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Symptoms and their causes: The troubleshooting of electronic circuits involves three steps, which should be done in a specific order. The first step is to identify the defect in the circuit. The second step includes fault analysis and determination of the possible causes. The third step is fixing the problem. First, it’s important to identify the problem. To do so, symptoms have to be recognized in the defective circuit. A defective circuit can be defined as one, where the output parameters are incorrect, although the input parameters are correct. For example, the input signal of the amplifier, depicted in is correct, but there is no signal at the output. In this case, the symptom is lack of voltage at the output. Amplifier Power supply 0.5V 0V 12V (a) (b ) (c) +12 V 240 V ++++10 Identifying the symptoms in a defective circuit. This particular symptom does not provide much information about the possible causes of the defect. The failure of various components in the circuit will result in the same symptom (zero voltage at the output). In other cases, a particular symptom points directly to a certain area where the fault is most likely to have occurred. For example, a DC voltage at the output, with the level equal to the supply voltage, indicates that there is a transistor in a cutoff condition. Starting from the stage that is closer to the output and going backwards, all transistors have to be checked for an internally open PN junction. The soldered joints and the values of the emitter resistors also have to be checked. If the amplifier is not defective, the amplified signal appears at the output. The amplitude of the output signal is approximately equal to the value of the rectified power supply. The waveform has to be an exact amplified replica of the input signal, without any kind of distortion. Troubleshooting techniques: Once the symptom is identified, the reasons that cause it have to be determined. The choice of which method to use depends on the circuit complexity, on symptoms, and on the personal preferences of the technician. The most common troubleshooting techniques are listed below: • Power check: It’s amazing how many times a simple issue such as a blown fuse or a flat battery is the cause of a circuit malfunction. Initially, therefore, ensure that the power cord is plugged in and that the fuses are not blown. If the circuit is battery powered, make sure that the voltage level is acceptable. If a power supply rectifier is present, check the level of the voltage at the output and make sure that the circuit is powered with the correct polarity. • Visual inspection: This inspection is part of the so-called sensory checks. Sensory checks rely on the human senses to detect a possible fault. The visual inspection of the PCB is the simplest troubleshooting technique (which is very effective in many of the cases). The soldered joints have to be inspected thoroughly. If any doubts exist about the quality of a certain joint, it has to be re-soldered. The PCB has to be inspected visually for any burnt components. Sometimes, components that overheat leave a brownish mark on the board. They can be used as 'starting points' in the troubleshooting process and the reasons why they overheat have to be determined. It’s bad practice simply to replace such components, without trying to find out what actually caused the component to overheat. In many cases, the reason is a faulty (or out of range) component near the failed component. It also has to be replaced. • Using a sense of touch (temperature) : This is another sensory check. Overheated components can be detected by simply touching them. However, this check has to be performed with extreme caution. The circuit has to be turned off, and some time allowed for the large capacitors to discharge. Always touch the components with the right hand only. This is important because in the case of electric shock it’s less likely that the current will pass through the heart. If possible, wear insulated shoes. In addition, care should be taken not to burn the fingers. Using the sense of touch is a very useful troubleshooting technique in circuits, where everything seems to work properly for a while, and then the circuit fails, due to overheating of a certain component. Identifying such components helps to detect the possible cause of the fault. Special freezing sprays are available, which allow instant freezing of components. If the circuit begins to operate properly immediately after the heated component is sprayed, this is an indication that this component is causing the circuit failure. Before replacing the component, further investigation is needed to determine what caused the overheating in the first place. • Smell, odor check: When certain components fail due to overheating it’s possible in most cases to detect a smell of smoke. This is usually the case, if the technician happens to be there at the time the accident occurred. If not, it’s usually possible to detect the failed component by visual inspection afterwards. • Component