Module 16—Introduction to Test Equipment
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The METER card is used to report changes, additions, or deletions to the user activity’s inventory. It is also used to report changes in custody of the item of test equipment. The procedure for filling out the METER card is outlined in the appendixes of the MEASURE Users Manual. Blank METER cards can be obtained through the responsible METCALREP.
A computer printout recall schedule is also generated by the MEASURE system. The purpose of this printout is to list those items of equipment that are due for calibration. Each recall schedule is composed of a set of four identical copies. One set is provided to the calibration activity as an aid to workload planning; a second set is sent to the user’s activity. The recall schedule is one of several products/formats sent automatically by the MEASURE Operation Control Center to the user activity on a regular basis. The MOCC automatically distributes the following products to user activities at the intervals shown:
TEST EQUIPMENT REFERENCES
Several publications that contain information concerning test equipment are required to be maintained aboard ship by type commander instructions. These requirements are usually found in the inspection checkoff list. Other publications, while not required by directive, are necessary to you as reference and study material so you will be able to administer an effective test equipment program. Technicians should become familiar with the publications/directives listed in appendix II of this module.
INTRODUCTION TO TROUBLESHOOTING
Our military forces increasingly rely on electrical and electronic equipment to help perform their mission. The effectiveness of our tactical forces depends on many types of electronic systems, such as communications systems, detection systems, and fire control systems. The reliability of such equipment is determined by many factors; however, the primary factors are the quality of the equipment in use, the availability of spare parts, and the ability of maintenance personnel to perform adequate maintenance.
Maintenance is work done to correct, reduce, or counteract wear, failure, and damage to equipment. Maintenance of electrical and electronic equipment is divided into two main categories: PREVENTIVE (routine) and CORRECTIVE maintenance. Preventive maintenance consists of mechanical, electrical, and electronic checks to determine whether equipment is operating properly. It also consists of visual inspections of cabling and equipment for damage and to determine if lubrication is needed. Corrective maintenance isolates equipment failure by means of test techniques and practices; it also replaces defective parts and realigns or readjusts equipment to bring it back to proper performance.
Q-9. What are the two main categories of maintenance?
Q-10. What type of maintenance involves isolating equipment troubles and replacing defective parts?
Testing and troubleshooting are the areas of maintenance that require the greatest technical skill. Testing procedures are referred to as measurements, tests, and checks. The definitions of these terms often overlap, depending on their use and the results obtained. For example, a power measurement and a frequency check could constitute a test of the operation of the same radio transmitter.
Troubleshooting is a term which we in the electronics field use daily. But what does it mean? Troubleshooting is sometimes thought to be the simple repair of a piece of equipment when it fails to function properly. This, however, is only part of the picture. In addition to repair, you, as a troubleshooter, must be able to evaluate equipment performance. You evaluate performance by comparing your knowledge of how the equipment should operate with the way it is actually performing. You must evaluate equipment both before and after repairs are accomplished.
Equipment performance data, along with other general information for various electronic equipments, is available to help you in making comparisons. This information is provided in performance standards books for each piece of equipment. It illustrates what a particular waveform should look like at a given test point or what amplitude a voltage should be, and so forth. This data aids you in making intelligent comparisons of current and baseline operating characteristics for the specific equipment assigned to you for maintenance. ("Baseline" refers to the initial operating conditions of the equipment on installation or after overhaul when it is operating according to design.)
Remember, maintenance refers to all actions you perform on equipment to retain it in a serviceable condition or to restore it to proper operation. This involves inspecting, testing, servicing, repairing, rebuilding, and so forth. Proper maintenance can be performed only by trained personnel who are thoroughly familiar with the equipment. This familiarity requires a thorough knowledge of the theory of operation of the equipment.
A logical and systematic approach to troubleshooting is of the utmost importance in your performance of electronics maintenance. Many hours have been lost because of time-consuming "hit-or- miss" (often referred to as "easter-egging") methods of troubleshooting.
GENERAL TEST EQUIPMENT INFORMATION
In any maintenance training program, one of your most important tasks is to learn the use of test equipment in all types of maintenance work. To be effective in maintenance work, you must become familiar not only with the common types of measuring instruments, but also with the more specialized equipment. Some examples of common types of typical measuring instruments are the ammeter, voltmeter, and ohmmeter; examples of specialized test equipment are the spectrum analyzer, dual-trace oscilloscope, and power and frequency meters.
