Troubleshooting Pulse Circuits

By

SAM WILSON, CET

80706-2

Edition 1

Troubleshooting Pulse Circuits Instructional Objectives

Pulse circuits are used in communications, digital sys­tems, microprocessor control systems, automated ma­chine control, and in many other applications. As a tech­nician, you must know how to logically troubleshoot these systems so that you can quickly find the cause of breakdowns.

In industrial systems, downtime for electronic equip­ment can be very, very expensive; thus, it's a must to employ fast troubleshooting procedures.

to: After you have studied this lesson, you should be able

• List the characteristics of pulses to be tested.

• Compare the troubleshooting of pulse circuits with that of other systems.

• Determine which part of digital and pulse circuits to test first in quick troubleshooting procedures.

• Explain the use of logic analyzers and the pur­pose of single-step testing.

• Understand the troubleshooting techniques for a microprocessor system.

Contents

Review of Troubleshooting Techniques

Troubleshooting Pulse and Logic Circuits Signal-Tracing Versus Signal Injection Tracing and Injecting Precautions Waveform Tests Triangular and Tone-Burst Tests Check Your Learning 1

Use of Pulsers and Probes

Logic Pulsers Basic Pulser Tests Logic Probes Use of Logic Probes Pulsers Versus Signal Generators Probes Versus VOMs Check Your Learning 2

Troubleshooting Aids

Probe and Pulser Connections Simple Logic Probes Inverter Circuit Probes Simple Logic Pulser Troubleshooting Equipment Removing ICs Logic Scopes Multitrace Oscilloscopes Troubleshooting Industrial Electronic Systems Check Your Learning 3

Self-Test

Self-Test Answers

1 2 3 5 6 7

8 9

11 12 13 14 16

18 20 21 22 23 24 25 26 26 29

30

34

Review of Troubleshooting Techniques

1J Troubleshooting Pulse and Logic Circuits

Some of the techniques for troubleshooting pulse and logic circuits, such as signal-tracing and signal injection, are already known to you. However, some other tech­niques, such as benchmark testing, which we'll take up now, have not been discussed in depth in previous les­sons. We'll begin with a brief review of what may be familiar material to some of you.

A benchmark program is a step-by-step procedure that is normally used to compare different system designs for such factors as speed, number of operations required, and power requirements. For example, two different types of computers may be compared as to which is suitable for a specific job. However, the benchmark program may also be used for testing a single system. The test proce­dure is to run through the program until the system fails; the failure step often indentifies the defective stage.

New technology and the need for fast troubleshoot­ing have resulted in some new types of test equipment for quickly locating circuit failures. Two examples of com­paratively new equipment are the logic probe and the logic pulser, which will be discussed in this lesson.

1

Fig. 1. When you use the signal­tracing technique, not only must you follow the signal through the amplification stages, but you must also look for gain in each stage.

2

~ Signal-Tracing Versus Signal Injection

Signal-tracing and signal injection are both meth­ods of troubleshooting logic and pulse circuits. It will be helpful to start the study of troubleshooting by reviewing these two techniques.

Fig. 1 shows the technique called signal-tracing. In this case, a signal generator injects a sine-wave signal input at point A. The oscilloscope is used to trace this signal from points A to B to C, and finally to D. When tracing by this method, the oscilloscope will show the presence of a signal until it probes past the circuit fault. For example, if the middle amplifier is not functioning, the oscilloscope will show a signal at point B, but NOT at point C.

INPUT

SIGNAL GENERATOR

/ (

/ /

I /

/

OSCILLOSCOPE

If you assume that conventional amplifiers are shown in Fig. 1, the signal amplitude at B should be greater than it is at A, and the amplitude at C should be even greater. You can't always expect gain, of course, because emitter­follower stages are sometimes used for d-e (direct current) level shifting between stages. (You may recall that the voltage gain at any emitter follower is less than one.)

One reason for starting signal-tracing at point A is to check the signal generator output. Another reason is to determine that the input circuit ofthe first amplifier has

not swamped the signal from the signal generator. How­ever, many technicians prefer to start at point D instead of point A when they probe for the signal. In most cases, this procedure is acceptable, but it is still good practice to look at the signal before you start using the test equipment.

Fig. 2 shows the technique of signal injection. Here, the oscilloscope is connected to the output of the last amplifier. The signal generator is first used to inject a signal at point Cto check the third amplifier. If the ampli­fier checks out, the signal generator is next moved to point B, and then finally to point A. When the generator probe is moved past the defective circuit, the signal will no longer be observed on the oscilloscope. For example, if a signal is displayed when the generator probe is at point C, but not when it is at point B, then the middle amplifier is defective.

INPUT

SIGNAL GENERATOR

OUTPUT

OSCILLOSCOPE

® Tracing and Injecting Precautions

A couple of precautions should be taken when you use the techniques of Figs. 1 and 2. For example, in the method of

Fig. 2. Signal injection differs from signal-tracing in that signal injection uses a probe from the signal generator (instead of the oscilloscope) to check for the output signal.

3

4

Fig. 1, you first set the gain of the oscilloscope so that you can view the waveform of the signal at point A. As you move the oscilloscope to point B, the amount of signal strength becomes much larger, and it continues to in­crease with each stage of amplification. Thus the higher­amplitude signals could overdrive the oscilloscope. Also, the amplitude could be so high that you won't recognize the signal.

