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The course starts from a beginner level, assuming no prior knowledge in these areas. The format of the book is that of a laboratory manual, which can be used as a stand-alone crash-course for a self-motivated student, or be directly adopted as a course textbook for an elective in a college or university context. The book includes various fun lab activities that increase in difficulty, and enough theory and practical advice to help complement the activities with understanding.
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This book uses modern televisions troubleshooting; however, all circuits and components of consumer electronics are very similar. This book describes very specifically the functions and purposes of various types of circuitry, electronic components, their functions and the malfunctions of televisions when they are faulty. The book includes everything that you will need to know for beginning television, computers and other electronic repair.
This book contains actual symptom, troubleshooting, diagnosis and repair procedures for all television problems. All essential knowledge, skills and procedures are in an articulated fashion, so that, no time will be wasted discerning the jest of each section. All sections are in the table of contents and in bold face for quick reference or study guide. Archaic but clear representation of conductors that cross paths but are not electrically connected to each other. Shielded cables require additional symbology along with the conductors.
Figure shows examples of shielded wire, often used to indicate the use of coaxial cable in an electronic circuit. Coaxial cable contains a single wire called the center conductor surrounded by a cylindrical, conduit-like conductive shield. An insulating layer, called the dielectric, keeps the two conductive elements isolated from each other. In most coaxial cables, the dielectric material consists of solid or foamed polyethylene. Tip Figure shows a symbol for coaxial cable when the shield con- nects to a chassis ground, such as the metal plate on which an electronic circuit is constructed.
At A, symbol for a coaxial cable with an ungrounded shield. At B, symbol for a coaxial cable with an earth-grounded shield. Symbol for a coaxial cable with a chassis-grounded shield. In some cables, a single shield surrounds two or more conduc- tors. Figure shows the schematic symbol for a two-conductor shielded cable.
This symbol is identical to the one for coaxial cable, except that an extra inner conductor exists. If more than two inner conductors exist, then the number of straight, parallel lines going through the elliptical part of the symbol should equal the number of conductors. For example, if the cable in Fig. Diodes and transistors Figure shows the basic symbol for a semiconductor diode. In this symbol, an arrow and a vertical line indicate parts of the diode, and the horizontal lines to the left and right indicate the leads.
The symbol in Fig. Under normal operat- ing conditions, a rectifier diode conducts when the electrons move FIG. Symbol for a shielded two-conductor cable, in this case with a chassis ground for the shield.
Diodes and transistors 47 FIG. Symbol for a general-purpose semiconductor diode or rectifier. Figure shows the symbols for some specialized diode types. At A, we see a varactor diode, which can act as a variable capacitor when we apply an adjustable DC voltage to it. At B, we see a Zener diode, which can serve as a voltage regulator in a power supply. At C, we see a Gunn diode, which can act as an oscillator or amplifier at microwave radio frequencies.
A silicon-controlled rectifier SCR is, in effect, a semiconductor diode with an extra element and corresponding terminal. Its sche- matic symbol appears in Fig. In the SCR representation, a circle often but not always surrounds the diode symbol, and the control element, called the gate, shows up as a diagonal line that runs out- ward from the tip of the arrow.
Figure shows the schematic symbols for bipolar transistors. The only distinction between the two is the direction of the arrow. In the PNP device, the arrow points into the straight line for the base electrode. In the NPN device, the arrow points outward from the base. Symbol for a silicon-controlled rectifier SCR. Besides the bipolar variety, many other types of transistors exist.
Tip Transistors can be made from various types of semiconductor materials and metal-oxide compounds, but the schematic symbol, all by itself, tells us nothing about the elemental semiconductor material used in manufacture. The symbol merely indicates com- ponent functionality. When you want to create the symbol for a vacuum tube, you should start by drawing a fairly large circle, and then you should add the necessary symbols inside the circle to symbolize the type of tube involved.
Figure shows the schematic symbols for the various types of tube elements commonly used in schematic drawings. Figure shows the schematic symbol for a diode vacuum tube. This two-element device contains an anode also called a plate and a cathode.
Just as with the semiconductor diode, the anode is nor- mally positive with respect to the cathode when the device conducts current. The cathode emits electrons that travel through the vacuum to the anode. A hot-wire filament, something like a miniature low- wattage light bulb, heats the cathode to help drive electrons from it.