TEST EQUIPMENT SAFETY PRECAUTIONS
The electrical measuring instruments included in test equipment are delicately constructed and require certain handling precautions to prevent damage and to ensure accurate readings. In addition, to prevent injury to personnel, you must observe precautions while using test equipment. You can find a list of applicable instructions in appendix II of this module.
To prevent damage to electrical measuring instruments, you should observe the precautions relating to three hazards: mechanical shock, exposure to magnetic fields, and excessive current flow.
MECHANICAL SHOCK.—Instruments contain permanent magnets, meters, and other components that are sensitive to shock. Heavy vibrations or severe shock can cause these instruments to lose their calibration accuracy.
EXPOSURE TO STRONG MAGNETIC FIELDS.—Strong magnetic fields may permanently impair the accuracy of a test instrument. These fields may impress permanent magnetic effects on permanent magnets, moving-coil instruments, iron parts of moving-iron instruments, or in the magnetic materials used to shield instruments.
EXCESSIVE CURRENT FLOW.—This includes various precautions, depending on the type of instrument. When in doubt, use the maximum range scale on the first measurement and shift to lower range scales only after you verify that the reading can be made on a lower range. If possible, connections should be made while the circuit is de-energized. All connections should be checked to ensure that the instrument will not be overloaded before the circuit is reenergized.
Other Instrument Precautions
Precautions to be observed to prevent instrument damage include the following:
· Keep in mind that the coils of wattmeters, frequency meters, and power meters may be carrying large quantities of current even when the meter pointer is on scale.
· Never open secondaries of current transformers when the primary is energized.
· Never short-circuit secondaries of potential transformers the primary is energized.
· Never leave an instrument connected with its pointer off-scale or deflected in the wrong direction.
· Ensure that meters in motor circuits can handle the motor starting current. This may be as high as six to eight times the normal running current.
· Never attempt to measure the internal resistance of a meter movement with an ohmmeter since the movement may be damaged by the current output from the ohmmeter.
· Never advance the intensity control of an oscilloscope to a position that causes an excessively bright spot on the screen; never permit a sharply focused spot to remain stationary for any period of time. This results in burn spots on the face of the cathode-ray tube (CRT).
· In checking electron tubes with a tube tester that has a separate "short test," always make the short test first. If the tube is shorted, no further test should be made.
· Before measuring resistance, always discharge any capacitors in the circuit to be tested. Note and record any points not having bleeder resistors or discharge paths for capacitors.
· Always disconnect voltmeters from field generating or other highly inductive circuits before you open the circuit.
Q-11. Which quantity (voltage or current) determines the intensity of an electrical shock?
Situations can arise during the use of test equipment that are extremely dangerous to personnel. For example, you may have an oscilloscope plugged into one receptacle, an electronic meter plugged into
another, and a soldering iron in still another. Also, you may be using an extension cord for some equipments and not others or may be using other possible combinations. Some of the hazards presented by situations such as these include contact with live terminals or test leads. In addition, cords and test leads may be cross connected in such a manner that a potential difference exists between the metal cases of the instruments. This potential difference may cause serious or fatal shocks.
Test leads attached to test equipment should, if possible, extend from the back of the instruments away from the observer. If this is not possible, they should be clamped to the bench or table near the instruments.
At times, you may use instruments at locations where vibration is present, such as near a diesel engine. At such times, the instruments should be placed on pads of folded cloth, felt, or similar shock- absorbing material.
WORKING ON ENERGIZED CIRCUITS
Insofar as is practical, you should NOT undertake repair work on energized circuits and equipment. However, it could become necessary, such as when you make adjustments on operating equipment. In such cases, obtain permission from your supervisor, then proceed with your work, but carefully observe the following safety precautions:
· DO NOT WORK ALONE.
· Station an assistant near the main switch or circuit breaker so the equipment can be immediately de-energized in case of an emergency.
· Someone qualified in first aid for electrical shock should be standing by during the entire operation.
· Ensure that you have adequate lighting. You must be able to see clearly if you are to perform the job safely and properly.