The same thing can happen with the signal injection method shown in Fig. 2. The signal is first injected at point C and observed on the oscilloscope. Then, if the signal is moved to B, the gain is much greater for the signal and the oscilloscope may be overdriven. Not only that, but with this technique, it's also possible to over­drive the output amplifier. The generator signal at point C may be quite large in order to drive the last amplifier (which is presumably a power amplifier stage). Moving the generator to B (which is a voltage amplifier stage) may cause a very large signal to be injected in to an am pli­fier that can only stand a low peak-to-peak input signal.

When an op amp (operational amplifier) is in a sys­tem under test, an excessive amplitude can also produce a condition known as latch-up. When the op amp is latched, its output will no longer change with a change in input. In other words, the amplifier output is a d-e voltage. (Special circuits can be used to avoid latching, but they're not always available. In some op amps such as the 741, the special circuits are part of the amplifier design.)

Some experienced technicians avoid overdriving amplifiers and latching op amps by always probing to­ward a lower gain. So, with the technique of Fig. 1, they prefer to move the oscilloscope probe from point D toward point A. Likewise, in the injection method of Fig. 2, they move the generator probe from point A toward point C.

The disadvantage of probing toward a lower gain is that you have to start tracing at a point where there is sure to be no output. For example, suppose you start trac­ing at point Din Fig. 1, and the center amplifier is defec­tive (no gain). The scope, of course, will show no output. At this time, you don't know if this condition is normal, or if the signal generator has, for some reason, stopped send­ing a signal.

As a technician, you'll have to determine which tech­nique you like best, and use it. However, you must al­ways be aware of the disadvantages, as well as the advantages, of the technique you're using.

Troubleshooting in logic circuits is easier, because there's usually no gain in the signal from stage to stage. Therefore, there's no danger of overdriving or latching a stage. However, be careful when using tracing or injec­tion techniques. Remember that linear amplifiers are often used in the input or output sections of digital and microprocessor systems.

~ Waveform Tests

You've already learned about square- and triangular­wave tests. These tests are especially important when you troubleshoot pulse and logic circuitry. Any distortion of the pulse or clock signals caused by poor amplifier response can result in a shutdown of the sys­tem. Therefore, you must learn to quickly check an ampli­fier's response to various waveforms and frequencies. Since these tests are so important in pulse and logic test­ing, they'll be briefly reviewed here.

Fig. 3 shows the basic square-wave test. In (a), a square-wave generator, or a function generator with a square-wave output, is delivering a signal for the amplifi­er under test. The output waveform of the amplifier is obtained on the oscilloscope.

SIGNAL GENERATOR

n.ru

OSCILLOSCOPE

(a)

Fig. 3. When an amplifier is operating normally (a), the square wave introduced by the signal generator will be increased in amplitude without distortion. Output waveforms indicating poor high-frequency and low­frequency responses are shown in (b) and (c), respectively.

(b) (c)

5

(a)

(b)

Fig. 4. The output of a normal amplifier appears as shown in (a). The peaks of the triangular waveforms will be clipped, as in (b), after passing through an overdriven amplifier.

(a)

(b)

Fig. 5. A typical tone-burst signal (a) appears as shown in (b) after passing through an amplifier with poor low- and high­frequency responses. The tone­burst test is rarely used in digital circuitry.

6

There are two major amplifier problems that you can observe with this test. If the vertical sides of the square wave are distorted, as shown in (b), then the problem is a poor high-frequency response. On the other hand, if the top and bottom of the square wave are distorted, as shown in (c), the problem is poor low-frequency response.

One precaution to observe with the square-wave test is to be sure that the amplifier is not overdriven. An overdriven amplifier will produce a square-wave output, even though a large amount of distortion may be occur­ring in the amplifier itself.

{§) Triangular and Tone-Burst Tests

When you suspect the results ofthe square-wave test, you can use the triangular test wave. The output of a properly operating amplifier will appear on the oscilloscope as shown in Fig. 4(a). An overdriven amplifier will produce a waveform similar to that shown in (b). If you use a function generator, you can switch back and forth be­tween square and triangular waveforms, and perform both tests quite easily with a single signal source.

The outputs of another type of waveform test is shown in Fig. 5. This test is called the tone-burst test. In this test, as shown in (a), a burst of sine-wave signals is applied to the amplifier. (The burst may be as short as 10 cycles with a relatively long time between each burst.) Fig. 5 (b) shows a typical burst distortion in an amplifier that has a very poor high-frequency and low-frequency response.

These methods of troubleshooting have been re­viewed for your convenience. They have been discussed in detail in earlier lessons, and you should review them peri­odically until you're very familiar with the troubleshoot­ing technique of waveform analysis.

Check Your Learning 1

Let's pause for a few minutes to review some key points on rapid troubleshooting and waveform tests. Answer each of the follow­ing questions to the best of your ability, then, compare your answers with ours. If any of your answers differ from ours, care­fully review the appropriate article(s) to find out why.

1. When you use the signal-tracing method of troubleshooting a system, you should NOT expect to find a voltage gain in ___ stages.

1.

