henrys and ohms remain unchanged.
The only recently-named SI unit that occurs in this book is hertz (Hz), the unit of frequency: one
hertz is a frequency of one cycle per second. Multiples of hertz are formed in the usual way:
1000 Hz =λ kHz (kilohertz)
1000 kHz - 1 MHz (megahertz)
1000 MHz = 1 GHz (gigahertz)
Logarithm to base 10 is written log, and logarithm to base e is In.
SYMBOLS
The authority for symbols to be used in circuit diagrams has, for some years, been a British
Standard, BS 530 :1 948 (Graphical Symbols for Telecommunications). This standard is being
superseded by BS 3939 (Graphical Symbols for Electrical Power, Telecommunications and Electronics
Diagrams), which, at the time of writing, is still being published in sections. The symbols used in this
book are up-to-date at the time of going to press, but it may be that some symbols will be changed as
more of those in BS 530 are superseded by BS 3939.
Some of the new symbols are shown below; the biggest change is undoubtedly in the new
internationally agreed symbols for resistors.
Screened conductor 1 )
Fixed resistor
Fixed resistor with fixed tapping
(voltage divider) -{"
Fixed resistor with variable tapping _j 1
(potentiometer) ~l ^ '
Variable resistor —| \-
Plug _
Socket
Microphone Q|-
Inductor nr>r\-
Switch
CHAPTER 1 THE NEED FOR MAINTENANCE PRINCIPLES
THE PROBLEMS OF COMPLEXITY particular unit. This can then be changed rapidly,
thus keeping the computer serviceable, and the
The high cost of maintaining modern electronic faulty component within the unit can be traced
equipment—retaining it in, or restoring it to, a and rectified separately. A price is paid for this
serviceable state—presents a major problem to reduction in the time that the computer is un
those who use the equipment. The problem is serviceable, and it is that the user has to keep a
aggravated by the tendency to build and use equip stock of units as well as of components.
ment of ever-increasing complexity. It is a strange
fact that the more reliable electronic components The computer is an example which illustrates
become, the greater is the problem of maintenance. the situation where a particular policy has been
The modern, more reliable components enable adopted with a view to making its operation and
equipments to be designed which, because of the maintenance of maximum efficiency. This approach
huge number of components that are incorporated, should be made to all equipment. The relative
were just not feasible a few years ago. Because of priority given to all the factors concerned in the
the large number of components used, and the maintenance of a particular equipment is referred
associated increase of interconnections between to as the for that equipment.
maintenance policy
them, the circuits are complex. This complexity The factors which have to be considered in
produces two disadvantageous effects. formulating a maintenance policy are discussed
in Chapter 4.
The first is that although the reliability of an
individual component may have improved with
advancing technology, the number of components MAINTENANCE TECHNICIANS
used is so large that the probability of
something The requirements for a maintenance technician
failing, and hence rendering the equipment un
which are generally accepted at present are a com
serviceable, is quite high during a fairly short
prehensive background of fundamental theory,
period of time. The reliability of the system may
basic manual skills such as the use of hand tools
therefore be no better than that of less complex
and soldering, some general guidance regarding
systems using less reliable components. For example,
fault location, such as the types of fault usually
a unit containing 10 components, each likely to fail
encountered in a particular equipment, and
twice in 100 hours of operation, could be expected
sufficient experience to develop the ability to
to fail 20 times in that period. If the number of
trace faults quickly. Emphasis has always been
components is doubled, then the likelihood of
placed on the theoretical aspect, and it is for this
failure of each component must be reduced to
reason that its relevance to the maintenance task
once in 100 hours if the unit is to maintain the
has been the subject of considerable research. Some
same failure rate. It is much easier to double the
of this research is discussed in Chapter 12.
number of components than to double their
reliability!
It is not intended in this book to discuss how much
theory is necessary for a technician, but only to note
The second effect is that due to the complexity
that as new electronic devices are discovered and
of the equipment the technician will take longer
applied the amount of supporting theory expands.
to trace a fault than on simpler equipment. The
reasons for this are that there are more possible Further, because the equipments being considered
sources of the fault, the relationships between the are late 20th-century, they are usually not easy to
components are more difficult to deal with, and understand. This results in the need to recruit
the technician has a much more extensive set of technicians of high intellectual calibre who are able
fault indications to analyze. to absorb the background knowledge, and in the
need to provide long and expensive training courses.
