Principles of Electronics Maintenance (PEM)

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Maintenance principles depend on several factors:

System type
Place and work of the system.
Environmental conditions.
The desired level of system reliability.

All of this is closely related to the expertise of the maintenance staff and the component equipment. There are two ways of maintenance:

Preventive Maintenance (maintenance for prevention): replacing parts/components that are nearly damaged, as well as calibration.
Corrective Maintenance (maintenance for repair): replacing damaged components.

In Preventive maintenance, replacement is done before the component is completely damaged (worn out due to use) so that the reliability of the system can be increased. For example, components of moving parts and used continuously should be replaced before they are damaged, for example, servo potentiometers, motors and brushes, contacts on relays and switches or incandescent lamps (filaments).

A system that uses a large number of indicator lights has a failure graph like Figure 2.4.

The failure graph shows that the peak failure occurs at 1000 hours. The probability of an indicator lamp failing before 1000 hours is 0.5 (50%). So, if the lamps are replaced after 1000 hours, the probability of each lamp failing during that time is 0.5 (50%). If all the lamps are replaced at the same time within a standard deviation before their mean life, then this will make the reliability level better.

The difficulty is in accurately estimating the wear period for internal components, making it uneconomical to carry out preventive maintenance.

The disadvantage is that disruptions occur during preventive maintenance work, which can cause damage to the equipment itself.

Corrective maintenance is an activity of servicing electronic systems during their use, if there is component damage that cannot be predicted and cannot be overcome by inspection. In reality, inspection of a damage is preferred over prevention.

As an illustration, Figure 2.6 shows the relationship between maintenance costs and repair costs and the availability of the equipment itself.

Information

The total cost of maintenance and repairs.
Repair costs + damage losses (production losses, unemployed workers, etc.).
Repair costs.
Maintenance costs.
The optimum strain at which the amount of charge should be given (the limits are not absolute and may vary from problem to problem).
Optimize the availability of equipment at any time.

It can be clearly seen that there is an optimum range where the maintenance effort can be determined economically. Otherwise, the maintenance cost increases so much that no one can afford it.

Maintenance & Repair

In this chapter, we will discuss the goals and objectives and principles of general maintenance and repair management. Occupational health and safety issues will be discussed at the end of this chapter. Maintenance & Repair Goals and Objectives Basically, the goals and objectives of maintenance & repair management are highly dependent on the mission (what they want to achieve) of an organization. Of course, this mission will differ between one organization (for example, a school) and another (for example, the mission of the car assembly industry).

The purpose of maintenance and repair in schools is generally only to extend the life of the equipment. Many schools do not yet have a special unit for handling maintenance and repair of equipment or other facilities.

For some industries, maintenance and repair issues are generally always associated with their responsibility for quality, timely and high economic value products. Some large industries or organizations even have missions that are always associated with assets and investments. So maintenance & repair activities of equipment & other facilities are considered as part of assets & investments.

Therefore, the maintenance & repair department or unit is a very important part of this kind of organization.

Maintenance & Repair Activities

Before discussing further about maintenance and repair management, it is first necessary to understand the nature of maintenance and repair work or activities in general.

Maintenance and repairs include various activities or actions, as shown in Figure 1.1. In general, these activities can be divided into two, namely: activities that can be planned and activities that are unexpected or unplanned. Routine maintenance & repair activities are activities that can be planned, while emergency activities, such as equipment damage due to accidents (eg falling. Hit by lightning, etc.) are activities that cannot be predicted. However, these kinds of things must be anticipated. At least we know what to do when such a disruption occurs.

Task

Make a list of all the measuring instruments you have used in the past month. Then, make a note or mark the measuring instruments that are not working properly, for example by giving a mark for measuring instruments that are experiencing minor problems (for example, one measurement range is not working properly), for measuring instruments that are often experiencing problems, and for measuring instruments that are not working. Give the notes to the teacher or technician who handles the laboratory equipment.