2. You have inadvertently overdriven an op amp with a high- 2. amplitude signal, thus causing the output to become a d-e voltage. Assuming the op amp has NOT been destroyed, the condition can be called an example of __ _

3. When you use the square-wave test, any distortion in the sides 3. of the output waveform indicates a problem with (high-frequency, low-frequency) response.

4. Which one of the following waveforms indicates clipping in an 4. amplifier?

B. \-; C. rv D . .__

Answers to Check Your Learning 1

v ·6!.:1 'g ·pv ~ ·o ·v £ "6!.:1 'p ·~J\f £ ·~J\f

f\ouanbaJ!-46!4 ·£ ~ ·~J\f

dn-4o~e1 ·~

J9M0110!-J9n!W9 . ~

Your Answers

7

Courtesy of Global Specialties Corporation

Fig. 6. Logic pulsers are designed to inject pulses into a circuit. Note that a logic pulser can be switched to either TTL (transistor-transistor logic) or CMOS (complementary metal­oxide semiconductor).

8

Use of Pulsers and Probes

® Logic Pulsers

The logic, or digital, pulser, Fig. 6, used for troubleshoot­ing pulse and logic circuits, is actually a signal injector. (It takes the place of a signal generator for the tests shown in Figs. 1 and 2.)

A pulser generally has two modes of operation -single pulse and pulse waveform. To obtain a single pulse, the push-button switch on the pulser is held down only for a moment. Each time the button is pushed, the pulser delivers a logic 1 to the metal tip. The width of the output pulse doesn't usually depend upon the length of time the pulser switch is depressed. The reason is that a one-shot multivibrator is used to generate the pulse. The switch simply delivers a momentary trigger pulse to the input of that one-shot circuit.

Single pulses are very useful for single-stepping. This troubleshooting technique permits the tech­nician to operate the circuit without a clock signal. (In some systems, the clock signal may be operating at a frequency of several megahertz; thus, it may be difficult to find the cause of a malfunction. However, by operating the circuit by injecting one pulse at a time, it's possible to make a number of measurements for each pulse.) Single­stepping is used in microprocessor and computer testing. It's especially convenient for troubleshooting counters.

To obtain a continuous pulse waveform, the switch on the pulser must be held down for a given period of time -usually from 2 to 3 sec (seconds). When the pulser is

(

used in this mode, it delivers a pulse waveform that takes the place of a clock waveform. Continuous pulses are useful for substituting a clock signal in a nonoperating logic system.

Before using a logic pulser in any system, you must make sure that it can produce a rise time and a decay time within the specifications demanded by the system you're testing. In fact, the following advice applies to all test equipment that you use: Know your test equipment! Become familiar with its capabilities and its limita­tions!

{/ Basic Pulser Tests

Fig. 7 shows a block diagram of a basic countdown cir­cuit. The circuit is often called a divide-by-16 system. The clock-signal frequency is divided by 2 at the first flip-flop, divided by 2 again at the second flip-flop, divided by 2 again at the third flip-flop, and finally divided by two again at the fourth flip-flop.

f(16

As an example of pulser (signal injection) testing, let's assume that the countdown circuit has no output signal, and you must locate the defective stage. You use the pulser to inject the signal at the positions shown in Fig. 7, starting with position A. If there is an output signal when the pulser is applied at B, but not at C, it follows that the second flip-flop, FFz, is not operating. (Note the similarity between this test and the one shown in Fig. 2.)

Fig. 7. In locating the defective flip-flop (FF) in a divide-by-16 circuit, inject the first pulse at position A to check out flip-flop 4; then proceed in succession to position 0.

9

Fig. 8. In this block diagram, a sine wave is shaped and amplified, and then used as a clock signal. (Note the symbol for a connector to the left of the shaper block.) In pulser testing of the system, the procedure is to test the logic system, then the amplifier, and finally the shaper.

Fig. 9. Waveform A is the signal on point A in Fig. 8, and waveform B, the signal from the logic pulser. Shaded areas represent out-of-phase signals that can cause a burnout .

10

As another example of pulser testing, consider the system of Fig. 8, which is used in digital clocks and other digital systems that operate with a sine-wave input. Again, assume that there's no output from the logic system.

A

>>- SHAPER AMPLIFIER

~....-....._ LOGIC SYSTEM

The first step is to disconnect the input sine wave. This must be done to avoid a serious burnout. (This point will be taken up in more detail after the test is discussed.) After the sine-wave signal has been discon­nected, the logic pulser is applied to test point A in Fig. 8. If the logic circuit is operating properly, there will be an output. In that case, the circuit fault must be in the first two stages. Of course, ifthere is no output when the pulser injects a signal at A, the system fault must be in the logic circuit.

Fig. 9 shows what can happen if you apply a pulser signal to point A in the system shown in Fig. 8 when there's a clock signal present. Note that signals A and B are 180° (degrees) out of phase. During the shaded period,

A

8

(

the circuit clock is at logic Ievell and the pulser output is at logic 0. Any time a logic 1 and a logic 0 are at the same point in a circuit, the result is a short circuit that is sure to damage either the circuit or the test equipment.

@ Logic Probes

The logic probe shown in Fig. 10 is a convenient device for troubleshooting pulse and logic circuits. The probe is actually a signal tracer. (For troubleshooting communi­cations receivers, there's actually a test instrument called a signal tracer. It's used to follow the signal from the antenna to the speaker. However, for most electronic analog (nondigital) systems, an oscilloscope, such as that shown in Fig. 1, is used for tracing the signal.) Of course, if a logic probe is not available, an oscilloscope can be used to trace digital circuits. The disadvantage of using a scope is that it is not as convenient as a probe.