· Be sure that you are insulated from ground by an approved rubber mat or layers of dry canvas and/or wood.
· Where practical, use only one hand, keeping the other either behind you or in your pocket.
· If you expect voltage to exceed 150 volts, wear rubber gloves.
· DO NOT work on any type of electrical apparatus when you are wearing wet clothing or if your hands are wet.
· DO NOT wear loose or flapping clothing.
· The use of thin-soled shoes and shoes with metal plates or hobnails is prohibited.
· Flammable articles, such as celluloid cap visors, should not be worn.
· Remove all rings, wristwatches, bracelets, and similar metal items before working on the equipment. Also ensure that your clothing does not contain exposed metal fasteners, such as zippers, snaps, buttons, and pins.
· Do not tamper with interlock switches; that is, do not defeat their purpose by shorting them or blocking them open.
· Ensure that equipment is properly grounded before energizing.
· De-energize equipment before attaching alligator clips to any circuit.
· Use only approved meters and other indicating devices to check for the presence of voltage.
· Observe the following procedures when measuring voltages in excess of 300 volts:
Turn off the equipment power.
Short-circuit or ground the terminals of all components capable of retaining a charge.
Connect the meter leads to the points to be measured.
Remove any terminal grounds previously connected.
Turn on the power and observe the voltage reading.
Turn off the power.
Short circuit or ground all components capable of retaining a charge.
Disconnect the meter leads.
· On all circuits where the voltage is in excess of 30 volts and where decks, bulkheads, or workbenches are made of metal, you should insulate yourself from accidental grounding by using approved insulating material. The insulating material should have the following qualities:
It should be dry, without holes, and should not contain conducting materials.
The voltage rating for which it is made should be clearly marked on the material. The proper material should be used so that adequate protection from the voltage can be supplied.
Dry wood may be used or, as an alternative, several layers of dry canvas, sheets of phenolic (resin or plastic) insulating material, or suitable rubber mats.
Care should be exercised to ensure that moisture, dust, metal chips, and so forth, which may collect on insulating material, are removed at once. Small deposits of such materials can become electrical hazards.
All insulating materials on machinery and in the area should be kept free of oil, grease, carbon dust, and so forth, since such deposits destroy insulation.
SAFETY SHORTING PROBE
A representative shorting probe is shown in figure 1-4. An approved shorting probe is shown in
NAVSEA 0967-LP-000-0100, EIMB, General, Section 3.
Figure 1-4.—Representative safety shorting probe.
Capacitors and cathode-ray tubes may retain their charge for a considerable period of time after having been disconnected from the power source.
Always assume there is a voltage present when working with circuits having high capacitance, even when the circuit has been disconnected from its power source.
An approved type of shorting probe should be used to discharge capacitors and cathode-ray tubes individually.
When using the safety shorting probe, always be sure to first connect the test clip to a good ground (if necessary, scrape the paint off the grounding metal to make a good contact). Then hold the safety shorting probe by the insulated handle and touch the probe end of the shorting rod to the point to be shorted out. The probe end is fashioned so that it can be hooked over the part or terminal to provide a constant connection by the weight of the handle alone. Always take care not to touch any of the metal parts of the safety shorting probe while touching the probe to the exposed "hot" terminal. It pays to be safe; use the safety shorting probe with care.
Some equipments are provided with walk-around shorting devices, such as fixed grounding studs or permanently attached grounding rods. When that is the case, the walk-around shorting devices should be used rather than the safety shorting probe.
Q-12. What tool is used to de-energize capacitors in a circuit that has been disconnected from its power source?
WORKING ON DE-ENERGIZED CIRCUITS
When any electronic equipment is to be repaired or overhauled, certain general safety precautions should be observed. They are as follows:
· Remember that electrical and electronic circuits often have more than one source of power. Take time to study the schematics or wiring diagrams of the entire system to ensure that all sources of power have been disconnected
· If pertinent, inform the remote station regarding the circuit on which work will be performed.
· Use one hand when turning switches on or off.
· Safety devices, such as interlocks, overload relays, and fuses, should never be altered or disconnected except for replacement. In addition, they should never be changed or modified in any way without specific authorization.
