Six Step Troubleshooting
transcribed from NAVPERS 93500
Troubleshooting Communications Systems dated June 1965
and revised by JJester 6/12/2017
The Need for a Logical Troubleshooting Procedure
Troubleshooting is a skill which must be developed if you are to be proficient in the operation of a station or equipment. Good troubleshooting is not a talkent with which a person is born. It is, however, a skill that can be acquired by anyone with a suitable background. You can become a good troubleshooter if you have:
- Sufficient background knowledge to learn, or be taught how the equipment works
- skill in reading and interpreting data contained in the equipment technical manuals
- skill in operating test equipment and interpreting test readings, and
- a logical approach to troubleshooting
Logical trouble shooting does not recognize "Easter-egging," "cook-booking," or "trial-and-error" methods. The "Easter-egger" makes unsupported guesses as to the tlocation of the trouble. The "cook-booker" looks for trouble-locating clues in the trouble chart of the technical manuals, and is lost if the manual doesn't cover the particular fault. The "trial-and-error" technician starts at one end of the equipment and works towards the other end with component checkers and test equipment. If any of the three finds the trouble in a rasonable length of time, they are lucky: finding the one bad part or wire or connection among hundreds or thousands is not easy to do by illogical methods.
Logical troubleshooting is a time-proven procedure used by all accomplished technicians. Most of them have applied the procedure so often that they no longer pay attention to it's fine points. Through habit and years of experience they may have forgotten its specific details.
Probably no two technicians would explain the procedure alike, but all would agree that logical troubleshooting consistes of sequential steps that systematically isolate the trouble to the faulty part. Some would list the procedure in three or four steps' others would count a dozen, fifteen or more. Regardless of the number, the principle would be the same. Six steps have been chosen as the easiest method of learning and applying this procedure. The steps in their sequential order are:
- Step 1 - Symptom recognition
- Step 2 - Symptom elaboration
- Step 3 - Listing the probably faulty functions
- Step 4 - Localizing the faulty function
- Step 5 - Localizing the faulty circuit
- Step 6 - Failure Analysis
The Six Step Troubleshooting process Block Diagram
Step 1 - Symptom Recognition
The first step is any troubleshooting problem is recognition of a trouble indication. recognizing that a trouble exists in an equipment is not always easy to do since conditions of less than peak performance are not always apparent. Decreased sensitivity in a superheterdyne receiver, lower transmitter power, slight distortion in audio equipment are ust a few of the hundreds of examples. Each of these is a trouble symptom that requires recognition.
There are many ways that the existance of a trouble can be brought to your attention. The obvious troubles will undoubtedly be noted during operational or preoperational checkout. These usually include complete or almost complete operational tests of the equipment. Malfunction of the equipment or any part of the equipment requires immediate attention. Troubles that are not easily noticed are those that cause degradation in equipment performance. A 125 mile radar that only reached out to 50 miles, a 100 watt transmitter only capable of 87 watts, a multimeter reading that provides readings that are 10 percent off, or a noisy telemeter record are examples of equipment faults that are difficult to recognize because there is no visible or audible indication that say they exist. The owners depend on full performance equipment; the technician must locate the trouble and effect repairs in a timely manner. If you make a point of looking for these "hidden" trouble symptoms every time you touch the equipment, most of the symptoms of degraded performance can be recognized. Compare performance between two similar equipments. Make the performance standard checks located in the equipment manuals. Verify changes in performance since the last time you repaired, tuned, calibrated, or aligned the equipment. While you are troubleshooting, you can look for, and probably find, symptoms that signify degraded performance. Trouble symptoms can be recognized if you look for them.
Step 2 - Symptom Elaboration
Breaking out the test equipment and equipment prints and proceeding headlong into troubleshooting on just the original identiy of a trouble symptom is a very doubtful procedure. It could also be an unnecessary expenditure of energy. A dead scope, noise in a receiver, a zero reading on a panel meter or a missing time pulse by itself is not sufficient identification of a trouble symptom. There is a tendency among noncompetent technicians to attempt a solution of a troubleshooting problem before they have completely defined it.