Commercial logic probes usually have the following modes of operation: display of logic 1; display of logic 0; display of a clock or a continuous pulse condition; and display of a glitch condition. Two or more modes of opera­tions may be combined in one probe. An LED (light­emitting diode) is generally used as an indicator for each of the modes of operation.

For both the logic pulser and the logic probe, there's often a method of switching the internal instrument cir­cuitry so that it will be useful for troubleshooting in dif­ferent families oflogic. The most encountered families are TTL (transistor-transistor logic), CMOS (complementary metal-oxide semiconductor), and ECL (emitter-coupled logic). An important difference between these families is that their logic levels are not the same; therefore, the logic probe must be switched when you change from one sys­tem to another.

A glitch is an unwanted voltage spike. It's char­acterized by having a very short duration and a very short rise time. In a logic system, a glitch can cause a circuit to operate at a time when no operation is supposed to occur. In most cases, it's difficult, if not impossible, to

+

TTL ECL

~ PULSE

0 LEVEL

0

Fig. 10. A typical logic probe can be switched for troubleshooting ECL, TTL, or CMOS logic families. The pulse LED indicates a pulse or glitch; the level LED indicates logic level.

11

Fig. 11. The LEOs in the probes represented in (a) are shown in the operating state by the solid dots. As shown by the pulse LED in (b), an output pulse signal is generated when the switch is in logic 1 position, but signals are not generated with the switch in logic 0 position (c).

12

see a glitch on an oscilloscope display. If the bandwidth of the oscilloscope is less the 30 MHz (megahertz), the oscil­loscope will not show the glitches on the screen.

One type oflogic probe catches a glitch by delivering it to a one-shot multivibrator. The multivibrator then produces an output pulse that is capable of operating a flip-flop. The flip-flop latches to a high condition and holds the glitch indicator of the probe in the on condition.

@ Use of Logic Probes

Fig. ll(a) shows how a probe is used to check an enable circuit. The probe has two LED displays. The one nearest the probe tip indicates either a logic 1 or a logic 0. The other one indicates the presence of a clock or recurrent pulse. It can also show the presence of a glitch.

The conditions for enable operation are shown in Figs. 11(b) and 11(c). If the switch is in the logic 1 position, as in (b), the signal passes through the AND gate. When

LOGIC 0 LOGIC 1 PULSE

(a)

LOGIC 1

LOGICO I (b)

LOGIC 1 ! LOGIC 0

(c)

the switch is in the logic 0 position, as in (c), the signal can't pass through the AND gate. (Recall that all inputs to an AND gate must be logic 1 to produce a logic 1 output.)

The following example problems will show you how the logic probe can be used to find defective logic stages.

Problem 1. According to the logic probe, is the circuit of Fig. 12 operating properly?

Solution. The probe shows a logic 1 at the output, but it should show that a recurrent pulse signal is present. Therefore, the circuit is not operating properly. Ans.

Problem 2. Since the probe in Fig. 12 shows that the re­quired output signal is not present, what is the next logical step in troubleshooting?

Solution. Measure the input signal to the AND gate. If there's no signal, go to the source ofthe signal. You must locate the point where the signal is lost. Ans.

Problem 3. Which one of the flip-flops in Fig. 13 is not operating properly?

Solution. The flip-flop in Fig. 13(a) is not operating proper­ly, since the probe shows a logic 0 at both Q and Q. Ans.

(Recall that a properly operating R-S flip-flop has only a set (Q = 1; Q = 0) or a reset (Q = 0; Q = 1) state.)

1J@ Pulsers Versus Signal Generators

In reality, nothing can be done with pulsers that can't also be done with a signal generator or function genera­tor. Use of the pulser, however, has some advantages.

When you're injecting a signal in a logic system, it's very important to maintain the amplitude of the signal generator waveform. Specifically, don't inject a pulse or square-wave signal that has an amplitude greater than the power-supply voltage. So, whenever you set up a signal generator or function generator to operate a logic system, you must be very careful to adjust the amplitude correctly before applying the signal.

When you're using a pulser, the amplitude ofthe sig­nal is automatically adjusted with the type oflogic family that you're troubleshooting. Since the pulser gets its power-supply voltage from the same supply used for the circuit under test, it's unlikely that the pulser can over­drive the system.

rt.I1.J1_

Fig. 12. Circuit diagram for example problems 1 and 2. Note that the logic LED of the probe is in operation.

(a)

(b)

Fig. 13. These are the flip-flop diagrams for example problem 3.

13

14

A second advantage of using the pulser is that you can use the push button to obtain one pulse at a time, or, by holding the push button down, you can get a continu­ous string of pulses. (With a signal generator or function generator, the output is always a continuous string of pulses or square waves.) As mentioned earlier, the single pulses can be used to single-step a logic circuit. That allows you to test each step, one at a time.

