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Electronics-World-1959-05

By WILLIAM C. CALDWELL

By WILLIAM C. CALDWELL Rational troubleshooting based on clear -cut rules can end the futility of hit- and -miss procedures. GET OUT your meter, put on your thinking cap, and let's see how we can put those increasingly popular transistor portables back into your customers' pockets almost before they miss them. It is possible to do this if a few simple rules and transistor principles are observed: Rule 1. Always keep a voltmeter handy and don't be afraid to use it. A 20,000- ohms -per-volt instrument is perfectly satisfactory in most cases. In fact, since it requires no periodic zero adjustments, it can prevent errors and save time when reading small voltages and biases. Rule 2. Use miniaturized tools for miniaturized components -small screw- transistors may also be divided into three general categories. Low -power types are generaly used in such stages as r.f., local oscillator, converter, i.f., a.g.c. amplifiers, and small audio amplifiers (under 50 milliwatts) like those found in hearing aids. Medium -power transistors may be used as audio drivers 150 milliwatts or more) or in audio -output stages of small portables. High -power (or simply "power") types are used in the audio -output stages of car radios or larger portables that are capable of one watt of output or more. Troubleshooting Procedure Now we are ready to take a typical portable and apply some of the prin- ciples involved. The first step always is to check battery voltage under load (radio on). This reading should be within 10 per -cent of rated voltage if standard zinc -carbon batteries are used or within 5 per -cent of rated voltage if mercury batteries are involved. Bad batteries can cause any number of conditions: weak, dead, or distorted sound, or even instability and oscillation. While at the batteries, it is a very simple matter to find out how much current the radio is drawing without unsoldering any leads if the batteries are mounted in clips. Simply insert a piece of paper as an insulator between one end of a battery and its clip. Switch the meter to the appropriate milliam- Save Time on Transistor Radios drivers, pliers, and a small (20 to 40 watt) soldering iron. Nothing like the right tool for the job. Rule 3. Use a system; never just "hunt or poke" around. "But what sort of system shall I use ?" you ask. Simply follow the rules already mentioned, along with some other common electronic principles that will be pointed out here, and you're in business. Before stating these principles, let's review some working facts about transistors: 1. There are two basic transistor types commonly used in portable radios, n -p -n and p -n -p. Of these two, the n -p-n may be more closely compared to a vacuum tube: one element emits electrons that are influenced (biased) by the base as they are attracted toward a collector that has a positive potential on it. 2. The p -n -p transistor differs from the n -p-n primarily in the direction that electrons flow through it. They move from collector to emitter, but this flow is still influenced by the potential (bias) between base and emitter. With this reversed internal operation, externally applied voltages are also turned around: the most positive potential is applied to the emitter. 3. In terms of their use in the radio, 112 l Fig. 1. Typical transistor stages. Fig. 2. Open condition in base circuit. Fig. 3. Open condition in collector. Fig. 4. Open condition in the emitter. pere range and measure across the open battery connection, with no signal and minimum volume on the receiver. If more than 25 per -cent over the normal drain is indicated you should be looking for a short somewhere, not an open condition. If several times the normal value of drain is noted, you probably have an "A" line short, such as in an electrolytic capacitor. Next, if any sign of instability is present -oscillation, motorboating, whistles, or howl on stations- substitute good electrolytics across the ones in the circuit. If not, isolate the trouble to a stage or section of the radio. Some manufacturers give a step -by- step procedure for stage isolation in their service literature. A good example is the Delco Radio Division of General Motors. This company describes both a "click" test and a signal - generator approach for auto -radio portables. The "click" test (touching a 10,000 -ohm protection resistor from certain transistor elements to ground while listening for a slight noise as this is done) is usually the fastest procedure for a completely dead radio. The signal -generator approach is often best for a set that is merely weak. A resistor installed in a test lead, with clips on both ends, has proved very ELECTRONICS WORLD