The procedures for sysmptom elaboration are dependent upon the available aids designed into the equipment and the nature of the orignal symptom. The aids include front panel controls and built in performance measuring indicators. Additional information can be obtained about most malfunctions as a result of systematic front panel check. If you have good knowledge of how an equipment works, manipulation of appropriate controls and switches and correstponding checks of the equipment meters and indications will reveal how the trouble is affecting the entire equipment. From these clues you can narrow down the probable areas of the equipment that could contain the trouble.
Step 3 - Symptom Elaboration
The third step requires that you make an "educated guess" as to the probably cause of the trouble. From the trouble symptoms. as you have identified them, you determine the most logical locations of the trouble. Locations are generally confined to the major subdivisions of the equipments, the functional units.
The term "function" or "functional unit" is used to denote and electronic operation performed by a specific area of an equipment; for example, functions may be entitled transmitter, receiver, modulator, or power supply. These functions combined together, make up an equipment set (transceiver set, radar set, sonar set, etc) and "unit" (a physical subdivision) are synonymous. However, there are occasions when one or more circuits for a particular function may be physically located on more than onr indicated unit or even more than one printed circuit board. "Educated gusses" are made from a knowledge of how the equipment works and a study of the equipment's functional block didagram.
As an example, assume there is no receiver audio from a remote amplifier. Your "educated guess" should include that audio path from the receiver to the remote amplifier as well as the remote amplifier itself. Making an educated guess that it is a bad tansistor (because the greater percentage of the problems are caused by transistors in that equipment) is not acceptable. You still have to find that bad transistor. The purpose here is to use valid reasoning to identify all probable, technically sensible functional areas which could contain the trouble. It may be well that the specific trouble is a bad transistor, but wholesale transistor substitution takes a lot of time and could introduce additional troubles, particularly in circuits with critical tolerances.
Don't worry if your find your lists of "suspects" are incomplete; even accomplished technicians may not be able to list all the functional units that are probably sources of the trouble. Your lists will improve with practice until you don't have to even write the suspects down. but instinctively know which units the trouble could be in. On the other hand, don't worry if you end up with a list that is only one item. In a well-designed equipment, you will often be able to name the one funtional unit causing the problem.
Step 4 - Localizing the Faulty Function.
In this step you select one of the "suspect" functions for testing. Your first choice is not necessarily the one you thought of first nore the one that past experience suggests as being the most probable. Selection of the first functional unit to be tested should be based not only on probablility but also the difficulties involved in making the necessary tests. Under some circumstances, you might delect to test the second most probable "suspect" rather than the most probable because the later might involve testing difficulties that should be initially avoided or require tampering with circuit adjustments that might later prove to have been unnecessary. Like all the others, this step in the troubleshooting procedure places an emphasis on sommon-sense thinking rather than resultant action. If you do your preliminary work properly, manual work in gaining access to tests points and using test equipment can be limited to a bare minimum.
After selecting the order in which you will check the units you have listed, you proceed to verify your first selection. The check wil normally be made an an output test point on the selected unit. The test equipment reading is compared with the normal signal described in the technical manual. A "no output" condition is relatively easy to recognize. A distorted or abnormal output, however, should be carefully verified before arriving at a technical conclusion.
Upon completing a verification of the probable faulty unit you have selected, you will arrive at one of several conclusions. the test will verify that:
(1) this is the unit in which the trouble lies;
(2) the trouble could be in this plus another unit(s) from which it recives signal or control voltages;
(3) the trouble is not in this unit;
(4) the output looks suspicious and further verifying tests need to be made.
Whatever your conclusion, you have discovered information that can be used to substantiate or eliminate suspected units on your list or provide evidence for adding another. Tests of suspected unit outputs are continued until the single faulty unit is identified. You have narrowed down the trouble to a fraction of the total number of circuits and parts in the equipment. If you have this much of the procedure properly, you can confine your search to the functional area you have isolated.
Note that this step can be eliminated if your symtom elaboration definitely identified one faulty unit.
Step 5 - Localizing the Faulty Circuit
The next step after you have isolated the faulty function or functional area is to identify the faulty circuit. The same narrowing down procedures are used as before. First you divide the funtional unit into circuit groups (IF strip, mixer, discriminator, amplifier, etc.) You then examine each circuit group as if it could contain the fault. Finally, you make tests to isolate the faulty circuit group without going through the unnecessary, time-wasting chore of test-point to test-point checking from one end of the unit to another.