The following example may help you with the mean­ing of single-stepping. Suppose a certain memory has 16 individual numbers stored, and you want to check and make sure that each of the numbers stored is correct. In normal operation, a clock signal is applied which auto­matically sequences through the 16 numbers. The rate of sequencing can easily be 2 million numbers per second! Obviously, you can't check and be sure each number is correct when all those numbers are flashing by at that rate. By using a logic pulser, you can apply one pulse at a time, and therefore, sequence through the numbers one at a time. For each position, you can then test and make sure each number is correct. Single-stepping is an impor­tant type of troubleshooting procedure used in mi­croprocessors and logic systems. One advantage of a signal generator or a function generator is that the fre­quency is adjustable. Another advantage is that the out­put frequency is very stable when compared to the fre­quency of a logic pulser.

1J1J Probes Versus VOMs

When probes are used to display a logic level of either 1 or 0, they do so by the use of a single LED. When the probe is touching a logic 1, the diode is on (lit). When the probe is touching a logic 0, the diode is off (unlit).

Some systems demand that a logic 1 be 5 V (volts) ±5%(percent). Other systems permit the voltage to be as low as 4.2 V and still be considered a logic 1. Thus, if you're using a VOM (volt-ohm-milliammeter) to deter­mine whether a logic 1 or a logic 0 is present, you would have to make a decision for each measurement. Inother words, you would have to answer the question: Is that

(

enough voltage to be considered a logic 1? On the other hand, when you use a logic probe, you don't need to make a decision.

If the light is on, you can consider the measurement to be a logic 1. Therefore, one advantage of using a probe is that no decision making is needed.

Another advantage is that the logic probe is much more convenient to use than a VOM, because the indica­tor is in the probe itself. With a VOM, every time you make a measurement you have to look up to the VOM scale to see if the necessary voltage is present. With a logic probe, the light at the end of the probe tells you immediately whether you have a logic 1 or a logic 0.

In some cases, however, it's necessary to know the exact value of the logic 1 voltage for the system. If the manufacturer of a logic system specifies that the voltage must be 5 V ± 0.5%, you can't use a logic probe to deter­mine whether the correct voltage is being supplied. For small voltage measurements such as these, you need an accurate digital voltmeter. (Also, since the power-supply output voltage is probably adjustable, you need the volt­meter to determine when the adjustment is correct.)

The overall conclusion is that the logic pulser and the logic probe do not replace the test equipment that you're familiar with. Pulsers and probes are used only to provide a very fast troubleshooting procedure for isolating malfunctioning com­ponents in a system.

15

Check Your Learning 2

Let's take another short break to review some key points on the use of pulsers and probes. As before, complete this short quiz before comparing your answers with ours. Be sure to review the appropriate article(s) for any of your answers that differ from ours.

1. When you use a logic pulser for troubleshooting a system, 1. you're using the equivalent of (a signal generator, an oscillo-scope) __ _

2. When you're troubleshooting a counter by operating one 2. pulse at a time, what procedure are you using?

3. A logic probe is an example of 3. A. an oscilloscope. B. a pulser. C. a tracer.

4. What is the usual name given to an undesirable short-duration 4. pulse?

5. True or False? The logic probe in the following diagram indi­cates that AND gate B is NOT operating properly.

16

o--0

5.

Your Answers

True False (Circle your choice.)

(

LI

1. a signal generator Art. 6, Figs. 1, 2

2. Single-stepping Art. 6 3. C. a tracer. Art. 8 4. Glitch Art. 8

5. False Art. 9, Fig. 12 (There is no way of know­ing which of the two gates is defective from the mea­surement shown. You must probe the lead between gates A ~nd 8 to determine the defective gate.)

c; 6u1uJea1 JnOA >t:>a4:> 01 sJaMsuv

Fig. 14. Most logic probes and pulsers obtain their operating voltages by the appropriate lead connections to the circuit being tested.

18

Troubleshooting Aids

lJ~ Probe and Pulser Connections

Logic probes and logic pulsers do not have their own power supply. Instead, they use the power supply of the system being tested. Fig. 14 shows how a typical logic probe is connected.

+

When troubleshooting any electronic system, it's a good idea to start by checking the power supply. Of course, when the logic probe is connected as in Fig. 14, you can tell immediately the state of the power supply. The

probe won't operate if there's no voltage at the B+ termi­nal. (The probe can also be used to check the common bus; however, this is more of a probe check than a circuit check.)

When you use the probe, be careful not to short­circuit between the pins on ICs (integrated circuits). Fig. 15 shows the right and wrong ways to use the probe.

PROBE TIP

RIGHT WRONG

Another important point to remember in checking an IC is to probe the leads (pins) of the IC rather than the printed-circuit leads. For example, Fig. 16 shows two ICs connected by a foil conductor on a printed-circuit board. The signal is generated by ICx and passed to /Cy.

The proper way to trace the signal is to probe at the IC lead as shown by probe B. This eliminates the possibility of the signal being lost at a cold solder joint or IC socket connector. If the signal is NOT present at probe position B, the next step is to move to probe position A to see if the signal is on the conductor. If the signal is not on the conductor, then the next step would be to probe at the output pin of ICxto see if the signal is leaving the source.

Pulsers are handled in the same way as logic probes. Also, they use the power supply of the system being test­ed. The connection for a pulser is the same as that for the logic probe shown in Fig. 14.

Fig. 15. The right way to use a probe or pulser in checking an IC is to place the very end of the tip of the instrument directly on the pin as close as possible to the IC housing. By using the probe in the wrong way, that is, by using the side of the pin tip, you can easily short-circuit on an adjacent pin.