0 MIXER Ca CONVERTER DIODE ST AUDIO AUDIO OUTPUT COLLECTOR DIODE COLLECTOR BASE ITER DIODE EMITTER Fig. S. Block diagram for deluxe transistor portables. Fig. 6. Internal elements (A) in an "n -p.n " transistor., (BI Points for checking individual transistor "diodes." IAI helpful in troubleshooting dead portables. The resistor prevents damage to transistors and i.f. transformers. For example, in the block diagram of Fig. 5, a click should be heard at the base of the first audio transistor unless the trouble is present in one of the audio stages. Then, moving toward the front end, a click should be heard at the collector element of the mixer stage. If not, one of the i.f. stages is at fault. In some radios, clicks may be heard within the i.f. stages, but in others the gain is not sufficient. The general rule is that, if two i.f. stages are present, a click at the first i.f. collector or base may be heard but, if the radio uses only one i.f. stage, the collector of the mixer stage is the first click test point after the first audio stage. Then, moving to the r.f. stage, a click should be heard at the base element and at the "hot" end of the loop antenna when it is touched with a screwdriver that is "grounded" by the grip of your hand. Since radios vary in their ability to pass these test signals, it is advisable to try clicks and signal -generator injections at various points in order to get the feel of normal responses and to gain self- confidence. The volume control should be set at maximum in these tests. One important point : when using a signal generator, always start with a low r.f. gain setting and increase gradually to prevent blocking the stages with excessive signal. The amount of signal required, of course, should decrease as one moves toward the front end. In -Stage Isolation After stage or section isolation, the next step is to pinpoint quickly the defective part, connection, or circuit point. This is where that voltmeter and those previously mentioned transistor principles come in handy. Voltages should be measured carefully in the suspected stage or stages. Assuming that the battery voltage is normal, stage voltages are usually accurate to ± 10 per -cent for voltages above one volt. Accuracy of voltages below one volt is usually .1 volt. If battery voltage is not quite at rated value, allowances would have to be made in measuring the stage voltages. This is another good reason for always measuring source voltage first. Fig. lA shows a typical n -p -n amplifier stage and Fig. 1B shows the same circuit revised for p -n -p operation. The voltages are "normal" ones, taken with May, 1959 respect to ground. (In this article, the negative pole of the battery is at ground potential. If a positive ground is used in a circuit being checked, the direction of change in voltage with each defect, would be reversed from that shown here.) Before launching into a discussion of defects and their symptoms, one point is worth clarifying. Reference is made in what follows to "open" transformers and resistors. Actually these conditions are seldom found in transistor radios except in the output stages of auto radios where the emitter resistor also serves as a fuse. In actual practice, apparently open conditions of this kind would end up as poor connections, usually repaired by heating solder joints slightly, rather than as true internal opens. However, the assumption of open transformers and resistors is useful for the purpose of explanation and clarification. Now for principles: Principle 1. With an open transformer in the base circuit, voltages around the transistor elements will change in the directions indicated in Fig. 2. Note that voltages for an n -p-n transistor tend to go in the same direction that a tube's voltages would go under similar circumstances. The p -n-p voltages change in the opposite direction. In either case, the floating base tends to assume the emitter voltage; thus there will be no difference in potential between emitter and base. Principle 2. With an open transformer in the collector circuit, transistor voltages will change in the direction shown in Fig. 3. In this case, the emitter measures the same as its source potential (ground or battery voltage, as a rule), indicating that no collector current is flowing through the emitter resistor. The collector assumes the same voltage, making this condition easy to detect. Principle .1. With an open stabilizing resistor in the emitter circuit (or an open in the printed wiring of the emitter circuit), transistor voltages will alter in the direction shown in Fig. 4. The emitter assumes the voltage of the adjacent element (in terms of internal construction) , and again there is no potential difference between base and emitter. Principle 4. When the lead to the transistor base is internally open, voltages will change in the direction shown in Fig. 7. Principle 5. With an internal short or high leakage between emitter and collector, voltages will change in the direction shown in Fig. 8. Note that the transistor that has become leaky or shorted in this way can cause the emitter -to-base voltage relationship to be reversed from normal. That is, if the emitter voltage is supposed to be higher than the base voltage, it will become lower, or rice rersa in this instance. Based on the typical arrangements shown in Fig. 1 for the two types of transistors, and the defects illustrated in Figs. 2 through 8, a summary of defect indications is given in Table 1. Until the symptoms described are memorized, it might be useful to keep a copy of this table handy for ready Fig. 7. Open lead in transistor base. Fig. 8. Emitter -to- collector leakage. Table 1. A summary of typical defect indications in transistor circuits. DEFECT N -P-N P -N-P Collector Emitter Base Collector Emitter Base Open Base Transformer High Low Low Low High Open Collector Coil Low Low OK High High OK Open Emitter Resistor High High" OK Low Low" OK Open Base (internal) High Low OK Low High OK Emitter -Collector Leak Low High OK High Low OK NOTES: All indications will be opposite to those listed if a positive ground is used. 'Reading depends on resistance of meter. "Small change only. Emitter assumes voltage on base. 113

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