replacement: This troubleshooting method relies mostly on the operator's skills and experience. Certain symptoms are an obvious indication of a particular component failure. This statement is especially true for an experienced electronic technician. For example, some TV service technicians can unmistakably identify the failed component in a TV set (even before opening it), by just briefly examining the symptoms. Component replacement is a good troubleshooting technique for an experienced electronics technician, as it saves a lot of time and money. Moreover, this technique guarantees the success of the repair, because if enough components are replaced, eventually the faulty one will be replaced too. However, it’s recommended that the amateur technician initially applies some logical thinking to the troubleshooting process. • Signal tracing: This troubleshooting technique is not the most common one, but it’s the most desirable, as it requires intelligent and logical thinking from the troubleshooter. This method is based on the measuring of the signal at various test points along the circuit. A test point in the circuit is the point, where the value of the voltage is known to the operator. This troubleshooting technique relies on finding a point, where the signal becomes incorrect. Thus, the operator knows that the problem exists in that portion of the circuit, between the point where the signal becomes incorrect, and the point where the signal appeared correct for the last time. In other words, the operator constantly narrows the searched portion of the circuit, until he finds what caused the fault. There are two basic approaches in conducting the signal tracing. In the first approach, the signal check starts from the input, checking consecutively the test points towards the output. The checks are carried out, until a point when an incorrect signal is found. The second approach is to start from the output and to work backwards towards the input in the same manner until a correct signal appears. Fault analysis: Fault analysis requires a good theoretical knowledge and analytical thinking. It’s not something which can be studied from books, but has to be acquired through constant troubleshooting and experimenting. The basic question in fault analysis is: 'What would the symptoms in the circuit be, if the component X is faulty?' For each specific application, there are no ready answers to this question. If there were, many books devoted to industrial electronics would be meaningless. However, there are certain rules, which can be adhered to, during the troubleshooting process. One of the tasks of this manual is to teach you some of these basic rules. As an example, let us examine a bridge rectifier, to illustrate the process of fault analysis. The block circuit of a bridge rectifier that is working properly. It consists of a transformer, a rectifier, and a filter. The voltages, taken with an oscilloscope at each test point are depicted. Transformer Rectifier Filter Output 240 V 220V 12 V 12 V 12 V ++++11 A block diagram of a rectifier in good working order; Rectifier Fuse 240V; Transformer Filter. ++++12 A circuit diagram of the bridge rectifier. The circuit diagram of the same bridge rectifier is depicted. A signal trace is conducted commencing from the output and working towards the input. An analysis of all possible faults in this circuit are given below: • Faulty capacitors, C: There are three possible problems. The capacitor could be shorted, opened, or leaky. If the capacitor is shorted, it effectively brings both terminals of the load resistor together and therefore the output voltage is zero. If the capacitor is open, it does not filter the output voltage supplied from the rectifier. The waveform of the voltage, at the output, remains the same as the waveform of the voltage, after the rectifier. Therefore, the waveforms at points C and D are identical. The only difference is that the amplitude of the voltage at the point D is smaller, due to the voltage drop across the resistor R_surge. Finally, if the capacitor is leaky the output voltage will appear with increased ripples on the output. A leaky capacitor appears as if there is a leakage resistor, connected to it in parallel. The leakage resistor decreases the time for a discharge, thus the voltage ripples increase at the output. ++++13 Symptoms of a faulty capacitor -- 240 V 50 Hz Transformer Rectifier Open capacitor. Shorted capacitor Leaky capacitor R; surge RL C A B C D Filter. • Faulty resistor, R_surge: There is only one possible faulty condition, namely a blown resistor Rsurge (Rsurge appears as an open circuit). This occurs, when an excessive current flows through it. An excessive current flows through Rsurge if the output terminals are short-circuited or if the capacitor is shorted. In both cases when Rsurge blows, it brakes the circuit and prevents the diodes (which are more expensive than the resistor) from burning too. The output voltage in this case is zero. Before replacing Rsurge, ensure that the capacitor, or the output terminals of the circuit, is not shorted