Fig. 16. Probing a circuit as shown by probe A will indicate only whether a signal from /Cx is carried by the conductor.

19

Fig. 17. These circuits for simple logic probes are suitable for hobbyists or for emergencies when better probes are not available. However, for professional troubleshooting, these probes are not adequate.

20

l.J~ Simple Logic Probes

A very simple logic probe can be fabricated by us­ing an LED and a 470-fl (ohm) resistor. Fig. 17(a) shows the circuitry for this simple probe. One end of the probe is connected to common (ground) and the other end (which comes from the anode ofthe LED) is used to probe for logic 1levels. The 4 70-fl resistor is needed to limit the current through the diode. However, the value of the resis­tor is not critical- resistor manufacturers recommend values from 330 fl to 4 70 fl.

~., 470 n

s """' l --- -PROBE (a)

+

...........

PROBE

In operation, when the probe is touched to a logic 1, a complete circuit goes through the LED, and the LED is turned on. If the probe is touched to a logic 0, no current passes through the system, and the LED remains off. The obvious disadvantage of this simple probe is that there's no difference between a logic 0 and an open circuit, since the LED is off for either condition.

Fig. 17(b) shows how the probe can be modified to look for logic 0 levels. In this modification, the anode of the probe is permanently connected to the positive volt­age in the circuit. The other end of the probe is used to probe for logic 0 levels. If the probe touches a logic 0 (or common), the circuit is complete, and the LED is on. If the pro be touches a logic 1, the circuit has no current passing through, and the LED is off.

Returning again to the probe of Fig. 17(a), if this probe is touched to a place where there's a square wave or a pulse, the LED will be on. However, the LED will not be as bright as when it is touched to a logic 1. The reason is that the LED glows with an average brightness, which is some point between maximum (when touched to a logic 1) and minimum (when touched to a logic 0). Thus, with a little experience, a technician can spot a pulse condition.

1]~ Inverter Circuit Probes

A logic probe with two LEDs and two inverters in the circuit is shown in Fig. 18. The LEDs indicate the logic level, either a 1 or a 0, and the inverters determine the

+ +

// y

PROBE

condition of the LEDs. In explaining the operation of this logic probe, keep this important fact in mind- an invert­er is simply a circuit that reverses the level of the logic input. Thus, a logic 1 input to the inverter produces a logic 0 output, and conversely, a logic 0 input produces a logic 1 output.

Suppose, for example, that the probe in Fig. 18 is touching a logic 1level in a circuit. The first inverter will convert this signal to a logic 0, and the LED marked X will be on. The logic 0 input to the second inverter will be converted to a logic 1, so the LED marked Y will be off.

Fig. 18. By using two LEDs and two inverters in a logic probe circuit, either a logic 1 or a logic 0 will be recognized by one of the LEDs.

21

22

If the probe in Fig. 18 is touched to a logic 0, the first inverter will have a logic 1 output. The LED marked X will have a logic 1 at both ends of its circuit, and no current can flow through it. So, it will not be lit. The second inverter will convert the logic 1 to a logic 0. That means the LED marked Y will be connected between a logic 1 and a logic 0. With a voltage across the LED, it will be lit. Thus, when the probe is touching a logic 1, LED X will be on; when the probe is touching a logic 0, LED Y will be on.

The probe in Fig. 18 solves the problem of having to reconnect the probe to find the logic 1 and logic 0, but it does not solve the problem of getting an erroneous read­ing with an open circuit. For example, when the probe is not touching anything, one ofthe two LEDs will be on. It isn't a simple matter to tell which one is on, because stray signals may be delivering a logic 1 to the first inverter. On the other hand, the first inverter may be looking at an open which it considers to be a logic 0.

Commercially available probes have an additional circuit that turns both LEDs off when the probe is at an open circuit.

1J@ Simple Logic Pulser

Fig. 19 shows a block diagram of an astable circuit with a pulsed output waveform. This simple circuit can be used as a pulser if a commercial pulser is not available. By connecting this simple pulser to the power supply of the circuit being tested, you're assured that the output wave­form will not have an amplitude that exceeds the safe limit for that circuit. In other words, ifthe circuit's power supply is 5 V, the amplitude of the output waveform on pin 3 can't exceed 5 V.

The disadvantage of the circuit in Fig. 19 is that it can be used only for low frequencies. It can't generate fre­quencies in the megahertz range, which are often used as the clock frequency in a microprocessor system.

+

8 4 n.nn...

6 3

2

5551C

7 5

GND

I - -

1]@ Troubleshooting Equipment

To be a good technician, you must avail yourself of all the equipment necessary to perform your job. You can't overhaul a jet engine with a screwdriver, so you can't expect to properly troubleshoot and repair a com­plex digital pulse circuit without having the proper tools and instruments available. Therefore, in the remaining articles of this lesson, we'll describe some of the more important instruments and tools that you should have available for troubleshooting.

A convenient troubleshooting tool is the IC lead ex­tender, shown in Fig. 20. This device fits over the IC being tested in such a manner that the signal on each pin of the IC is transmitted to the external pins of the device. An obvious advantage of the IC lead extender is that clip-on leads from the probe or pulser are easily attached to the external pins.