The equipment list that you create can be used as a report on the results of monitoring the performance of equipment in the laboratory. With this kind of monitoring, maintenance time and costs can be reduced to a minimum. If minor damage or disruption is not handled properly, it can result in more serious disruption or damage. If this happens, the cost used for repairs is more expensive, and the repair time is also longer. Overall, this will certainly interfere with the learning process. In industry, monitoring equipment conditions is very important, because if a larger disruption occurs, it will not only interfere with productivity, but will also increase costs, both equipment repair costs and production costs, because to replace lost time workers must work overtime.

Unit Reliability & Failure

You must have known that every electronic equipment after some time will experience a decline in performance or even damage, because no equipment can work perfectly all the time, even though its quality and technology are sophisticated. For example, satellites require very high reliability so that they continue to work well until the specified time limit, because damage to the satellite will be very difficult to repair and requires very high costs. But still the satellite must be replaced with a new one after the time limit before the damage occurs, so that all types of communication are not disrupted.

Quality is the ability of an item to meet its specifications, while reliability is the development of quality over time.

The reliability and quality of a device will affect the working life of the device. An electronic device that is made by maintaining quality factors will operate well for a longer period of time than a system device that is made with less attention to quality factors.

To be able to predict how far the reliability of a tool is, the definition of reliability itself must be known. Reliability is the ability of an item to perform a required function under a specified condition within a specified period of time.

In this case, item means component, instrument or system. Reliability figures cannot be predicted without specifying the time and operating conditions. More detailed matters concerning reliability will be discussed in a separate sub-chapter in this book. To get a more complete picture, because reliability is very closely related to failure, it is necessary to consider a definition of failure. Failure is the end of an item's ability to perform its required function.

From the two definitions above, we can see the relationship between reliability and failure. If an item shows a decrease in its reliability, then this indicates a symptom of failure.

1. Failure stage

There are three stages of failure during the life of a piece of equipment.

The first stage is called early failure (infant mortality)

Namely, the failure of equipment shortly after the equipment is manufactured and shipped to the customer. Failure during this stage is caused by damage to components that have been installed on the equipment. Usually the operating conditions of the equipment do not last long. The equipment is usually still under company warranty and repairs are the responsibility of the company. Another cause of premature failure is a design error that places too much emphasis on one part of the equipment. This is only possible in newly designed products and the company's inability to resolve all of the product's weaknesses.

The second stage is the normal failure of the equipment working life

The failure rate at that time was the lowest.

The third stage is the period when a piece of equipment experiences the highest failure rate.

This is because the working life of the tool has ended. During this time, everything seems to go wrong.

How quickly a piece of equipment enters this stage depends on how the equipment is maintained during use. For example, if it is known that a component has reached the end of its service life, then the component should be replaced quickly before it causes failure of the equipment.

2. Partial or Partial Failure

It is a failure due to deviation of characteristics or parameters outside the specification limits, but does not reduce the overall function of the tool. For example: a function generator that can still produce signals, but the frequency does not match the measurement limit position, a TV that loses its green color, etc.

3. Complete or Total Failure

Caused by the deviation of characteristics or parameters outside the specification limits so that it completely reduces the function of the equipment. For example, a function generator that cannot produce all waveforms, a TV that won't turn on, etc.

4. Causes of failure

Misuse failure is an error caused by use beyond the capabilities of the component or tool. For example: a multimeter used to measure AC voltage but installed in the DC voltage position.

Weaknesses in items (components, equipment or systems) even when operated within their capabilities can also be the cause of failure. For example, a multimeter that is being used to measure voltage suddenly breaks down even though it has been used correctly.

Failure times can be grouped into two, namely:

Sudden failure, which is a failure that cannot be predicted through previous testing. For example: a TV that is being operated and suddenly breaks down without any clear cause.
Gradual failure, which is a failure that can be predicted through previous testing. For example: TV volume starts to make noise when the volume potentiometer is turned up or down.