In Fig. Symbols for tube elements and characteristics. A: Filament or directly heated cathode. B: Indirectly heated cathode. C: Cold cathode. D: Photocathode. E: Grid. F: Anode plate. G: Deflection plate. H: Beam-forming plates. I: Envelope for vacuum tube.
J: Envelope for gas-filled tube. Tip All tube elements are surrounded by a circle, which represents the tube envelope. Figure shows two versions of a triode vacuum tube, which consists of the same elements as the diode previously discussed, with the addition of a dashed line to indicate the grid.
Can you see it? Look closely FIG. Schematic symbol for a diode vacuum tube with an indirectly heated cathode. Although a filament exists, it is often omitted to reduce clutter in symbols for tubes with indirectly heated cathodes.
Symbols for a triode tube with a directly heated cathode A and an indirectly heated cathode B. The tube at A has a directly heated cathode, in which the filament and the cathode are the very same physical object! We apply the negative cathode voltage directly to the filament wire; no separate cathode exists at all. In this symbol, the fila- ment is inside the cathode, which comprises a metal cylinder running along the central vertical axis of the tube. Tetrode vacuum tubes have two grids. To represent one of them, we need an additional dashed line, as shown in the drawings of Fig.
In the tetrode, the upper grid, closer to the anode, is called the screen. Figure shows symbols for the so-called pentode tube, which has three grids and a total of five elements. In the pentode, the second grid going from the bottom up is the screen, and the third grid just underneath the plate is called the suppressor. In both Figs. Symbols for a tetrode tube with a directly heated cathode A and an indirectly heated cathode B.
Symbols for a pentode tube with a directly heated cathode A and an indirectly heated cathode B. Follow the flow In all the vacuum tube symbols shown here, electrons normally flow from the bottom up.
They come off the cathode, travel through the grid or grids if any , and end up at the plate. In that sort of situation, you can simply remember that the electrons go from the cathode to the plate under normal operating conditions.
Some vacuum tubes consist of two separate, independent sets of electrodes housed in a single envelope. These components are called dual tubes. If the two sets of electrodes are identical, the entire compo- nent is called a dual diode, dual triode, dual tetrode, or dual pentode.
Figure shows the schematic symbol for a dual triode vacuum tube with indirectly heated cathodes. In some older radio and television receivers, tubes with four or five grids were sometimes used. These tubes had six and seven elements, respectively, and were called hexodes and heptodes. These esoteric devices were used mainly for mixing, a process in which two RF signals having different frequencies are combined to get new signals at the sum and difference frequencies.
The schematic symbol for a hexode is shown in Fig. Schematic symbol for a dual triode tube. At A, symbol for a hexode tube. At B, symbol for a heptode tube, also known as a pentagrid converter. Some engineers called the heptode tube a pentagrid converter. Both of these symbols show devices with indirectly heated cathodes. Cells and batteries A cell or battery is often used as a power source for electronic circuits.
A single-cell com- ponent such as this usually has an output of approximately 1. Schematic symbol for a single electrochemical cell. Schematic symbol for a self-contained multicell electrochemical battery. The multicell battery symbol is simply a number of single-cell symbols placed end-to-end without any intervening lines.
If a circuit calls for the use of three individual, discrete single-cell batteries in a series connection, you might draw three cell symbols in series with wire conductor symbols between them Fig. Standard practice calls for polarity signs to go with the symbols for cells or batteries. Unfortunately, some draftspeople neglect this detail.
Logic gates All digital electronic devices employ switches that perform specific logical operations. These switches, called logic gates, can have any- where from one to several inputs and usually a single output. Logic devices have two states, represented by the digits 0 and 1. It reverses, or inverts, the state of the input.
If the input equals 1, then the output equals 0. If the input equals 0, then the output equals 1. Symbol for three single electrochemical cells connected in series to form a battery. Logic gates 55 output equals 0. If any of the inputs equals 1, then the output equals 1. If both, or all, of the inputs equal 1, then the output equals 1.
If any of the inputs equals 0, then the output equals 0. If both, or all, of the inputs equal 0, then the output equals 1. If any of the inputs equals 1, then the output equals 0. If both, or all, of the inputs equal 1, then the output equals 0. If any of the inputs equals 0, then the output equals 1. If the two inputs have the same state either both 1 or both 0 , then the output equals 0.
If the two inputs have different states, then the output equals 1. Figure illustrates the schematic symbols that engineers and tech- nicians use to represent these gates in circuit diagrams.
