
This manual will give you a solid understanding in common electronic terminology and symbols, as well as the construction and operation of common electronic components. The general troubleshooting process is explained followed by a brief study of various hand tools and electronic test and measuring instruments. You will learn to implement procedures for the testing of electronic components as well as skills for carrying out simple repair procedures for the correction of faults on printed circuit boards with confidence.
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1 Introduction to Troubleshooting
1 Introduction to Troubleshooting
Electronic equipment can develop a wide variety of problems. The act of troubleshooting arises in order to make the problems disappear so that the equipment works as per the expectation. This introductory chapter provides an overview of troubleshooting processes and various troubleshooting techniques. It also emphasizes how to prepare and read a circuit diagram, as a first step for troubleshooting.
Learning objectives
In general, for any application, equipment is designed and manufactured to work trouble free within its specified limits during its useful time. However, sometimes there is a conflict between the expectations of the user and the performance of the instrument. Thus develops the need for troubleshooting and maintenance.
What is Troubleshooting?
Troubleshooting is the process of isolating and correcting a problem in malfunctioning equipment so that it returns to its expected performance level. The process of troubleshooting requires a systematic fault finding approach. Whenever a fault occurs, two things can happen:
The second type of fault can be further sub-divided into:
The basics which can be applied to troubleshooting are given below:
The following table shows a list of component failure in their order of probable occurrence:
Table 1.1
Order of occurrence of component failure
Order of occurrence of failure |
Component |
1 |
Mechanical and electromechanical devices such as relays, switches, plugs and sockets.
|
2 |
Components that get hot in their normal operation, like power amplifiers and rectifiers.
|
3 |
Electrolytic capacitors of small versions and those subjected to high voltage.
|
4 |
Active devices like transistors and SCRs.
|
5 |
Passive devices like resistors and capacitors.
|
The Troubleshooting Process
The process of troubleshooting comprises the following steps:
Let's address these steps in brief:
Fault Establishment
It is important to establish the presence of a fault in equipment before taking any other action. In some cases a system may be reported faulty, but it could be a case of faulty operation or a system failure may be reported with either very little or misleading information. It is essential that a functional test, checking the system's actual performance against its specification must be made and all fault systems must be noted.
It is also important to check the history of the equipment and repair and servicing work carried out earlier by any other person.
Fault Location
This involves pin-pointing the cause of the fault by studying the literature relevant to servicing, maintenance and repairs. The fault is located first in subsystem and then in a single component in the sub system.
Fault Correction
Fault correction consists in replacing or repairing the faulty component. This is followed by a thorough functional check on the whole system.
Figure 1.1 Troubleshooting Procedure
Troubleshooting Aids
Troubleshooting aids help in quickly analyzing a malfunction and taking corrective action. The following points are discussed in this aspect:
Tools
A basic set of tools and test equipment like multimeter and oscilloscopes are necessary. Sometimes specialized equipment are required, such as a high speed scope. The maintenance technician is required to have all this knowledge.
Documents
A complete set of documentation is a must. Most manufacturers supply the following documents with their manuals:
Data Manuals
A good list of data manuals is essential. There are data books from all major component manufacturers which can be collected.
Thumb Rule in Troubleshooting:
The various troubleshooting techniques given below are used in the majority of electronic systems. The type of system being handled will decide which technique should be adopted.
Functional Area Approach
An electronic system comprises several functional parts such as power supplies, amplifier, signal converters, etc. When the system fails to give the expected performance, the problem could be in any of these functional areas. Therefore, it is essential to troubleshoot the system in order to isolate the fault to the failing functional area and then to the failing component. The logical approach of isolating a fault is through a process of elimination of the functional areas that are performing properly. Once a failure is isolated, further analysis of the circuitry within this area is carried out to isolate the malfunction to the faulty component. This functional area approach is also called the Block- Diagram approach to troubleshooting.
Split Half Method
In this technique, as the name suggests, the circuit is split in half and the output is checked at the half-way point in case of an absence of an output. This helps to isolate the failing circuit in the first or second part. When the faulty half is determined, the ageing circuit is split into half for further isolation of failure. This splitting is continued until the failure is isolated to one function or component.