5. Combination of Failures

Catastrophic failure = sudden + complete failure. Example: A TV that is being operated and suddenly breaks down.
Degradation failure = gradual + incomplete (partial) failure, for example: a TV whose volume starts to make noise when the volume potentiometer is turned up or down.

Factors Affecting Unit Reliability

The reliability of an electronic device or instrument cannot be separated from the factors that influence it during the equipment's life cycle. In more detail, to achieve the desired reliability target, the following steps are taken:

1. Design and Development Stage

At this stage, the reliability to be achieved must be prepared, so that in the next step, the design experts will be directed to achieve the target. The work at this stage includes:
Designing the circuit determines the component layout, and thoroughly tests the prototype.
Designing the circuit and selecting the right components, so that there will be no emphasis on only one component. To select the right components, each component is checked for its failure potential in the designed circuit. This step is called Error Analysis and Emphasis.
Determine the layout of components, assemblies and panels. Installation of components should be carried out carefully so as not to experience excessive mechanical stress and heat.
The influence of the environment in which the equipment will be operated must be taken into account and protection must be made against it. These protective measures include tight closure, cold air compression, anti-vibration installation or installation of insulating compounds.
Comprehensive prototype testing is carried out to see whether the design meets the specified reliability and workability specifications.
Components must be well secured and stored as short as possible. For small quantities, a complete inspection can be carried out. But for large quantities, the inspection can be carried out by taking product samples and using statistical analysis methods.
Employee cooperation and skills. Every employee, toolmaker, production and method expert, assembly operator, test and inspection expert forms a production chain and can help produce quality products.
A good training framework will ensure that employees are able to use production techniques correctly and more effectively.
Production equipment meets required standards and is well maintained.
Comfortable working or assembly environment conditions, such as good air ventilation, good lighting, comfortable room temperature for workers and tools, and free from dust to ensure comfortable conditions.
Automatic test equipment can be used to check for open or short circuits in circuit lines. Soak tests are necessary when the instrument is operated at varying temperatures, and temperature cycling will help identify weak components.

2. Storage and Transportation

The storage method will affect the reliability of the instrument's operation.
The packing method must be taken into account in the reliability specification. The packing must be able to protect the instrument from corrosion and mechanical damage hazards, storage temperature and humidity levels must be constantly controlled.
Transportation When sold, the instrument must be transported and this will be subject to vibration, mechanical shock, changes in temperature, humidity and pressure. It must be special.

3. Operation

Suitable environmental conditions.
Correct operation method. Well-written operating instructions should ensure that there will be no errors in use.

Law of Unit Reliability Exponents

The relationship between reliability (R) and system failure rate ( ) is written by the equation:

With:

t = operating time (hours)
= system failure rate is the sum of all component failures (per hour);
e = base of logarithm,
R = reliability in time t.

The meaning of the formula is that the probability of no system failure in time t is an exponential function of that time. In other words, the longer the system is operated, the less reliable it will be and the higher the probability of failure.

Figure 2.29 below shows a graph of R against t divided into m intervals, showing that when t = m, that is, the operating time is the same as the MTBF, the probability of successful operation will drop to nearly 0.37 or 37%. Only when the operating time is relatively shorter than the MTBF, the reliability becomes high.

For example, a naval radar system has an estimated MTBF of 10,000 hours. What are the success probabilities for mission times of 100, 2000, and 5000 hours?

For:

So R cannot be 1 because that would mean it never fails. Some ways to improve Reliability (R) are by:

Derating

operating a component below its maximum limit. For example: using a ½ Watt resistor for a circuit that actually only needs a ¼ Watt resistor.

Redundancy

Connecting one unit to another unit with the same function, so that if one fails the other will take over its function. Usually these units are installed in parallel.