Computers provide a good illustration. Before
the transistor was invented one of the biggest Improved performance in the maintenance task
factors affecting the reliability of electronic can be expected if time is spent in analyzing the
equipment was the high failure rate of valves, and knowledge and skill required for the task, in
systems employing tens of thousands of valves developing aids to the task, and in working out
were not feasible on the grounds of unreliability methods of imparting the necessary knowledge and
alone. Transistors are, however, very much more skills efficiently. New methods are, of course, only
reliable than valves, and computers employing acceptable if they prove themselves in realistic
tens of thousands of transistors are feasible, and situations. Work of this nature has been carried out
are made. When a fault does occur, however, it in recent years, but the results are not generally
takes a long time to localize it. To avoid having available. It is undoubtedly for this reason that'
the whole computer unserviceable for long periods, most existing courses for maintenance technicians
modern design is such that the computer is built of include instruction neither in the principles of
replaceable units, each consisting of a number of maintenance, nor in systematic, logical procedures
components, and the fault is first localized to a supported by efficient and instructive maintenance
aids. This book is intended to help to fill the gap.
1
A LOGICAL APPROACH ledge of the relationships between the components
of the equipment. It is possible to prepare diagrams
The problems of training technicians to maintain which emphasize this information. These are termed
modern complex electronic equipment have been functional diagrams, and they are described in detail
most acute in the armed Services, particularly in Chapter 8. It is also possible to prepare rules
where recruitment is voluntary and sufficient which define the best method of narrowing down
numbers of recruits of high ability are difficult the faulty area; these are based on the fundamental
to obtain. This has led to a lot of research into half-split rule which is described in Chapter 6.
maintenance methods and training being under Having decided upon the information which is
taken, and some of this book stems from this required for maintenance and the best method of
research and the application of the results. displaying it, and having decided upon the best
Although the origins may be military, the results method of using this information, the following
are directly applicable to any industry which is questions arise. How is this knowledge best
faced with the same problems. conveyed to the technician? Should the maintenance
be automated or should a man be trained to carry
Investigations have shown that the most impor it out? These questions are also fully discussed in
tant aspect of fault diagnosis is knowledge of the this book.
fundamental structure of equipment, i.e. know
CHAPTER 2 RELIABILITY
INTRODUCTION The nearest approach that has been made to
eliminating maintenance completely is in the
Maintenance cannot be divorced from relia manufacture of unmanned satellites, for when
bility; if an equipment could be made perfectly they are in orbit maintenance is extremely diffi
reliable no maintenance would be necessary at all. cult. It is undoubtedly due to the development
It is essential therefore for the maintenance tech of earth satellites that great interest has been
nician to know something about reliability and its taken in reliability in recent years. However, the
relationship with the problems of maintenance. astronomical cost of achieving the standard of
A knowledge of which components are most reliability which is required is well known, as is
reliable, and which least reliable, assists in the
the fact that failures still occur.
maintenance task. The effects on reliability, and
hence on maintenance, of such factors as tem
A point is therefore reached, in the quest
perature, humidity and vibration are also impor
for better reliability, where a balance is struck
tant, as are special methods, such as redundancy,
between the increasing costs of development
by which reliability can be improved at the
and production and the savings made in main
design stage.
tenance personnel and spares. The position of this
optimum balance point depends upon a number
This chapter discusses the connection
of factors. The cost of improving the reliability
between reliability and maintenance, and also
of resistors, for example, may soon be recovered
defines the terms 'failure rate', Yeliability',
since they are manufactured and sold by the
'mean time between failures', and 'availability'.
million; for travelling-wave tubes, however, this
The connection between these quantities is
is not so.
developed, and examples are given using typical
component and equipment reliability figures.
The method of calculating the reliability of a RELIABILITY AND MAINTENANCE
system is shown, major causes of unreliability are
Within reasonable limits increased reliability
stated, and methods of improving reliability are
pays off in reduced costs of maintenance personnel,
described.
spares and test equipment. More reliable com
ponents, however, tend to generate more complex
WHY PERFECT RELIABILITY
equipment, using more components and thus to
CANNOT BE ACHIEVED
create an overall reliability problem again. In
creased reliability of components has enabled
In theory maintenance could be avoided
automatic test equipment to be developed. This
completely by making equipment perfectly
is discussed further in Chapter 10.
reliable. In practice, of course, perfect reliability
cannot be achieved. No-one has ever made any
In the Royal Air Force, studies show that at
thing that never wears out, never has a defect,
station and squadron level total maintenance costs
and that would last for ever. Perfect reliability
for aircraft are about 40% of the total station
is like perpetual motion — interesting to imagine,
costs, and that one-quarter of this is in respect of
but not practical.
electronic equipment. These figures include
personnel and spares. If the reliability of all air
But it may be thought that a piece of equip
craft electronic equipment were doubled, it is
ment could be made of components that were so
estimated that the consequent saving in personnel
reliable that maintenance would be unnecessary
and spares costs would be just under 2% of the
before it became out-of-date. Again, in theory
total cost of running the Royal Air Force. This
this is possible, and, in fact, development of new
estimate ignores the increased cost of providing
components which are more reliable than the old
the electronic equipment which is twice as reliable,
is a ceaseless activity. However, we have already
and is therefore highly unrealistic, but it does give
mentioned the fact that increases in component
an ideaof the magnitude of the sums involved and
reliability usually lead to an increase in complexity.
of the possible savings. Even modest improvements
Also, the development of more reliable components
in reliability can produce savings of the order of
costs money; research and testing have to be paid for.
millions of pounds.