Fig. 19. When this astable multivibrator is used as a logic pulser, pin 8 and GND of the 555 IC are connected, respectively, to the positive voltage and the common of the system being tested. While adequate for occasional troubleshooting, this circuit is not recommended as a replacement for commercially available pulser units.

Fig. 20. Technicians often refer to this IC lead extender as a glomper.

23

HOOK

Fig. 21. The obvious advantage of a clip-on lead is that it frees the hands of the technician to make adjustments on the test instruments or on the circuit being tested.

Courtesy of Hewlett-Packard Company

Fig. 22. This type of clip-on IC tester permits the troubleshooter to see simultaneously all the logic levels on the IC pins.

24

An example of a clip-on lead from a probe or pulser is shown in Fig. 21. It is spring-loaded so that when the end ofthe lead is squeezed, the small wire hook extends. This hook is looped around a pin on the lead extender, so when the end ofthe lead is released (and the hook retracts), the lead remains firmly fixed to the pin being tested.

Another type ofiC tester, referred to as a logic moni­tor, is shown in Fig. 22. In this device, LEDs are located in a convenient position so that the logic level of each lead on the IC can be seen.

Removing ICs

One mistake technicians often make when troubleshoot­ing pulse and digital circuits is trying to remove an IC from a socket by pulling it out with their fingers. This procedure can result in broken (or badly bent) pins. The proper procedure for removing an IC is to simulta­neously pry it loose at both ends; of course, the best proce­dure is to use one of the commercially available IC extractors.

A simple tool like the one shown in Fig. 23 can be used to remove small ICs from their sockets. The tool looks like a tiny crowbar, and that's the way it's used. The flat end is placed under the IC end and it is pried loose from the ends. Remember that two of these tools must be used simultaneously, and the IC must be pried from both ends at the same time to prevent damage to the pins.

You must be very careful when you use the type of tool in Fig. 23 to remove large ICs (40 to 64 pins). With a great deal of care, twoofthesetoolscan be used, butyoumustbe careful to exert a slow, steady pressure to free the IC. Large ICs will crack and break if pried from one end. However, in truly professional troubleshooting, only a commercially available extractor should be used for removing the large ICs.

(

1]@3 Logic Scopes

When troubleshooting digital and pulse circuits, the technician is often interested only in the presence of a logic 1 (maximum amplitude) or a logic 0 (minimum or zero-volt levels) signal. When this is the case, the techni­cian may use a device such as the logic analyzer shown in Fig. 24. Although the instrument uses two rows ofLEDs instead of a CRT (cathode-ray tube), it's often referred to as a logic scope or a digital scope. In operation, a logic 1 is indicated by an on LED. A pulse signal is shown by an LED on time that is twice the off time.

One advantage of the logic analyzer is that it is less expensive than a standard oscilloscope. Another advan­tage1 is that, like the logic probe, the logic analyzer dis­plays only the information of importance to the techni­cian; that is, it displays only the presence or absence of a pulse, a logic 1, or a logic 0.

A disadvantage of the logic analyzer is that its LEDs are hard to read, so the technician must take some time to learn to use this instrument properly.

Fig. 23. If an IC extractor is not readily available, small ICs can be removed by using two of these crowbarlike tools.

Fig. 24. The logic analyzer shown here displays only the relationship between highs and lows in a waveform.

25

Fig. 25. Here you see a typical waveform display, or timing diagram, as it would appear on a multitrace oscilloscope.

Fig. 26. This multitrace oscilloscope is a 48-input channel logic analyzer. Sampling rates are from direct current to 50 MHz.

26

1]@ Multitrace Oscilloscopes

Multitrace oscilloscopes are used for observing combina­tions of waveforms, such as those shown in Fig. 25, which are often referred to as timing diagrams. The type of oscilloscope used for displaying these diagrams is shown in Fig. 26.

Timing diagrams are important because they show the relationship between the signals at various points in the system. As a simple example, assume that several points in the system all go to a logic 1 (pulse) at exactly the same instant of time. Since the multitrace oscilloscope can display all of the pulses simultaneously, the techni­cian can thus determine if the circuit is operating properly.

Multitrace oscilloscopes are very expensive and they are generally used only for bench testing and experiment­ing with prototypes. As with the logic scope, the techni­cian has to spend a certain amount of time learning to use the instrument properly.

~@ Troubleshooting Industrial Electronic Systems

From studying the troubleshooting procedures discussed so far, you could easily get the idea that all ofthe trouble­shooting done in industrial systems involves looking for

I defective components in pulse and digital circuits. How-ever, always keep in mind that an industrial electronic system usually involves complex machinery and often employs high-level technology. Therefore, the digital or pulse circuitry represents only one small part of the total system. Look at the block diagram in Fig. 27 to see what an industrial electronic system involves. (The type of re­sistance welder used in this welding system was dis­cussed in an earlier lesson.) The actual beginning of the system is the power transformer that delivers the alter­nating current for operating the equipment. A circuit breaker of some type is next in line, to protect the power transformer against an overload that could destroy it. The power to the actual welding transformer is con trolled by an SCR (silicon-controlled rectifier). The amount of time that the SCR delivers power to the welding tips is controlled by the synchronous control circuitry and the welding timer.