There are two ways of redundancy:

1. Active: when a standby unit comes on following a failure. For example: UPS attached to a computer, AC emergency light that is always ready to turn on when the AC voltage goes off etc.

2. Passive: when the elements are combined to share the load or perform their functions separately. For example: a generator in an office building that is available but not operated and not automatic.

To calculate Reliability ( R ) if two units/systems each with reliability Rx and Ry are installed:

For example

A power supply, an oscillator and an amplifier, all used in a simple system are connected in series. Calculate the reliability of each unit and the system for a period of 1000 hours of operation, if the MTBF is 20,000 hours, 100,000 hours and 50,000 hours.

Unit Damage Tracking Method

You know that many damage finding techniques can be applied in the field of electronics. These techniques include: component testing, checking the input output of each block. Another method is to do it yourself by checking the input and output of each function block. Which method is good? It depends on the type of system damage being observed. The important thing to note is how to find damage efficiently (quickly and accurately) because here Time is Money.

How to Choose the Right Method

The method chosen to find the damage will determine the efficiency of the work. You should try to find as much damage or irregularity as possible. To save time, it is better if we ask people who know about the disturbance in the tool, through several questions as shown in Table 2.5 below.

Table 2.5: Questions

What is actually wrong?
What are the physical characteristics of damage?
Does it always happen like this?
If it is true, under what conditions?
Is there any abuse? (vibration, shock, heat, etc.)
Did the damage occur suddenly or gradually?
Did any damage occur during operation of the equipment?
Does the damage appear to affect other functions?
Is there any additional information?
Has anyone tried to fix it?

When the owner of a hi-fi set says that the device is not working properly, this is very little information.

So, to clarify the problem, ask questions as follows:

When what is the device not working properly or what part is not working properly? For example, one of the stereo system channels is weaker than the others. This will narrow the problem down to one of the channel amplifiers to measure.

The second question aims to focus on the error. In the example above, we asked the owner if he had tried adjusting the volume, loudness control, tone control, or balance.

The third question aims to find out whether the damage occurs continuously or only occasionally, whether it depends on external influences. Whether the damage is total.

The fourth question, to find out under what conditions the damage occurs. Often damage occurs when there is vibration, high temperature, getting a shock (falling, being hit) or some other effects.

The fifth question is our help to find out whether the damage only appears after falling, being exposed to vibrations (when being transported by car), being exposed to excessively high temperatures, etc.

The sixth question helps us to find out whether the damage is due to age or sudden damage.
The seventh question is to find out whether the damage occurred when the tool/system was operating or off.
Eighth question, Sometimes damage to one function can also affect other parts. For example, interference in the power supply (poor filter) will affect other parts.
The ninth question will help us to determine the location of the fault, by adding details of the device, for example, a picture defect on a TV is similar to the operation of a vacuum cleaner.
Finally, the tenth question is to address the damage.

Using the right technique for a particular problem is very efficient in the troubleshooting process. There are several techniques that can be used:

Symptom-function: to isolate damage to a particular part.
Signal-tracing: to find the specific block causing the usage failure.
Voltage and resistance method to isolate faulty components or specific circuit areas.
Half-splitting method: for circuits with blocks arranged in series.
Loop Breaking Method: for closed loop systems in industries.
Substitution method: trying to solder the same component to the damaged part.

When & How to Use the Symptom Function Technique?

Symptom-function (symptom function) has been used in everyday life.

For example, when we turn on the study lamp and it doesn't light up (the symptom), then what is checked (its function) is:

The power cable is connected or disconnected,
The lights are either on or off,
if it still doesn't turn on, maybe the switch isn't working properly and so on.

By looking at the symptoms of damage, the type and location of damage to the tool can be estimated by knowing the working principle of the tool and based on observations of the tool's work, it is possible to know the damage, without using measuring tools and without taking measurements.

In Figure 2.36a, a number of different inputs leading to one output (convergence) are shown. For example: a complete HI-FI system.