Appendix A is a comprehensive table of schematic symbols. In addition to the ones already discussed, you will see symbols for jacks and plugs, piezoelectric crystals, lamps, microphones, meters, antennas, and many other electronic compo- nents.
It might, at first thought, seem like a massive chore to memorize all of these symbols, but their usage and correct identification will come to you with practice and with time. Schematic symbols are the fundamental elements of a commu- nication scheme, like the symbols in mathematical expressions or architectural blueprints. Most schematic symbols in electronics are based on the structure of the components or devices they represent.
Schematic symbols often appear in groups, each of which bears some relationship to the others. Minor symbol changes portray variations in internal structure, but all can be easily identified as some type of transistor. The same rule applies to the symbols for diodes, resistors, capacitors, inductors, transformers, meters, lamps, and most other electronic components.
One school feels that you should learn to read diagrams before you learn to draw anything. The other school feels that you should learn to read diagrams as part of the process of drawing them. Certainly, you must start out by learning the basic component symbols.
Then you should try to read as many schematic drawings as you can find. When this business starts to grow boring, you can draw your own simple diagrams. I recommend that you devote half of your study time to reading and the other half to drawing. Getting started This chapter deals with reading and drawing diagrams of some simple electronic circuits shown in pictorial and schematic form. Using this method, you can actually see the physical layout for a device, and then see how the schematic representation derives from it.
Some com- mercial schematics are produced in this manner. However, in most instances, a circuit is designed schematically first, and then built and tested from the schematic. When these changes are made to the test circuit, the results are noted, and the schematic is changed accord- ingly. In the end, the finished and corrected schematic is a product of design theory, actual testing, and modification.
Figure shows a simple circuit that everyone has used at one time or another. The device consists of a single electro- chemical cell and an electric light bulb.
This pictorial representation also shows the conductors, which attach to the light bulb and the battery. The conductors provide a current path between the battery and the light bulb. Follow the flow In the circuit of Fig. But some physicists will tell you that the cur- rent actually goes from the positive cell pole to the negative cell pole. In order to draw a schematic diagram of the flashlight drawn in Fig.
These represent the electrochemical cell, the conductors, and the bulb, as shown in Fig. Pictorial drawing of a flashlight circuit using a single electrochemical cell or single-cell battery , some wire, and an incandescent bulb. Schematic symbols for an electrochemical cell A , an electrical conductor such as wire B , and an incandescent bulb C. Start by drawing the cell symbol. Next comes the symbol for the light bulb, which you can draw at any point near the cell. Using this example, you should try to make the schematic symbols fall in line with the way the pictorial diagram appears.
This layout places the light bulb above the cell. Notice that the pictorial drawing shows two conductors. Therefore, the schematic diagram also has two conductors. Figure shows the completed schematic drawing, which is the symbolic equivalent of Fig. Schematic diagram of the single-cell flashlight. Alternative arrangements for the flashlight schematic.
Figure is by no means the only way that you can represent this simple circuit in schematic form. But any schematic representation will require the use of the same three basic symbols: cell, bulb, and conductors. The only changes that can occur involve the positioning of the component symbols on the page.
Figure shows two dif- ferent alternatives for portraying the same circuit. All three of these diagrams the one in Fig. Figure shows the same basic flash- light circuit, but an additional cell and a switch have been added. Pictorial drawing of a flashlight circuit using two single cells in series, some wire, a switch, and an incandescent bulb. By examining this pictorial drawing, you can see that any schematic rep- resentation will need symbols for the cells, the conductors, the light bulb, and the switch.
Again, you should draw the symbols in the same basic order as the com- ponents are wired in the circuit. Figure shows the resulting sche- matic. Note that the two cell symbols are drawn separately, connected in series, with polarity markings provided for each one.
Schematic diagram of the two-cell switched flashlight. The same two conductors are used from the cell terminals, but you need a third one to connect the switch to the light bulb, and you might also need a fourth one to connect the two cells together to form a battery unless the cells rest directly against each other, a common state of affairs inside commercially manufactured flashlights. Figure shows the switch in the off position. Now you know what a common two-cell flashlight looks like when represented with schematic symbology.
The next time that you switch one of those things on, you can imagine the switch symbol in Fig. Figure is a pictorial representation of a device called a field-strength meter.
Wireless communications engineers sometimes use this type of meter to see whether or not an RF electromagnetic EM field exists at a given location. The circuit consists of an antenna, an RF diode, a microammeter a sensitive current meter graduated in millionths of an ampere , and a coil.