· Fuses should be removed and replaced only after the circuit has been de-energized. When a fuse "blows," the replacement should be of the same type and have the same current and voltage ratings. A fuse puller should be used to remove and replace cartridge fuses.
· All circuit breakers and switches from which power could possibly be supplied should be secured (locked if possible) in the OPEN or OFF (safe) position and danger tagged in accordance with procedures in the Standard Organization and Regulations of the U.S. Navy, OPNAVINST
· After the work has been completed, the tag (or tags) should be removed only by the same person who signed it (them) when the work began.
· Keep clothing, hands, and feet dry if at all possible. When you must work in wet or damp locations, place a rubber mat or other nonconductive material on top of a dry, wooden platform or stool; then use the platform or stool to sit and stand on. Use insulated tools and insulated flashlights of the molded type when you are required to work on exposed parts.
GROUNDING OF POWER TOOLS AND EQUIPMENT
The possibility of electrical shock can be reduced by ensuring that all motor and generator frames, metal bases, and other structural parts of electrical and electronic equipment are at ground potential.
Normally, on steel-hull vessels, such grounds are inherently provided because the metal cases or frames of the equipment are in contact with one another and with the metal structure of the vessel. In some instances where such inherent grounding is not provided by the mounting arrangements, such as equipment supported on shock mounts, suitable ground connections must be provided.
The grounding wire used for this purpose is generally made of flexible material (copper or aluminum) that provides sufficient current-carrying capacity to ensure an effective ground. In this manner, equipment cases and frames that are not intended to be above ground potential are effectively grounded; also, the possibility of electrical shock to personnel coming in contact with metal parts of the equipment is minimized. The secondary purpose of grounding equipment is to improve the operation and continuity of service of all equipments.
Paint, grease, or other foreign matter can interfere with the positive metal-to-metal contact at the ground connection point. Therefore, all bonding surfaces (connection points or metallic junctions) must be securely fastened and free of such matter. In all instances where equipment grounding is provided, certain general precautions and preventive maintenance measures must be taken. A few of these precautions are listed below:
· Periodically clean all strap-and-clamp connectors to ensure that all direct metal-to-metal contacts are free from foreign matter.
· Check all mounting hardware for mechanical failure or loose connections.
· Replace any faulty, rusted, or otherwise unfit grounding strap, clamp, connection, or component between the equipment and the ground to the ship’s hull.
· When replacing a part of the ground connection, make certain that the metallic contact surfaces are clean and that electrical continuity is re-established.
· After completing the foregoing steps, recheck to be sure that the connection is securely fastened with the correct mounting hardware. Paint the ground strap and hardware in accordance with current procedures.
Because of the electrical shock hazards that could be encountered aboard ship, plugs and convenience outlets for use with portable equipment and power tools normally are standard three-prong type. Both plugs and outlets are keyed so that the plug must be in the correct position before it can be inserted into the receptacle. To ensure that the safety factors incorporated in these devices are in serviceable condition and are safe for use, you must perform the following precautions and inspections:
· Inspect the pins of the plug to see that they are firmly in place and are not bent or damaged.
· Check the wiring terminals and connections of the plug. Loose connections and frayed wires on the plug surface must be corrected and any foreign matter removed before the plug is inserted into the receptacle.
· Use a meter to ensure that the ground pin has a resistance of less than 1 ohm equipment ground.
· Do not attempt to insert a grounded-type plug into a grounded receptacle without first aligning the plug properly.
Never use a power tool or a piece of portable test equipment unless you are absolutely sure that it is equipped with a properly grounded conductor.
Electronic measurements involve the fundamental electrical quantities of voltage and current and the inherent characteristics of resistance, capacitance, and inductance. In circuits being tested, voltage and current are dependent upon resistance, capacitance, and inductance for their distribution; therefore, voltage and current measurements are valuable aids in determining circuit component conditions and in the evaluation of symptoms. Practically any reading obtained from the use of test equipment will depend on these basic measured quantities of resistance, capacitance, and inductance.