In narrowing down the trouble to a single functional group of circuits, you can employ a process called "bracketing." In this process brackets are placed on the schematic at the inputs to the unit that are known to be good and at the outputs known to be bad. Next you select points at you can test to isolate or eliminate portions of the unit. As each test is made, an input or output bracket is moved to the point in the block diagram where the test was made. In this manner the closing brackets systematically narrow the fault to a single circuit.
In selecting a point on a detailed block diagram to which one of the brackets is to be moved, you must consider two things - the characteristics of the improper output signal and the types of signal paths contained in the unit. The waveshape of a signal has certain characteristics - voltage, rise-time, noise content, frequency, etc - that can be measured or observed. When these characteristiccs are in accordance with the designated standards, the signal is considered to be good. In a bad signal characteristics that are improper can reveal clues that will help to identify a circuit group whose function is to originate or control that portion of the waveshape. For example, the output of a unit is suppose to be a sawtooth waveform with six pulses equally spaced on its slope. If the pulses re there but the slope is nonexistent or insufficient, the sawtooth geneerating and shaping circuits would be suspected. If the proper slope was there but there were no pulses or an incorrect number of pulses, the pulse generating or controlling circuit groups probably contain the trouble.
The type of signal path contained in the unit is the other item to be considered before moving a bracket. There are four general types - linear, switching, convergent/divergent, and feedback. In a linear signal path the signal is processed through circted that are connected in series. when identification of the faulty circuit group is difficult or impossible (that is when the waveshape characteristics do not indicate the faulty circuit function), brackets are moved to successively smaller half-points in the linear string. signals from two or more circuit channels that meet at a common point or a signal that leaves a common point to enter two or more channels are examples of convergent and divergent paths respectively Moving a bracket (after making the appropriate test) to the common point will seperate the bad from the good signal paths. In the same manner, a test and bracket at the point where signal paths are connected by a switch will reveal the same informaiont. the remaining type - feedback - is the hardest type to troubleshoot. Negative feedback circuits (automatic gain or frequency control, active filtering, etc) can be opened - by removing the component in the feedback path - or in some cases grounded to remove the feedback effect. Positive feedback loops (regenerative receivers, oscillators) are more difficult since the regeneration will not occur if the positive feedback is removed. Signal insertion plus disabling of the feedback loop is effective in most cases.
There are no hard-and-fast, step-by-step procedures for bracketing, but there are some realistic rules. Examine the characteristics of the faulty output to determine the circuit group function that either generates or controls the improper characteristic. study the detailed block diagram to determin the least number of bracket moves that will isolate the faulty circuit. such moves will be dependent on the types of signal paths contained in the unit and the electronic functions of the circuit groups that may be responsible for distortions contained in the unit's output. Move only one bracket at a time after verifying the suitablility of the signal by making a test. If the test does not reveal sufficient information for a valid bracket move, make another "educated guess"." Determing which bracket to move is dependent upon circuit configuration within the unit and the number of circuits that it will enclosed. Figure 1-1 illustrates a logical sequence for isolating a fault by this procedure.
The servicng block diagram can serve as the instrument for the complete bracketing process. In some cases it will be necessary to refer to a schematic diagram for information regarding location of test points. There is sufficient diagram information in the technical manual to support the bracketing procedure and preclude wasteful, unreliable, circuit-to-circuit checking; you must know the organization and content of the manual to make effective use of your time.
The bracketing step is ompleted when you have isolated the trouble to a single circuit and verified that the output of this circuit is the cause of the original symptoms.
Step 6 - Failure Analysis
The troubleshooting procedure thus far has narrowed the trouble to a single circuit consisting of a single active device. If there is no output from the circuit, voltage and reistance checks are your best bet. However, such checks can be minimized if there is an output that can be examined for distortions that will reveal the circuit parts that are most likely to be at fault. Quite often the waveform will identify the malfunction to be in a specific portion of the circuit. Such a study should be made before any parts are replaced.
When the faulty part has been identified, it should not be replaced until you substantiate that it is causing the actual trouble. a suspected open reistor, shorted capacitor, detuned coil, or weak tube may not be the reason (or the only reason) for the faulty output of the circuit. such a defect may have resulted from another trouble. If you replace the part without an adequate technical reason, you may not cure the problem. Analyzed the failure before making the repair.