Pictorial representation of a field-strength meter circuit. Schematic symbols for the components in the field-strength meter: Antenna A , coil B , microammeter C , and diode D. Using the same method as before, you can draw the schematic by connect- ing the symbols in the same geometric sequence as the components they represent appear in the circuit. Figure is a schematic of the field-strength meter shown pictori- ally in Fig.
This drawing involves nothing more than substitution of the schematic symbols for the pictorial symbols. As before, the parts need not be physically placed in the same positions as the sche- matic diagram suggests, but they must be interconnected precisely as indicated in the schematic. When you build a circuit from a schematic diagram that you trust, you should double-check and triple-check your actual component interconnections to make sure that they agree with the schematic.
If you try to build the circuit shown in Fig. Schematic diagram of the field-strength meter. In more sophisticated devices and systems, wiring mistakes can cause component damage and, once in awhile, give rise to dan- gerous situations! Follow the flow In the circuit of Figs.
That current is high-frequency AC. The microammeter registers this current. As the strength of the EM field increases, the current increases, and the meter reading goes up.
Previously, we compared schematic drawings to road maps. A road map is supposed to indicate exactly what a motorist will experience in practice. The schematic drawing does the same thing. The high- way lines that interconnect towns on a map correspond to the con- ductor lines that interconnect components in a schematic.
Figure is a schematic diagram of a power supply that produces pure, battery-like DC from utility AC. At the top of the transformer secondary winding the one on the right , a rectifier diode is connected in series.
Following the diode, an electrolytic capacitor note the polarity sign is connected between the output of the rectifier and the bottom of the transformer second- ary. A fixed resistor is connected in parallel with the capacitor.
The DC output appears at the extreme right. The physical size and weight of a real-world power supply, which you can build on the basis of Fig. Schematic diagram of a simple DC power supply. Any power supply that uses a single diode, capacitor, and resistor will have this same basic configuration. Whether the output is 5 V at 1 A or V at 50 A, the schematic drawing will look the same.
Figure says nothing about how many volts or amperes the transformer, diode, capacitor, and resistor are meant to handle. The AC travels through the fuse and flows in the transformer pri- mary. In the secondary, AC also flows, but the voltage across the transformer secondary might be higher or lower than the voltage across the primary depending on the transformer specifications.
The diode allows current to flow only one way; in this case the electrons can go only from right to left against the arrow. As a result, pulsating DC comes out of the diode. The capacitor gets rid of the pulsations, called ripple, on the DC output from the diode. The resistor discharges, or bleeds, the capacitor when you unplug the whole device from the utility outlet. These designators all refer to a components list at the bottom.
Now you can see that this power supply uses a transformer with a primary winding rated at V and a secondary winding that yields 12 V. The circuit has a diode rated at 50 peak inverse volts PIV and a forward current of 1 A; a microfarad, V capacitor; and a 10,ohm, 1-W carbon resistor. The fuse is rated at 0. The letters that identify each component are more or less standard. Notice that each letter is followed by the number 1.
If this circuit had two transformers, then one of them would bear the label T1 and the other one would bear the label T2. The numbers reference the posi- tion or order on the components list; they serve no other purpose. The diode carries the reference designator D1, with D serving as the standard abbreviation for most diodes. Standardization is not univer- sal, though! In some instances, the diode might bear the label SR1, where the letters SR stand for silicon rectifier.
Schematic diagram of the power supply with component designators and specifications. Component labeling 67 sponding symbols. If you replaced the designation D1 with SR1, your readers would still know that the abbreviation went with the symbol for the diode, as long as you made sure to put the abbreviation close enough to the symbol.
In the situation of Fig. You could simply write P for the plug, F for the fuse, T for the transformer, D for the diode, C for the capacitor, and R for the resistor. Or, if you had con- fidence that your readers knew all the symbols, you could leave out designators altogether!
Nevertheless, standard diagramming practice requires that you always include a letter and a number, even if only one of a certain component type exists in the whole circuit. In complicated electronic systems, several hundred components of the same type resistors, for example might exist, many of which come from the same family. For instance, if you see the designation R, then you know that the system contains at least resistors. If you want to know the type and value of resistor R, you will have to look up R in the components list to find its specifications.
