Troubleshooting electronic devices is a complex and multi-step process. Quickly, you can find only the simplest faults. Although these materials are written on the example of electronic devices, but there are materials that carry and are rather abstract in nature. I bring to your attention a cycle of materials: Strategy for troubleshooting. and Troubleshooting methods. As well as the reasons for the inoperability of electronic devices.

Fault finding strategy.

Introductory word.

A few years ago, I wrote a note: "Methods for finding and repairing faults, as well as the causes of inoperability in CEA.". Recently got his hands to bring it into relative order. Then this article was conceived. The fact is that, having even the most perfect tool, you can not be able to use it. Accordingly, this note is about the correct, in my opinion, the use of tools for troubleshooting. Naturally, I do not pretend to build an exhaustive, flawless scheme by which one could find any malfunction and solve any problem. This text is only an attempt to summarize my experience.

Main part.

Procedure. The procedure always wants to be reduced to a simple and clear scheme. I first had the idea to express all this in a universal flowchart, but the search process is always too multivariate and requires attention to detail. Even a minimal, randomly obtained bit of information can push the solution of the problem. Also, the initial information is always of poor quality, and the choice of the factor on the basis of which one should begin to act is largely intuitive. Naturally, this does not mean that repair is identical to fortune-telling on the coffee grounds, or random sorting, but the random factor is always present. Of course, with a careful and consistent approach to the analysis, we will get a positive result. First of all, you need to remember that every action should be aimed at localizing the fault. AND, although in the early stages localization may not be accurate, but you still need to understand what you are striving for and what you expect from this or that dimension. First, you should always use quick methods, such as "visual inspection" and "finding out the history of the occurrence of a malfunction." Do not neglect these things. They are very effective. Actions during the repair of a used product and when a new product is launched are somewhat different. When we have a product out of service, we can assume that it worked, in other words, there are no errors in volumetric installation, there are no errors in installation on printed circuit boards, all elements of the correct denomination and type. At the same time, in the new product, before turning it on, we must check the printed wiring, check the correctness of the surround installation. And only then try to turn on the product (in more or less complex and mass products). Of course, it is impossible to create a universal procedure, but you can draw a scheme in general terms. The figure in general terms shows the scheme of actions:Explanations to the picture: Preliminary checks imply checks for compliance with the documentation, such as: correct installation, no closures, no pollution, and so on, depending on the production conditions. As you can see, the troubleshooting is cyclical, except for the simplest case. With a negative result on the next cycle, a deeper study of the product should be applied. In this case, a health check can be carried out depending on the situation by the most convenient method. After the stated problem has been stranded, the product should be tested in full.

Application of methods.

When we use one or another method, we pursue several goals: collecting information, testing a hypothesis of a malfunction, localizing a malfunction. And at each step of our actions we get new information, and test the hypothesis, and localize the fault. It should be understood that - the primary in the current step, and that - again. At the same time, one should not neglect secondary effects. For example, when the main purpose of an action is to test a hypothesis, then the information obtained during this test may serve to refine the hypothesis put forward or allow a new assumption to be put forward. For faster localization of the fault, apply the principle: "Divide and conquer". To do this, you must first gain control over the key points of the scheme. Knowledge of the main, the main will save you from having to go into unnecessary details. I will try to describe the general scheme of action on an example. There is a big system that does not work correctly. First, isolate a separate block in the system. We include the block outside the system and isolate a separate module in the block. Then we turn on the module and exit to the element. In this slim system, there is one problem. For example, the module itself works correctly, and errors begin in the block. This is a problem of non-compliance of the working conditions of the module (block, element - not important) in the system and on the test bench. There are always such differences! Do not deceive yourself. First of all, of course, they think about differences in electrical signals, differences in temperature, cooling. In practice, these differences are not always obvious. For example, in my practice there was an interesting case when vibration was a significant factor. And, the board did not have vibration-sensitive elements, the board was strong enough, etc. But the replaced triacs burned out time after time. The reason turned out to be in graphite dust, which huddled under the connectors in the power circuits and in vibration conditions created a short circuit, while at the stand everything worked fine. The isolation of key points is quite complex and requires a good knowledge of the principles of operation of the device and its structure. In the simplest cases, the device should be divided into star structures and successive structures. In star structures, they first investigate (if possible) the central node and, based on its work, make a conclusion about the operation of its and other nodes. If the study of the central node is difficult, then "first we chop off the branches, if we can not immediately cut down the trunk." That is, we first investigate the periphery, which is more laborious, but will provide information necessary for the study of the central node. In successive structures, you should make sure that the input signals are correct, the output signals are incorrect. Hence, the structure somewhere does not work. To fix the problem as quickly as possible, split the sequence of cascades in half and check the signals in the middle cascade. This way we will find out where the fault is at the beginning or at the end. Then divide the intended area in half again, and so on. But it happens that such an action strategy is inconvenient for technological reasons, or we have to analyze signals for each stage based on the signals in the previous stage.