The Half- split method is extremely useful when the system is made up of a large number of blocks in the series:
Figure 1.2
Split Half Method
Many electronic systems do not involve only series connected blocks. They may have feedback loops or parallel branches in a part of the circuit. Hence use of this method is rather restricted.
Figure 1.3
Divergent Path
Here the output from one block is fed to two or more blocks. In such systems, it is best to start by checking the common feed point. Alternatively if output is normal (at A or B in fig. 1.3), check after the divergence point. Conversely, if one output is abnormal, check before the common point. The most common example is that of the power supply circuit which supplies dc power to various subsystems in equipment.
Convergent Path
In a convergent path two or more input lines feed a circuit block:
Figure 1.4
Convergent Path
In order to check such a scheme, all inputs at or before the point of convergence must be checked one by one. If any of the inputs is incorrect (at C or D in fig. 1.4), then the fault lies in that particular input circuit. If all are found to be correct, the fault lies beyond the convergent point. For example, if C and D are correct and there is no output at E, the fault lies in unit 3. But if input at C is faulty, the fault lies in block 1 or before that.
Feedback Path
The feedback loop usually corrects the output of some block with the input of an earlier block via some network called feedback network. Since the circuit behaves as a closed loop, any fault within the loop will appear as if all the output blocks within the system are at fault:
Figure 1.5
Feedback Path`
Before starting the troubleshooting of a system having feedback loop, the type of the feedback and its use should be well understood. Feedback paths are basically provided for the following functions:
Having identified the type of feedback circuit, one can proceed as follows regarding locating the fault.
For the first type, i.e. modifying feedback, it may be possible to break the feedback loop and convert the system to a straight linear data flow. Each block can then be tested separately without the fault signal to be fed around the loop. In some cases instead of completely breaking the loop, the feedback can be modified at or near the point where it rejoins the main forward path. If the output appears normal, check the feedback path, otherwise, check the forward path.
For the second type, i.e. sustaining type, feedback is disconnected from the output and a suitable test signal is injected to check the performance of various circuit blocks.
Figure 1.6
Fault Location in Switching Path (rsk40)
If a system has switch-able parts and if the circuit function is found faulty in one position of the switch then throw the switch to another position. If the problem persists, check the switch in common circuitry. If the problem disappears with this action, check that the circuitry switched out.
1.3 Gaining Circuit Familiarity
A circuit diagram is a graphical representation of the interconnections of various components constituting the equipment. It is the most important document for the maintenance technician. Usually every assembly in electronic equipment is assigned an assembly number which appears on the circuit board and on the diagram. Commonly used symbols in electronic circuits are shown below in Figure 1.7.
The maintenance technician should be well versed with the circuit of the system before actually starting troubleshooting. A circuit diagram is the most important document for the technician. Many-a-time the circuit diagram of the system or equipment is not ready or not provided by the manufacturer. In that case, the technician has to prepare the circuit diagram. The circuit diagram makes the fault finding process easy.
Preparing a Circuit Diagram
The technician should be experienced enough to draw a circuit diagram. Usually, it is not recommended for larger systems. A larger system is broken into parts (subsystems) and then circuit diagrams for the smaller, suspected systems is drawn to trace the fault. The following points must be noted when preparing a circuit diagram:
Reading a Circuit Diagram
A circuit diagram is a graphical representation of interconnections of various components constituting the equipment. It is the most important document for the maintenance technician. Usually every assembly in electronic equipment is assigned an assembly number which appears on the circuit board and on the diagram. Commonly used symbols in electronic circuits are shown below:
Table 1.2
Various Symbols in electronic circuits
Circuit symbols to DIN EN, NEMA ICS/ANSI/IEEE/CSA
(The following comparison of circuit symbols is based upon the following international/national specifications:
-IEC 60617 graphic symbol database (DIN EN 60617-2 to DIN EN 60617-12
-NEMA ICS 19-2002, ANSI Y32.2/IEEE 315/315 A, CSA Z99 )
A circuit diagram illustrating some symbols is also shown below:
Figure 1.7
Electronic Circuit Diagram
The Making of an Electronic Circuit
An electronic circuit makes use of both active and passive components. These components are physically interconnected with each other to form any electronic circuit. There are three major techniques to interconnect the components. Let us have a brief overlook of these methods:
Solder
This method makes use of a solder and a wire to interconnect electronic components. It is a very slow method and is very cumbersome if a large number of devices are to be connected.