Of course you can isolate the fault effectively, if you know which inputs are not showing any output symptoms.

Figure 2.36b shows the working principle of a device with one input and has several different outputs (divergence). For example: color TV.

Here you can also isolate the fault effectively by observing which outputs are working and which are not working.

When & How to Use Signal-Tracing Technique?

Figure 2.37 illustrates the principle of signal-tracing in a simple amplifier. The signal generator with internal resistance RG provides the input signal to the amplifier, and it can be seen whether the amplifier will amplify DC, audio, video or IF signals. The amplitude of the input signal is measured by Vi when measured across the input impedance R1. The output of the amplifier is measured by Vo when measured across the load resistor RL.

By comparing the Vi and Vo readings, we can determine the gain. This method is also called the Input-Output Method / Output-Input Method.
By changing the output amplitude of the signal generator, we can see whether the amplifier is linear in the input signal region.
By varying the load impedance RL, we can see whether the gain is linear to the change in load.
By changing the frequency of the signal generator, we can determine the frequency response of the amplifier.

With this simple arrangement, the important characteristics of the amplifier can be measured by a signal-tracing system, in amplitude and frequency, from the input to the output of the amplifier.

In some electronic devices, this external signaling is not always necessary, especially if the signal that should be on the device can be easily identified. This method is called the passive signal tracing method. For example: checking a power supply as in Figure 2.38 below:

The mains voltage is measured with an AC voltmeter at the wall socket, at the fuse, and at the switch. If there is 220 V AC voltage at the primary end of the transformer, then it can be ascertained that the plug, cable, fuse and switch are in good condition.
The AC signal on the transformer secondary can be measured on each side (the transformer secondary has CT) to ground. If there is a voltage on the transformer secondary that is of the appropriate magnitude, then it can be ascertained that the transformer is in good condition.
Next, use the meter switch on the DC scale. Measure the voltage on C1 and on C2. If there is no DC voltage on C1 or C2, it means that the capacitor is short-circuited. If the L coil is open, then there is only DC voltage on C1, but none on C2. If C1 and C2 are open (broken), or if the rectifier CR1 and CR2 are open, or both are short-circuited, then the measured DC voltage is incorrect. In such conditions, resistance measurements need to be carried out to determine the damaged component.
The second method is the opposite of the first method, namely starting from measuring the DC voltage on capacitor C2, followed by measuring the DC voltage on capacitor C1, etc. The results are the same because the measurement only uses a voltmeter.

The following example is an FM radio whose block diagram is shown in Figure 2.39 does not work. The power supply and voltage checks on the static condition of the circuit have been carried out. The fault is in the area between the antenna and the audio amplifier. In the passive method, the normal signal is assumed to be present or known. However, because the antenna and tuning (which are assumed to provide a normal signal to the system) are inside the system itself, an external signal must be given as a normal signal and the speaker must be used as a signal indicator. This method is called the active signal-tracing method or signal injection.

First way:

The signal generator is connected to the RF tuner, and the antenna is removed; the signal generator and the tuner are set to the same frequency. If nothing is heard in the loudspeaker, move the signal generator to point A. Change the signal generator frequency to 10.7 MHz (Standard for FM radio). If now you hear sound (tone from the signal generator), this means the damage is in the RF tuner.
If nothing is heard, move the generator signal to the output of the middle amplifier (IF amplifier), namely at point B. At this point, the amplitude of the generator signal must be increased to compensate for the gain of the middle amplifier.
At point C, the normal signal is an audio signal. Therefore, the generator signal fed through this point must be at an audio frequency.
At point D the generator signal should be strong enough to drive the loudspeaker. The loudspeaker can be tested by checking the voltage across the amplifier driver and reducing it momentarily with a suitable resistor between the voltage and ground. This should produce a clicking sound in the loudspeaker.