Good food.

Always first (or as soon as possible) you should check the supply circuit and the quality of the supply voltage. Keep in mind the goal and to the place to change it. It is necessary to understand what this or that action is directed to. On the collection of general information, on testing a hypothesis, on checking a particular block. At the same time, it is necessary to assess the result of the action in time and proceed to the next stage in time. When you collect information, you need to assess in time that the necessary information has already been collected and it is necessary to proceed to the analysis. No need to get hung up at any stage. This is not always easy and not always obvious.

Results:

We formulate the main strategic principles:

• First, use fast and inaccurate methods, then we specify;
• Cyclicity with a negative result, with an increasingly in-depth study on each cycle;
• Assessment of feasibility;
• "Divide and rule";
• Control of key points;
• "You can not cut down the trunk, cut down the branches";
• Do no harm

The problem is that you can not make a clear scheme of action. We are always forced to keep in mind all the principles and constantly look for what to use at the moment.

Troubleshooting methods. As well as the reasons for the inoperability of electronic devices.

The first version published 2007-09-01 and was called: Methods for troubleshooting. As well as the causes of inoperability in CEA.

Here I plan to describe practical methods of troubleshooting in electronics, if possible, without reference to specific equipment. Under the causes of inoperability means the failure of the element, the mistakes of developers, installers, etc. The methods are interconnected with each other, and their complex application is almost always necessary. Sometimes the search is very closely related to the elimination. In the process of working on the text, it became clear that the methods are very interrelated and often have similar features. Perhaps it can be said that the methods overlap. However, it was decided not to combine similar methods into one, in order to highlight problems from different sides and more fully describe the process of troubleshooting.

Basic troubleshooting concepts.

1. The action should not harm the device under investigation. 2. The action should lead to the predicted result: - hypothesis about the health or malfunction of the unit, element, etc. - confirmation or refutation of the hypothesis put forward and, as a result, localization of the malfunction; 3. It is necessary to distinguish between a probable malfunction and a confirmed (detected malfunction), a hypothesis put forward and a confirmed hypothesis. 4. It is necessary to adequately assess the maintainability of the product. For example, boards with elements in a BGA package have a very low maintainability due to the impossibility or limited ability to apply basic diagnostic methods. 5. It is necessary to adequately assess the profitability and the need for repair. Often the repair is not profitable in terms of costs, but is necessary in terms of working out the technology,

The scheme of the description of methods:

• The essence of the method
• Method Capabilities
• Advantages of the method
• Disadvantages of the method
• Application of the method

1. Clarification of the history of the fault.

The essence of the method: The history of the appearance of a malfunction can tell a lot about the localization of the malfunction, which module is the source of the system malfunction, and which modules have failed due to the initial malfunction, and the type of the malfunctioning element. Also, knowledge of the history of the appearance of a malfunction can greatly reduce the time of testing the device, improve the quality of repair, reliability of the corrected equipment. Figuring history allows to find out, is not the problem the result of external action, such as: climatic factors (temperature, humidity, dust, etc.), Mechanical stress, pollution by various substances, and so on. The possibilities of the method: This method allows very quickly to hypothesize a fault localization . Advantages of the method:

• No need to know the details of the product;
• Superefficiency;
• No documentation required.

Disadvantages of the method:

• The need to obtain information about events stretched in time for which you were not present, inaccuracy and inaccuracy of the information provided;
• Requires confirmation and clarification by other methods; in some cases, the probability of error and inaccuracy of localization;

Application of the method:

• If the malfunction first appeared rarely, and then began to manifest itself more often (within a week or several years), then the electrolytic capacitor, electron tube, or power semiconductor element is most likely faulty, excessive heating of which leads to deterioration of its characteristics.

• If the malfunction appears as a result of mechanical impact, then it is quite likely that it will be able to be detected by an external inspection of the unit.

• If a malfunction occurs with a slight mechanical impact, then its localization should begin with the use of mechanical effects on individual elements.
• If a malfunction has appeared after any actions (modification, repair, revision, etc.) above the device, then special attention should be paid to the part of the product in which the actions were performed. Should verify the correctness of these actions.
• If a malfunction occurs after climatic influences, exposure to humidity, acids, vapors, electromagnetic interference, power surges, it is necessary to check the compliance of the operational characteristics of the product as a whole and its components with the working conditions. If necessary, take appropriate action. (change of working conditions or changes in the product, depending on the tasks and opportunities)
• On the localization of the malfunction can tell a lot of manifestations of malfunction at different stages of its development.

2. External examination.

The essence of the method: External examination is often neglected, but external examination allows localizing about 50% of faults, especially in small-scale production. External inspection in terms of production and repair has its own specifics. Method features:

• The method allows superfunctional detection of a malfunction and localization of it with an accuracy of an element in the presence of external manifestation.

Advantages of the method:

• Superefficiency;
• Accurate localization;
• A minimum of equipment is required;
• No documentation required (or minimum availability).

Disadvantages of the method:

• Allows you to identify only faults that have a manifestation in the appearance of the elements and parts of the product;
• As a rule, requires disassembly of the product, its parts and blocks;
• Requires performer experience and excellent vision.

Application of the method:

• In terms of production, special attention must be paid to the quality of installation. The quality of installation includes: the correct placement of elements on the board, the quality of solder joints, the integrity of printed conductors, the absence of foreign inclusions in the board material, the absence of closures (sometimes the closures are visible only under a microscope or at a certain angle), the integrity of the insulation on the wires, reliable attachment of contacts in connectors. Sometimes an unsuccessful constructive provokes closures or breaks.
• In terms of repair, you should find out if the device ever worked correctly. If it did not work (the case of a factory defect), then you should check the quality of the installation.
• If the device worked normally, but failed (case of repair itself), then you should pay attention to traces of thermal damage to electronic components, printed conductors, wires, connectors, etc. Also, during the inspection, you must check the integrity of the insulation on the wires, cracks from time cracks due to mechanical stress, especially in places where conductors work on bending (for example, sliders and flips of mobile phones). Special attention should be paid to the presence of dirt, dust, electrolyte leakage and odor (burning, mold, faeces, etc.). The presence of pollution can cause the inoperability of CEA or an indicator of the cause of the malfunction (for example, leakage of electrolyte).
• Inspection of printed wiring requires good lighting. It is desirable to use a magnifying glass. As a rule, short circuits between soldering and poor soldering are visible only from a certain angle of view and illumination.

Naturally, in all cases, you should pay attention to any mechanical damage to the housing, electronic components, circuit boards, conductors, screens, etc., etc.

3. Paging.

The essence of the method: The essence of the method is that with an ohmmeter, in one form or another, the presence of the necessary connections and the absence of unnecessary connections (closures) are checked. Method features:

• Prevention of malfunctions during production, quality control of installation;
• Testing the hypothesis of the presence of a fault in a particular circuit;

Advantages of the method:

• simplicity;
• high qualification of the performer is not required;
• high reliability;
• exact fault location; Disadvantages of the method:
• high labor intensity;
• restrictions on checking boards with mounted elements and connected wiring harnesses, elements included in the circuit.
• the need to get direct access to contacts and items.

Application of the method:

• In practice, as a rule, it is enough to check the presence of the necessary connections. Lack of short circuits is checked only on power circuits.
• The absence of unnecessary connections is also provided by technological methods: marking and numbering of wires in the harness.
• Checking for the presence of extra connections is carried out in the case when there is a suspicion of specific conductors, or a suspicion of a design error.
• Checking for the presence of unnecessary connections is extremely time consuming. In this regard, it is carried out as one of the final stages, when a possible area of ​​the circuit (for example, there is no signal at the test point) is localized by other methods.
• It is possible to precisely localize the closure with the help of a millimeter, with an accuracy of several centimeters.
• Although this technique has certain disadvantages, it is very widely used in small-scale production, due to its simplicity and efficiency.
• It is better to call according to the dialing table, made on the basis of the electric principle circuit. In this case, possible errors in the design documentation are corrected and there is no error in the dial.

4. Removal of performance

The essence of the method. When using this method, the product is included in the working conditions or in conditions simulating workers. And check the characteristics, comparing them with the required characteristics of a working product or theoretically calculated. It is also possible to take characteristics of a separate unit, module, element in the product. Method features:

• Allows you to quickly diagnose the product as a whole or a separate unit;
• Allows you to roughly estimate the location of the malfunction, to identify a functional unit that does not work correctly, in case the product does not work correctly;

Advantages of the method:

• Rather high efficiency;
• Accuracy; Adequacy;
• Evaluation of the product as a whole;

Disadvantages of the method:

• The need for specialized equipment or, at a minimum, the need to assemble a wiring diagram;
• The need for standard equipment;
• The need for a fairly high qualification of the performer;
• It is necessary to know the principles of operation of the device, the composition of the device, its block diagram (to isolate the fault).