Production of more reliable components involves the
introduction of new methods and machinery for manu
facture, and of more stringent production testing. HOW RELIABILITY IS MEASURED
A stage is reached where the cost of producing So far we have discussed some aspects of
more reliable components outweighs the savings reliability but have avoided the question 'What is
that are made by reduced maintenance costs and meant by reliability?' Reliability is connected
with failure and there isa straightforward relation
reduced holdings of spares. When this stage is
ship between failure and reliability. Let us consider
reached, improving the reliability of the equipment
failure first.
becomes too expensive to be practical.
FAILURE RATE failures are finished a period of low, constant
failure rate follows.
Failure of a component, unit or equipment
occurs when it fails to perform its intended The constant failure-rate period is known as
function under operating conditions. A failure the useful life, and during this period failures are
may be loss of output, or a change in output to random in nature. This means that failures are due
a value outside the specification limits. Spec- to chance alone, and that the chance of a failure
fication limits are necessary because, in a complex occurring is the same at all times during the useful
equipment, the tolerances in the various circuit life. This is the period of most interest to us, for
outputs must be small so that errors accumulated it covers the normal life-span of the equipment
in a large number of circuits do not become too or component.
great.
Finally, the wear-out period shows a rising
The failure rate of a component or equipment failure rate as the effects of age, usage and chance
is the number of failures divided by the time combine to give an increasingly unreliable equip
during which they occur. Thus it may be found ment.
that in a piece of equipment 33 resistors fail per
year. This figure is not helpful as it stands since These three periods apply to motor cars as
the total number of resistors is not mentioned, well as to electronic equipment, and help to
and failure rate is often quoted as percentage determine such factors as the best time to sell or
of failures in a given time. Suppose there are 1000 buy, and secondhand values.
resistors; the failure rate can then be stated as
3.3% per year. If in the same equipment there are
RELIABILITY MEASURED AS A PROBABILITY
120 switches, and every year six of these fail, the
failure rate for switches is 5% per year. This When failures are due to chance alone, and
method of expressing failure rates enables the
thus occur during the period known as the useful
reliability of resistors and switches to be compared
life of the component or equipment, the reliability
directly as far as their use in this particular equip
is also dependent on chance; it is dependent on the
ment is concerned. Note, however, that in a
chance of failure occurring. The term reliability
different equipment these figures would not apply;
expresses the chance that a component or equip
the switches may have to carry more current, for
ment will function normally for a specified period
example, and thus would probably show a higher
of time; it is the probability that it will perform
failure rate.
its proper function under normal operating
conditions for a specified period of time.
The failure rate for components is often
expressed as percentage failures per 1000 hours,
Probability expresses the chance of a certain
but for relays and switches it may be expressed as
event happening. It may be, for example, that
percentage failure per 1000 operations.
over a large number of years it is found that it
snows on Ist January one year in eight. The chance,
Study of electronic components and equip
or probability, that it will snow on 1st January
ments shows that under normal conditions the
next year can then be said to be one in eight, or g,
failure rate varies as in Figure 2.1.
or \ T/2%, Probability can be quoted asa number
or as a percentage. It can never be greater than 1,
or 100%, because these figures represent absolute
certainty that an event will occur.
If we say that an equipment has a reliability
of 70% for a 100 hour period, we mean that if we
operate it for 100 hours, seven times out of ten it
will function correctly with no failures.
Initial
Useful life Wear-out
failure
Reliability as defined above involves time.
Time
A statement that a certain equipment is '95%
Figure 2.1 Variation of failure rate with time reliable' is meaningless unless the time is quoted.
There is, as has been mentioned, a close
There is an initial period of high failure rate
relationship between reliability and failure rate,
during what is known as the burn-in, initial-failure
and it is a very simple relationship. Taking our
or infant-mortality period, during which time the
equipment which is 70% reliable over a 100 hour
failure rate falls to a steady value. The initial-failure
period, this means that seven times out of ten it
period is the fairly short period during which failure
will function properly when operated for 100 hours.
due to faulty manufacture occurs. When these