Second way:

The check is done from the speaker to the tuner. To determine whether to use the first or second method, an initial check can be done. For example, by shorting the input to the audio amplifier with ground, using a screwdriver or the tip of a clip. This should produce a clicking sound on the loudspeaker, if the loudspeaker and audio amplifier are working properly.
If there is no sound, then the second method is the best choice, because the damage must be between the loudspeaker and the audio amplifier.
If you hear a click, you can still continue the inspection using the second method starting from point C, or the first method, because both have the same inspection speed opportunity.

CONCLUSION:

Signal-tracing methods require an input signal to the suspected area and a precise output measurement. Signal-tracing always requires at least one test instrument and usually two.

Troubleshooting with SYMPTOM-FUNCTION Technique (Symptoms of Function)

Substance:

Confidence in properly determining which is a symptom and which is a function.
Troubleshooting servo systems is a very specialized area.

In TV repair, the symptoms can be seen on the screen or heard through the speakers. In digital equipment, the symptoms can be determined from the output. The difficulty is that when feedback is considered, it becomes more difficult to determine which is the symptom and which is the fault. The first example is illustrated by Figure 7.7 as a mixing tank in a food manufacturing plant.

Figure 7.7 Electronic Control For A Mixing Tank

The work of this system is as follows:

There are two liquids to be mixed. Each liquid comes from a different storage tank and is pumped through pipes of different lengths and diameters into a mixing tank.
The flow of fluid through the pipe is controlled in each case by a motor-controlled tap.
If it is desired for the same fluid in gallons per minute to flow through both pipes, the output of flowmeter 1 must be the same as the output of flowmeter 2. A signal comparator and control section compares the two voltages together for the flow of fluid through both pipes.
If the voltage from the flowmeter becomes large, the servo motor drives the amplifier connected to the motor driver valve no.1 will activate the motor to rotate the lower valve. If meter 2 shows excessive output, the valve controlled by motor 2 will be turned off.
Specific setting of maximum and minimum flowmeter signal level is done by signal divider. Without limitation, an increase in servo gain can cause motor driven valve no.1 to turn off or completely close. When this is compared with flowmeter no.2, this servo gain will turn off or close valve no.2, in a short time both valves can be completely closed.
The electronic reference voltage regulation portion, is the same as the speed control input for the difference amplifier in Figure 7.3, to prevent excessive closing or opening of the valve.

Damage that occurred:

The valve motor driver no.1 has a tendency to shut off the flow in the pipe after the equipment has been operated for several hours. The valve motor driver no.2 is working properly.

Steps taken:

The difficulty is the circuit that controls the flow through pipe no.1. Due to this failure both pipes are closed at the ends. We cannot check the output of both flowmeters since nothing is flowing through the pipes.
By applying the symptom function, we can reduce the damage to the pipe controller part no.2. Remember that only the motor drive of valve no.1 has a tendency to close. Also remember that this damage is only visible after the equipment has been operated for a while. Experienced troubleshooters immediately identify temperature problems. A damage that is usually only visible after a sufficient period of work.
A visual inspection of the circuitry on the signal comparator and controller, especially the servo motors that drive the amplifiers, may reveal overheating of the resistors or other signs.
By cutting, we can eliminate flowmeter 1 or flowmeter 2 as the source of damage. Both will not be hot and even if one of them is hot, it will not cause the motor to drive valves no. 1 and no. 2. Damage to that part will cause damage to both. If the comparator itself is not balanced, it will tend to close one valve and fully open the other valve. We already know that the motor that drives valve no. 2 is working properly. It does not look like the motor drives the valve by itself, an electro-mechanical combination fails in this case. In many cases, the position of the valve is controlled by a servo motor driven by a drive amplifier.
The most likely suspect without doing more detailed testing is the servo motor drive amplifier to valve no.1.

The second example is a cable thickness control device as shown in Figure 7.8 which shows in simple schematic form a thickness control system for a cable pulling machine.

Figure 7.8 Cable Thickness Control System