Application of the method: For example:

• The TV check the presence of the image and its parameters, the presence of sound and its parameters, power consumption, heat dissipation. According to the deviation of certain parameters, the health of functional blocks is judged.
• The mobile phone on the tester checks the parameters of the RF path, and by the deviation of certain parameters, it is judged that the functional units are in good condition.
• Naturally, it is necessary to be sure of the health of all external units and the correctness of the input signals. For this, the work of the product (element, unit) is compared with the work of the serviceable one in the same conditions and in this switching circuit. This means not theoretically the same scheme, but practically the same “iron”. Or you need to compare all input signals.

5. Observation of the passage of signals in cascades.

The essence of the method: With the help of measuring equipment (oscilloscope, tester, spectrum analyzer, etc.) observe the correctness of the propagation of signals through the stages and circuits of the device. To do this, carry out measurements of the characteristics of the signals at the control points. Method features:

• performance evaluation of the product as a whole;
• performance evaluation by cascade and functional blocks;

Advantages of the method:

• high accuracy of fault localization;
• the adequacy of the assessment of the state of the product as a whole and in cascades;

Disadvantages of the method:

• great difficulty in evaluating circuits with feedback;
• the need for highly skilled performer;
• labor intensity;
• the ambiguity of the result with improper use;

Application of the method:

• In schemes with a sequential arrangement of stages, the disappearance of the correct signal in one of the control points indicates a possible malfunction of either the output, or the input short circuit, or the communication fault.
• At the beginning, isolate the embedded signal sources (clock generators, sensors, power supply modules, etc.) and successively find a node in which the signal does not correspond to the correct one described in the documentation or determined by simulation.
• After checking the correct functioning of the built-in signal sources, the input signals (or inputs) give test signals and again control the correctness of their distribution and conversion. In some cases, for a more efficient application of the method, a temporary modification of the scheme is required, i.e. if necessary and possible - breaking the feedback circuits, breaking the communication circuits of the input and output of the suspected cascades
• .

Fig.1 Temporary modification of the device to eliminate the ambiguity of finding a fault. Crosses indicate a temporary connection break.

• In circuits with feedback it is very difficult to get unambiguous results.

6. Comparison with serviceable unit.

The essence of the method: It consists in comparing the various characteristics of a known-good product and a faulty one. The differences in appearance, electrical signals, electrical resistance are judged on the localization of the fault. Method features:

• Operational diagnosis in combination with other methods;
• The possibility of repair without documentation.

Advantages of the method:

• Online troubleshooting;
• No need to use documentation;
• Eliminates modeling and documentation errors;

Disadvantages of the method:

• The need for a serviceable product;
• The need to combine with other methods

Application of the method: Comparison with the serviceable unit is a very effective method, because not all product characteristics and signals are not documented in all circuit nodes. It is necessary to begin a comparison with a comparison of the appearance, arrangement of elements and configuration of the conductors on the board, the difference in the installation indicates that the product design has been changed and, quite likely, an error has been made. Then compare the various electrical characteristics. To compare the electrical characteristics, the signals are viewed at various points in the circuit, the operation of the device in different conditions, depending on the nature of the fault. It is sufficient to effectively measure the electrical resistance between different points (peripheral scanning method).

7. Modeling.

The essence of the method: The behavior of a serviceable and faulty device is simulated and, based on modeling, a hypothesis about a possible malfunction is put forward, and then the hypothesis is checked by measurements. The method is used in combination with other methods to improve their effectiveness. Method features:

• Prompt and adequate hypothesis on the location of the fault;
• Preliminary testing of the fault location hypothesis.

Advantages of the method:

• Ability to work with disappearing faults
• The adequacy of the assessment.

Disadvantages of the method:

• high qualification of the performer is necessary,
• combination with other methods is needed

Application of the method: When eliminating a periodically occurring fault, it is necessary to apply a simulation to find out if the element being replaced could have provoked the fault. For modeling it is necessary to present the principles of operation of the equipment and sometimes even know the subtleties of work.

8. Splitting into functional blocks.

The essence of the method: To pre-localize the fault is very effective to break the device into functional blocks. It should be borne in mind that often the design division into blocks is not effective from the point of view of diagnostics, since one building block can contain several functional blocks or one functional block can be constructively implemented as several modules. On the other hand, the structural unit is much easier to replace, which allows you to determine which structural unit the fault is in. Method features:

• Allows you to optimize the use of other methods;
• Allows you to quickly determine the location of the fault;
• Allows you to work with complex faults

Advantages of the method:

• Accelerates the troubleshooting process;

Disadvantages of the method:

• A deep knowledge of the circuitry of the product is required;
• It takes time to thoroughly analyze the device.