VOLTAGE AND CURRENT MEASUREMENTS
Voltage measurements may be made as part of either preventive or corrective maintenance. These measurements are made using a voltmeter. When compared with voltage charts, these measurements are a valuable aid in locating a trouble quickly and easily. However, if the sensitivity of the test voltmeter differs from that of the voltmeter used in preparing the chart, the voltage measurements must be evaluated before the true circuit conditions can be determined. (Sensitivity in voltmeters was discussed in NEETS, Module 3, Introduction to Circuit Protection, Control, and Measurement.)
Since many of the troubles you find in equipments and systems are the result of abnormal voltages, voltage measurements are a valuable aid in locating trouble. You can measure voltage with a voltmeter without interrupting circuit operation.
Point-to-point voltage measurement charts, usually found in equipment technical manuals, contain the normal operating voltages found in the various stages of the equipment. These voltages are usually measured between indicated points and ground unless otherwise stated. When you begin recording voltage measurements, it is a smart and safe practice to set the voltmeter on the highest range before measuring. This ensures that excessive voltages existing in the circuit will not cause overloading of the meter.
Q-13. On what range should you set the voltmeter prior to taking a voltage measurement?
To increase accuracy, you should then set the voltmeter to the appropriate range for the proper comparison with the expected voltage in the voltage charts. When checking voltages, remember that a voltage reading can be obtained across a resistance, even if that resistance is open. The resistance of the meter itself forms a circuit resistance when the meter probes are placed across the open resistance. Therefore, the voltage across the component may appear to be normal or near-normal as you read the meter, but may actually be abnormal when the meter is disconnected from the circuit.
If the internal resistance of the voltmeter is approximately the same value as the resistance being tested, it will indicate a considerably lower voltage than the actual voltage present when the meter is removed from the circuit. The sensitivity (in ohms per volt) of the voltmeter used to prepare the voltage charts is provided on those charts. If a meter of similar sensitivity is available, you should use it to reduce the effects of loading.
The following precautions are general safety measures that apply to the measurement of voltages. Remember that nearly all voltages are dangerous and have often proved fatal to careless technicians. When measuring voltages, be sure to observe the following precautions:
· Set test equipment to the HIGHEST range.
· Make sure safety observer knows where to secure power for the equipment under test.
· Connect the ground lead of the voltmeter first.
· Use only one hand to take measurements (when possible), and put the other hand in your pocket or behind your back.
· If the voltage to be measured is less than 300 volts, place the end of the test probe on the point to be tested; use the polarity switch to select positive or negative readings.
· If the voltage to be measured is more than 300 volts, proceed as follows:
1. Shut off circuit power.
2. Discharge all filter capacitors with a shorting probe.
3. Temporarily ground the point to be measured.
4. Connect (clip on) the proper test lead to the high-voltage point.
5. Move away from the voltmeter.
6. Turn on circuit power and read the voltmeter.
7. Turn off circuit power.
8. Discharge all capacitors before disconnecting the meter.
Q-14. When taking a voltage measurement, which lead of the voltmeter should you connect to the circuit first?
Current measurements are not often taken in the course of preventive maintenance or testing. This is because the ammeter (or other current-measuring instrument) must become an actual part of the equipment being tested. The circuit must be opened (desoldered) to connect the ammeter in series with the circuit being tested. Usually, you can take a voltage measurement and use this factor to calculate the circuit current by applying Ohm’s law.
Q-15. Is an ammeter connected in series or in parallel with the circuit under test?
Resistance measurements are a valuable aid to you in locating defective circuits and components during corrective maintenance. Maintenance handbooks for the equipment can often be used to help you take these measurements. These handbooks often contain resistance charts that are referenced to accessible test points within the equipment. Without these charts, taking resistance measurements in a complex circuit is a slow process. The process is slow because one side of the circuit component must often be desoldered to get a true resistance measurement. However, resistance tolerances vary so widely that approximate resistance readings are adequate for most jobs.
Once the most accessible test point is found, an ohmmeter is usually used to take the resistance measurement. Because of the degree of accuracy needed when an ohmmeter is used, proper calibration and understanding of the meter scales is a must. (Topic 2 of this module will discuss these requirements in detail.) When using an ohmmeter, you must observe the following precautions:
· The circuit being tested must be completely de-energized.
· Any meters or transistors which can be damaged by the ohmmeter current must be removed before any measurement is made.
Q-16. What must be done to a circuit before you can use an ohmmeter for testing?
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics