Because the term "distortion" is widely used and could mean any symptom except a completely dead receiver, the conditions referred to in this section will be defined as:
1. Uncontrollable 60-hz hum.
2. Tunable hum.
3. Squeals and motorboating.
4. Rattle or mushy sound from defective speaker.
5. Audio clipping.
Each of these has a distinctive sound which the technician learns to recognize quickly, and test routines are modified accordingly.
UNCONTROLLABLE 60-hz HUM
5-1
This is the most easily recognized form of distortion because it cannot be changed by any adjustment of tuning or volume control. It is nearly always due to an open filter capacitor in the power supply. In such a case, the filters can be checked very simply by shunting a good unit across them of sufficient capacitance and voltage rating. In all other cases of defective filters, it is necessary to disconnect the original unit before substitution of a new one. But in the case of uncontrollable 60-hze hum it is clear that the capacitor is open.
SERVICING CHART III shows two other causes of uncontrollable hum which may need to be checked. Heater-to-cathode leakage in the audio output tube produces 60-hz hum which cannot be controlled by the volume control, and this tube should be checked before the chassis is removed. The 50C5 output tube is a frequent offender in this way.
An open connection from the volume control to ground causes this symptom in rare cases. This is easily checked with the ohmmeter. The resistance from grid to ground in the out-put stage should also be checked. The failure may be in printed wiring associated with the control, and this should not be over looked.
TUNABLE HUM
5-2
This form of distortion differs from the preceding one in that the hum is not as loud and is present only when a station is being received. When the receiver is tuned to a quiet place on the dial, the hum disappears. In some cases, it will be present on only one or two of the stations and this sometimes leads the inexperienced technician to suspect trouble at the station as the cause.
Three common failures which cause tunable hum are shown on SERVICING CHART III. Of these three, the mixer-oscillator tube is the most frequent cause. A new tube should be substituted, rather than testing the suspected tube, because the checker may not show the defect. Heater-to-cathode leakage in the tube puts 60-hz modulation on the oscillator signal that is generated in the cathode circuit. The 60-hz signal is amplitude modulation of the oscillator signal and, as such, will not appear in the grid of the IF stage unless the radio is tuned to an incoming carrier so that conversion to 455 khz can take place. When a carrier is present, the 455 -khz beat will contain the 60-cycle modulation from the oscillator as well as the modulation from the station.
Another, less well known, cause of tunable hum is an open line-bypass capacitor. This is the capacitor connected directly across the 120-volt AC input line in transformerless receivers.
Its purpose is to bypass radio signals which are present on the electric lines, and thus prevent them from entering the receiver through the common B-line.
In some receivers, the A VC filter capacitor can cause a type of tunable hum which sounds the same as the others, but is due to audio variations in the grid-bias voltages of the mixer and IF stages.
SQUEALS AND MOTORBOATING
5-3
The sound of this symptom is unmistakable. The receiver may emit a high-pitched squeal when it is tuned through an incoming carrier, or it may produce a slow "put-put" sound between stations. There is sometimes a low-pitched growl resembling the noise of a large truck laboring under a heavy load. These sounds are the result of oscillations caused when a signal from one part of the receiver is fed back to another and re-amplified.
The oscillation may be confined to a single stage, or it may involve several stages. Technicians frequently use a simple trick to help locate the stage giving trouble. As explained in Section 4, oscillation always develops a negative charge in some part of the grid circuit, and this charge is affected by placing the finger on the grid pin of the tube socket. So the technician touches each grid with his finger while listening for a change in the oscillation. If no change occurs, the stage being checked is not likely to be part of the feed-back loop. Testing is therefore confined to components in stages where touching the grid affects the symptom.
Oscillation often results when the input and output trans formers in an IF stage are tuned to exactly the same frequency. This is due to feedback from the plate to the grid through the internal capacity of the tube or through magnetic coupling between wires connected to the input and output circuits. When a single stage oscillates, the technician can frequently trace the trouble to displacement of the wiring during an earlier repair job.
In receivers using more than one IF stage, magnetic coupling may take place between the tubes themselves when tube shields have been removed. Although it is not as common be cause of the different frequencies involved, this kind of coupling can occur between the mixer stage and the IF stage.
Often a slight "touch-up" of the alignment of the IF trans formers is all that is needed to subdue an oscillating IF stage.
It may seem detrimental to detune the transformers away from the 455- khz IF frequency, but it should be remembered from the description of the diode detector in Section 2 that the stage must also pass the audio sidebands and, therefore, it was not designed to peak sharply at 455 khz. For good fidelity, the IF bandpass should range from at least 450 khz to 460 khz.
So, it is not unreasonable to tune the input near 450 khz and the output close to 460 khz. The tendency for oscillation is greatly reduced when the input and output transformers are tuned to slightly different frequencies.