SYNCHRONOUS TIMER CONTROL

WELD I NG TIPS

POWER ---+

CIRCUIT r---+ SCR I

TRANSFORMER BREAKER NETWORK

The digital troubleshooting techniques discussed in this lesson are applicable only to the two sections marked timer and synchronous control. The SCR is, of course, a thyristor, and the rest of the system is primarily a high­power, high-current operation. As a technician, you must be able to test the overall system - not just the pulse circuitry.

Suppose, for example, that the welder does not deliver current to the welding tips. As with servicing any other electronic equipment, the first place to start looking for trouble is the power supply. In this example, the

Fig. 27. Shown here is the block diagram of a typical electronically controlled resistance welder.

27

28

power transformer delivers a-c (alternating-current) pow- ( er to the SCR system through a circuit breaker or circuit disconnect. Usually, this type of circuit breaker has a visual indicator when it has been tripped. So an expe­rienced technician would look to see if the system has been overloaded in some way that causes the circuit breaker to trip out. (If there is no visual indicator on the circuit breaker, it would still be a good idea to reset the circuit breaker to make sure that everything is OK at that point.)

The next step in troubleshooting the system is to see that a-c power is actually being delivered to the SCR circuit. If the SCR system is receiving a-c power, then the next step is to check that the timer is turning the SCR on, to deliver output power to the welding transformer.

This step-by-step procedure is nothing more than the signal-tracing procedure described at the beginning of this lesson. Even though the signal is an a-c voltage, it can be followed with an oscilloscope or other measuring equipment to determine where the defective component is.

Now, assume that all components leading to the timer and synchronous control are functioning properly. Since the timer and synchronous control in this system contain logic and pulse circuitry, your next steps would be to locate the problem by following troubleshooting proce­dures for that type circuitry.

~Check Your Learning 3

Take a few minutes to complete this short quiz on troubleshoot­ing aids before going on to the Self-Test. Don't shortchange yourself by failing to review the appropriate article(s) for any of your answers that differ from ours.

1. When you use a logic probe or pulser to troubleshoot an electronic system, what should you check first?

Question 2 is based on the following diagram.

'' PROBE

1.

2. When the LED in the diagram is off, you have located 2. with the probe. A. a logic 0 B. a logic 1 C. a common ground

3. A 555 IC in a logic pulser is connected so as to obtain a 3. (multivibrator, phase-locked loop) circuit.

4. How do you recognize a pulse signal when you troubleshoot a 4. system with a logic scope?

Answers to Check Your Learning 3

9 ~ 'lJ'v' '9W!l HO 9lH 90!Ml S! lBlll 9W!l UO 031 e Aq IBU6!S as1nd e MOljS sapO!P 941 ·v 9 ~ 'lJ'v' JOlBJq!A!llnW '£

(q)H ·6!.:1 '£~ 'lJ'v' ~ :>!601 e '8 ·c

G ~ 'lJ'v' Alddns J9M0d · ~

Your Answers

29

30

Self-Test

Read each one of the following statements carefully and be sure you understand II. Each statement is followed by four words or phrases, only one of which Is correct. Draw an X through the letter that indicates your answer.

1. You're using the signal-tracing technique in troubleshooting the system shown here.

GENERATOR

Assuming that amplifier Cis defective, you'll receive an oscilloscope signal

A. at point 2, but not at point 3. C. at point 4, but not at point 3. B. at point 3, but not at point 4. D. at point 5, but not at point 2.

2. One advantage of the triangular-wave test over the square-wave test is that the triangular-wave test can better detect

A. clipping. C. poor low-frequency response. B. ringing. D. poor high-frequency response.

3. Which one of the following devices will you find most convenient for trouble­shooting a system such as that shown in Fig. 7?

A. Function generator C. Logic pulser B. Oscilloscope D. DVM

4. You'll probably use ___ as a signal tracer in analog systems.

A. a function generator C. a logic pulser B. a logic probe D. an oscilloscope

5. You're using a logic probe to test an R-S flip-flop. If the flip-flop is operating properly in the reset mode, the probe will show

A. logic 0 at both outputs. C. logic 0 at output Q.

B. logic 1 at both outputs. D. logic 1 at output Q.

~ 6. One advantage of using a pulser instead of a function generator for signal

injection is that the pulser provides

A. an automatic adjustment of amplitude. B. a greater amount of signal power. C. a triangular waveform.

1 D. a wider frequency range.

7. A logic probe is rarely used for

A. measuring logic 1 voltage. C. tracing a clock signal. B. locating a glitch. D. troubleshooting a TTL device.

Question 8 is based on the following illustration.

8. Assume that one of the ICs in the illustration is defective. IC 1030 is receiving an input signal, but there's no output signal from IC 6849A. By probing for the signal at , you can immediately determine that IC 1030 is good or defective.

A. point 1 C. point 3 B. point 2 D. point 4

Question 9 is based on the following schematic.

+V

c D

PROBE

B

9. Which one of the following will occur with a logic high on the probe?

A. Point 8 will be low, and C will turn on. B. Point 8 will be low, and 0 will turn on. C. Point A will be high, and 0 will turn on. D. Point A will be low, and C will turn on.

31

10. You'll find a multitrace scope most useful for

A. observing timing diagrams. B. probing amplifier systems. C. rapid signal injection. D. single-stepping clocking devices.

Please turn the page for the self-test answers.

32

34