Tip You can use the schematic of Fig. But before each component was referenced, the schematic had no practical use. Some of these designations can vary in real-world documentation, depending upon the idiosyncrasies of the person making the draw- ing or designing the circuit. If the component has a complex name, such as silicon- controlled rectifier, the first letters from each of the three words is used, so you get SCR1.
Conflicts do arise, of course. If you want to designate a relay, you need to use some letter other than R because R indicates a resistor! A power supply that uses full-wave bridge rectification four rectifier diodes and Zener-diode voltage regulation. The circuit of Fig. Figure shows a voltage-doubler power supply. The two capaci- tors, C1 and C2, charge up from the full transformer secondary out- put after the current goes through diodes D1 and D2.
Because the two capacitors are connected in series, they act like two batteries in series, giving you twice the voltage. A voltage doubler power supply works well only at low current levels. In Figs. So, for example, in Fig. A voltage-doubler power supply. All the other components have only one of each type. The transformer is all alone, so you see only the number 1 following the letter T.
Tip Even though multiple components might all have the same value ohms, for example, or 50 microfarads , they must neverthe- less get separate numerical designations when two or more of them exist in a single circuit.
The schematic diagram for a device allows engi- neers and technicians to make the correct electrical connections when putting it together, and to locate the various components when testing, adjusting, debugging, or troubleshooting it. If you find all this talk overly philosophical, maybe a real-world example will clear things up. Remember that solid lines in schematic drawings repre- sent conductors. It might be part of a component lead, or perhaps a foil run on a printed circuit board the latter-day equivalent of a connecting wire.
Whether or not a separate length of wire is needed to inter- connect two components will depend on how close together those components are in the physical layout. Examine the simple schematic of Fig.
The circuit contains three resistors, all of which go together in a parallel arrangement. A simple circuit comprising three resistors in parallel.
Pictorial diagram of three resistors in parallel with leads intertwined. Taking the schematic literally, a conductor connects the left-hand side of R1 to the left-hand side of R2. Another conductor goes between the left-hand side of R2 and the left-hand side of R3. Two other conductors connect the right-hand sides of the components. In practice, the connections might be made with wires attached to the resistor leads, but if the components are close enough together, the leads themselves can form the interconnections.
Then Fig. Of course, in the above example, if the three resistors had to go in different parts of the circuit separated by some physical distance, then you would need to use interconnect- ing conductors between them. However, as you design the physical layout of a circuit, you should try to minimize the overall length that is, the total length of all the interconnecting wires or foil runs combined. Troubleshooting with schematics Engineers and technicians use schematic diagrams to create elec- tronic devices, but these diagrams can also prove invaluable for troubleshooting equipment when problems develop.
Knowing how to read schematic diagrams, however, is not enough. No matter how proficient you might get at electronics troubleshooting, seemingly simple repair jobs can explode into major headaches with- out complete, accurate, and clear schematic representations of the hardware. Schematic diagrams clarify circuits. They present the circuit ele- ments in a logical and easy-to-understand manner. They tell you very little, if anything, about the component layouts in actual devices.
When you build a circuit from a schematic drawing, the physical object rarely bears much physical resemblance to the schematic. The diagrams purposely spread out the components on the page for easy reading. Schematic diagrams are two-dimensional, whereas real-world electronic components are three-dimensional.
If you know a fair amount about electronic components and how they operate in various circuits, then you can use a schematic diagram to get a good idea without any equipment testing where a particu- lar problem might occur.
Then, by testing various circuit parameters at these critical points and comparing your findings with what the schematic diagram indicates should be present, you can make a quick assessment of the trouble.
For example, if a schematic diagram shows a direct connection between two components in a circuit, and a check with an ohmmeter reveals a high resistance between the two, then you can assume that a conductor is broken or a contact has been shaken loose. Troubleshooting with schematics 73 Beginners to electronics troubleshooting and diagram reading sometimes assume that a professional can instantly isolate a problem to the component level by looking at the schematic.
This idealized state of affairs might prevail for a few simple circuits, but in complex designs the situation grows a lot more involved. Often, the schematic diagram allows a technician to make educated guesses as to where or what the trouble might be, but an exhaustive diagnosis will nearly always require testing.
A particular malfunction in an electronic device will not necessarily have a single, easy-to-identify cause. Often there are many possible causes, and the technician must whittle the situ- ation down to a single cause by following a process of elimination. Suppose that a circuit will not activate, and no voltage can be detected through testing at any contact point indicated by the sche- matic. Chances are good that no current is passing through the circuit at all.