Application of the method: Two options are possible:

• If the product consists of blocks (modules, boards) and their quick replacement is possible, then, by changing the blocks in turn, they find the one which, if replaced, fails;
• In another version, analyzing the documentation, they make up the functional diagram of the device, model the product on the basis of the functional diagram (usually mentally) and put forward a hypothesis about the location of the fault.

9. Temporary modification of the scheme.

The essence of the method: To eliminate mutual influence and to eliminate ambiguity in the measurements, sometimes it is necessary to change the product design: break connections, connect additional connections, unsolder or solder elements. Method features:

• Localization of faults in the circuits with the OS;
• Exact fault location;
• Elimination of the mutual influence of elements and chains.

Advantages of the method:

• Allows you to clarify the location of the fault.

Disadvantages of the method:

• The need to modify the system
• The need to know the details of the device

Application of the method: Partial disconnection of chains is used in the following cases:

• when the chains have a mutual influence and it is unclear which of them is the cause of the malfunction;
• when a faulty unit can disable other units;
• when there is an assumption that the wrong / faulty circuit blocks the operation of the system.

It is necessary to disconnect the protection circuits and negative feedback circuits with special care, since turning them off may cause significant damage to the product. Disconnection of the feedback circuits can lead to a complete disruption of the operation mode of the cascades and, as a result, not to give the desired result. Disconnection of the POS circuit in the generators naturally leads to the breakdown of generation, but it can allow to take the characteristics of the cascades.

10. The inclusion of a functional unit outside the system, in conditions simulating the system.

The essence of the method: In fact, the method is a combination of methods: Breaking up into functional blocks and Removing external performance. Upon detection of faults, the “suspect” block is checked outside the system, which allows either to narrow the search range if the block is healthy or to localize the fault within the block if the block is faulty. Method features:

• testing the hypothesis about the health of a particular part of the system

Advantages of the method:

• the ability to test and repair a functional unit without a system.

Disadvantages of the method:

• the need to assemble a verification scheme.

Application of the method: When applying this method, it is necessary to monitor the correctness of the conditions created and the tests applied. Blocks can be poorly coordinated with each other at the design stage.

11. Preliminary check of functional blocks.

The essence of the method: The functional unit is pre-checked outside the system, on a specially made stand (workplace). When repairing this method makes sense if the unit does not require too many input signals or, in other words, it is not too difficult to simulate the system. For example, this method makes sense to apply when repairing power supplies. Method features:

• Testing the hypothesis about the health of the unit;
• Warning of possible malfunctions in the assembly of large systems.

Advantages of the method:

• The ability to check the basic characteristics of the unit without interfering effects;
• The ability to pre-check the blocks.

Disadvantages of the method:

• The need to assemble a validation scheme

Application of the method: It is widely used for the prevention of system malfunctions in the production of new products.

12. Replacement method.

The essence of the method: The suspected unit / component is replaced with a known-good one, and the functioning of the system is checked. The results of the test are judged on the correctness of the hypothesis regarding the malfunction. Method features:

• Testing the hypothesis about the health or non-health of a block or element.

Advantages of the method:

• Efficiency.

Disadvantages of the method:

• The need for a block for replacement.

Application of the method: Several cases are possible: when the behavior of the system has not changed, this means that the hypothesis is incorrect; when all faults in the system are eliminated, it means. the fault is indeed localized in the replaced block; when a part of the defects disappeared, this may mean that only the secondary malfunction has been eliminated and the serviceable unit will again burn out due to the primary defect of the system. In this case, perhaps the best solution would be to re-install the replaced unit (if possible and appropriate) and continue troubleshooting with that. to eliminate exactly the root cause. For example, a power supply failure may result in unsatisfactory operation of several units, one of which fails due to overvoltage.

13. Check item operation mode.

The essence of the method: Compare the values ​​of currents and voltages in the circuit with supposedly correct ones. They can be found in the documentation, calculated when modeling, measured in the study of a good unit. On this basis, make a conclusion about the health of the element. Method features:

• Localization of the fault to the element.

Advantages of the method:

• Accuracy

Disadvantages of the method:

• Slowness
• Requires highly qualified performer;

Application of the method:

• Check the correctness of the logic levels of digital circuits (compliance with standards, and also compared with the usual, typical levels);
• check the voltage drop across the diodes, resistors (compare with the calculated or with the values ​​in the serviceable uni

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Часть 1 Troubleshooting. Methods of troubleshooting As well as the reasons for the inoperability of electronic devices.