Wire-wrap
This technique tightly winds a small gauge wire around a wire-wrap metal post or terminal. There are special wire-wrap metal post sockets for the ICs that have longer posts for wire-wrapping the wire. Also, special tools are needed for wrapping and un-wrapping the wire.
Printed Circuit Board (PCB)
This technique includes interconnections between points printed in metal on the non-conductive board. The circuit is printed on the board by a series of photographic and chemical procedures. Most of the equipment in practice make use of PCB. They are generally made for completely checked out and working boards, as it is difficult to make wiring changes on the PCB.
A Quick Glance at Electronic Equipment
There can be one or more circuit boards inside of electronic equipment. They are mounted inside a wooden or metallic cabinet with some arrangement of interconnecting the circuit boards. This arrangement is called Edge Connectors.
The purpose of edge connectors is to bring signals and powers to and from the circuit boards without having to connect a wire to the circuit board itself. This arrangement provides easy installation and removal of the circuit board in equipment:
Figure 1.8
Electronic Equipment
Also inside of electronic equipment there is an assembly called card rack for the compact placement of the PCBs. But it is difficult to put a test probe on the circuit board for making any type of measurement or for troubleshooting. To eliminate this problem, special circuit boards called 'extender cards' are inserted into the card rack and the circuit board is extended into the extender card.
An extender card is just a wiring extension to make the circuit board accessible for testing.
Classification of System
In the literature, terms such as component, equipment or system have been used. The following table distinguishes between these terms:
Table 1.3
Classification of System
Serial Number |
Nomenclature |
Description |
Example |
1 |
System |
Collection of equipment arranged to perform a function. |
Television, Missile.
|
2 |
Equipment |
Collection of components arranged to operate without the need for other components. |
Radio Transmitter, the central part of a missile.
|
3 |
Assembly |
Collection of components in a prescribed order not all of which have, as yet, been so arranged. |
Terminal board with components parts attached.
|
4 |
Component |
Collection of elements arranged in a prescribed order. |
Resistor.
|
5 |
Element |
A simple object which can not be further sub-divided. |
Filament, a relay contact.
|
1.4 Getting Prepared for Troubleshooting
Where do you start?
A close visual inspection is a good and quick start for troubleshooting. It gives a clue for problems such as burned spots and places where high voltage arc has occurred. A quick look to the circuit also gives an idea of the condition of fuses and circuit breakers.
The troubleshooting technician should collect the history of the system. He/she should know whether the problem had occurred before and what is the frequency of the occurrence of the problem.
On the basis of the knowledge of how the system works, the kind of failure can be detected. This would lead you to select the troubleshooting technique.
If the system is not producing the desired end result, look for what is doing it correctly. You can locate where the problem is not present so that you can then focus on another location for troubleshooting.
If the system has been having problems immediately after some kind of maintenance or other change, the problems could be linked to those changes.
After all, the choice of techniques and strategies for troubleshooting totally depends upon the technician. The following points would be helpful for effective troubleshooting:
Preliminary Precautions
For the process of troubleshooting, preparing the circuit diagram is the initial and basic process performed by the maintenance technician.
The components can be physically interconnected to each other using solder, wire-wrap and printed circuit board methods.
Any electronic system consists of an element, component, assembly and equipment. All these parts together make a complete electronic system.
The troubleshooting process consists of fault establishment, fault location and fault correction.
Functional area approach, Split half method, Divergent path, Convergent path, Feedback path, Switching path are the common troubleshooting techniques. Which technique has to be applied totally depends on the type of system.
To start the process of troubleshooting, first the technician should have a close visual inspection of the system. He should understand the basic functionality of the system. Then he can proceed to analyze the cause of the trouble.
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