Has one of the components in the power supply become defective? Has the line cord been accidentally pulled from the wall outlet? Has a conductor broken between the output of the power supply and the input to the electronic device? Has the fuse blown? In a scenario of this sort, you will almost certainly want to consult the schematic diagram as you go through all of the standard test pro- cedures. You might wish to find the contact point that serves as the power supply output, indicated on the schematic.
If you test the volt- age at this point and it appears normal, then you can assume that the problem lies somewhere further along in the circuit. The schematic diagram and the test instrument readings allow you to methodically search out and isolate the problem by starting at a point in the circuit where operation is normal and proceeding forward until you get to the point where the circuit shows some abnormality.
Continuing with the same example, if no output comes from the power supply, you know that you must search backward toward the trouble point. You will continue testing until you reach a point of normal operation and then proceed from there.
Chances are good that this narrowing process will isolate the trouble to a single com- ponent or circuit connection. As you become more experienced in the art of electronics troubleshooting, the information contained in schematic drawings becomes increas- ingly valuable. Go back and look again at the flashlight circuit of Fig. Although the schematic diagram does not say so, the two batteries in series should yield a DC potential of 3 V because a typical flashlight cell provides 1.
Suppose that the flashlight has stopped working, and you decide to test the circuit with a volt-ohm-milliammeter VOM , also called a multimeter, with the help of Fig. First of all, you can measure the individual voltages across the cells. If both read zero, then you know that both cells have lost all their electrical charge. If one cell reads normal and the other one reads zero, then in theory you should only have to replace the one that reads zero.
If both cells read normal, then you can test the voltage across the bulb. Here, you should expect a reading of 3 V under normal operation with the switch closed.
If you do indeed observe 3 V here, then you can diag- nose the problem by looking at the schematic. The bulb must have burned out! The schematic shows you that current must go through the light bulb if the bulb can conduct, so it must light up. If voltage is available at the base of the light bulb, then current will flow through the element unless it has opened up.
In fact, with a burned-out bulb, no current will flow anywhere at all in the circuit. Three conductors are involved here: one between the negative terminal of the battery and one side of the bulb, another between the positive battery terminal and the switch, and another between the switch and the other side of the bulb. Obviously, one of the conductors has broken or a contact has been lost where the conductor attaches to the battery , or maybe the switch is defective.
While you keep an eye on the schematic, you can test for a defective switch by placing the negative meter probe on the negative battery terminal and the positive probe on the input to the switch.
If you see a normal voltage reading, then the switch must be defective. If you still get no voltage reading, then one of the conductors has come loose or broken. Admittedly, the scenario just described presents only a basic exam- ple of troubleshooting using a schematic diagram—almost as simple as things can get!
But imagine that the flashlight circuit is highly complex, one you know nothing about. Then the schematic diagram becomes an invaluable aid and a necessary adjunct to the standard test procedures with the VOM. This same basic test procedure will be used over and over again when testing highly complex electronic circuits of a similar nature. But if you have to do a comprehensive troubleshoot- ing operation, you might have to test each and every one of those circuits individually.
A more complex circuit Figure shows a diagram for a somewhat more complicated, real-world electronic circuit presented in a form intended to assist a troubleshooting technician. The circuit has a single NPN bipolar transistor along with some resistors and capacitors. Note that test points abbreviated TP exist at three different locations: TP1 at the emitter of the transistor, TP2 at the base of the transistor, and TP3 at the collector of the transistor. Schematic diagram of an amplifier circuit that includes component des- ignators and three test point TP locations.
Follow the flow The circuit of Fig. The general signal flow goes from left to right. The original AC signal enters at the input terminals, passes through capacitor C2, and reaches the base left-hand electrode of transistor Q1.
The resistors R2 and R3 have val- ues carefully chosen to place precisely the right DC voltage, called bias, on the base of Q1, ensuring that the transistor will work as well as it possibly can in this application. If the readings obtained are within this known error range called the component tolerance , then you can tentatively assume that this part of the circuit is working prop- erly. However, if the readings obtained are zero or well outside of the tolerance range, then you have pretty good reason to suspect a problem with the associated circuit portion, or possibly with other circuits that feed it.
As a further aid, the literature might include pictorial diagrams that show you where each part belongs on the circuit board or chassis. That way, you can follow the circuit not only according to its electrical details, but along the physical pathways as they actually look.
However, a few alternative labeling forms are also acceptable. Figure shows the same circuit as the one in Fig.
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