and that the conductive paths of the PCB are not shorted out. • Shorted diode: shorted diode appears as a jumper between the points of the connection, as it conducts the current in both directions. ++++ the current that flows in the circuit, when the diode D4 is shorted out. During the positive half-period, the current flows through D3 and D4 as normal. The shortened diode exhibits a zero resistance in both directions and it appears for the circuit, as if it’s simply forward-biased. Thus, the positive half-period appears as normal at the point C. However, during the negative half-period the picture changes. The current now flows through D1 and D4 instead of flowing through the rest of the circuit, because these two diodes, connected in series, provide a path of least resistance. Effectively the secondary winding is short-circuited and an excessive current flows through it. Thus, the diode D4 can be damaged quickly, due to overheating. The increase in the current in the secondary winding increases the current in the primary winding. If the circuit is properly fused, the fuse on the primary winding should blow. If this is not the case, the diode D1 overheats (and even possibly burns) and the voltage at the test point C has the form. Analytical thinking is required to analyze what happens in the circuit when some other diode shorts out, or when two or more diodes short out simultaneously. D2 D1 D3 D4 A B C D RL; Transformer; Filter 240 V 50 Hz; Fuse (possibly burnt); Shorted diode ++++14 Symptoms of a shortened diode. • Open diode: Let us assume that the same diode (D4) is open. No current flows through an open diode in both directions. During the negative half period, this diode appears to the circuit to be reverse-biased, and therefore it has no impact on the output voltage. However, during the positive half-period, the path for the current is broken and no voltage appears at the output. In other words, the circuit works as a half-wave rectifier. This can be detected by, the larger ripples in the output voltage. In addition, the frequency of the ripples is 50 Hz instead of 100 Hz. Similarly, the circuit can be analyzed for other open diodes. A B 240 V 50 Hz Transformer Open diode Filter C D D1 D3 D4 D2 ++++15 Symptoms of an open diode. • Faulty transformer: This is not a common fault, though if the rest of the circuit appears in a good working order, the transformer has to be checked. Several faults are possible: the primary or the secondary windings can be open or partially shorted. If one of the windings is open, no voltage is applied to the rest of the circuit. This obviously results in 0 V at the output. If the primary winding is partially shorted, the turns ratio of the transformer is effectively increased. The voltage on the secondary winding is also increased; thus, the level of the voltage at the output of the circuit is higher. A partially shorted secondary winding decreases the turn ratio of the transformer. The voltage supplied to the rectifier is lower; thus, the level of the circuit output voltage is also lower. • Blown fuse: As was mentioned earlier, this occurs when one of the diodes is shorted. Thus, before replacing the fuse, the diodes have to be checked. A partially shorted primary or secondary winding of the transformer can also increase the current to a level, where the fuse blows. Thus, the transformer also has to be tested before replacing the fuse. • Power supply: It sounds trivial, but if the power cord is not plugged in, or the power supply knob is not turned on, the circuit obviously won’t function. If you have checked the whole circuit from the output just to find out that this is the case, we recommend that next time you start from the input. Summary: Troubleshooting of electronic circuits involves three steps, which should be performed in a specific order. The first step is to identify the defect in the circuit. A defective circuit can be defined as one, where the input parameters are correct, but which has incorrect output parameters. The second step includes a fault analysis and a determination of the possible causes. Here, various techniques can be applied. The most common are: the power supply check, sensory checks (visual, using the sense of touch, smell, etc.), component replacement, and signal tracing. Component replacement is a very effective troubleshooting technique and works well for experienced electronic technicians. Signal tracing involves measuring the voltage at test points, until a voltage with an incorrect value is found. This is the most desirable technique, as it requires a good knowledge of the theory, and some logical thinking. The third step of the troubleshooting process is fixing the problem. This can be done by component replacement, re-soldering a dry joint, etc. Quiz: 1. What are the three steps in the troubleshooting process? 2. What does a defective circuit mean? 3. What is the first thing to be checked in a defective circuit? 4. Which are the most common troubleshooting techniques? 5. What does fault analysis mean? |
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