The above procedure applies only to ordinary broadcast band receivers and should not be attempted on communications receivers designed with very sharp IF response. In these receivers, the shielding is much better, and oscillation is probably the direct result of a component failure. Alignment of communications receivers should be attempted only by technicians who have experience with this type of equipment and only when manufacturer's data and the proper test instruments are available.
Components To Be Tested
5-4
Directly under the suggestion to touch-up alignment in SERVICING CHART III are listed components which often cause the symptom of squeals and motorboating.
The output filter capacitor in most 5-tube receivers is also the screen bypass capacitor for all the stages. When this capacitance decreases, it permits coupling between the stages through the common screen supply, resulting in oscillation.
Many technicians overlook this capacitor as a cause, assuming that an open filter will always cause 60-cycle hum. But in this case, the capacitor is not open-it has only decreased in value.
Shunting with a good capacitor will identify the defective unit.
It is good practice to replace all the filters at this time, especially if they are combined in a common container.
In the better receivers, multistage phono amplifiers, and in tape recorders, special networks are used to improve the isolation between stages. These are called decoupling networks; some examples are shown in Fig. 5-1. Resistor Rand capacitor C hold the voltage on the top plate of the capacitor constant when the plate current goes through large changes at an audio rate. The reactance of the capacitor to audio signals is about 1/10 the value of the resistor, and audio current is thus shunted to B- without passing through the power supply. In this way, the B+ voltage of the power supply does not vary due to large changes in current which would otherwise pass through it. An open decoupling capacitor usually causes distortion on low audio notes only, producing a kind of low pitched growl when the signal contains a large proportion of low frequencies.
Plate and screen bypass capacitors in the RF, IF, and first audio stages in a receiver are supposed to remove RF and IF signals from the following stages. If they fail to do so because of an open condition, oscillation frequently results. The capacitors can be checked by shunting across them, because, with normal volume present, it is certain they are not leaking or shorted. Capacitors in the A VC filters can cause oscillation in a similar manner and should be checked by shunting them also.
When an IF or audio transformer has been replaced, oscillation may result from an error in wiring which does not affect the receiver in other ways. When this happens, oscillation can be cured by reversing the leads to the windings of the trans-former. This may mean exchanging the primary for the secondary or simply switching the ends of a single winding. In stubborn cases where oscillation results after replacing an IF transformer, and no amount of detuning will help, the efficiency of the tuned circuits can be lowered by shunting the windings with resistors of 50K to 100K. While this method cannot be correctly called a "repair," it may be the last resort in cases where the original IF transformer is not available for replacement, and a more efficient substitution has been used.
Fig. 5-1. Examples of decoupling networks.
Fig. 5-2 shows a type of feedback used in high-fidelity audio systems. The polarity of the feedback signal taken from the output transformer is intended to be such that the feedback subtracts from the signal input. By inadvertently reversing the connections to the secondary, the technician could obtain a feedback signal which adds to the incoming signal and thus produces oscillation.
TEST POINT 1, CHART III CLIPPING AND MUSHY AUDIO
Fig. 5-2. Inverse feedback.
The types of distortion discussed earlier in this section are easily recognized and isolated because each one has a distinctive sound. Mushy audio, however, is a broad category covering all the kinds of distortion not previously mentioned, and the causes may be in a number of different parts of the receiver.
Technicians learn to recognize the "tinny," flat response or rattle of a defective speaker. One type of speaker failure which has been difficult for many technicians to find causes the sound to be distorted at low volume only. The sound of audio clipping when an audio stage is driven to saturation can also be recognized with experience. When the sound of the distortion is familiar and can be classified at once, further testing is restricted to the suspected area, but there are cases where it is difficult to guess which section of the receiver is at fault; it is in these cases that TEST POINT 1 is used.
5-5
Many repair shops keep handy a small record player that is equipped with a shielded cable leading from the crystal pickup and having clips on the free end. This is used to inject music of known quality into the volume control, the purpose being to determine whether the cause of the distortion is in the audio section or in the RF and IF sections. If the sound from the phonograph record is distorted when it is played through the receiver's audio system, tests will continue in the audio section only.
TEST POINT 2, CHART Ill INJECTED AUDIO IS DISTORTED
5-6
On the right side of SERVICING CHART III, below TEST POINT 1, are listed some things to check before moving on to TEST POINT 2. These are the audio tubes (which should be substituted rather than checked in a tube tester), and an ohmmeter check of the volume control, which may not be grounded as it should be.
After these tests, the music from the phonograph is injected into the grid of the audio output stage or into the next audio stage. The sound from the speaker will be weaker now, but it should be possible to tell whether or not it is distorted.
Further Tests When Sound Is Clear
Fig. 5-3. Checking coupling capacitor for leakage.
5-7
This proves that the first audio stage is responsible for the distortion. The coupling capacitor between the plate of the voltage amplifier and the grid of the following stage is the most common cause, and a new one should be tried. It is possible to test the capacitor using the method shown in Fig. 5-3.
Here, the VTVM is used as a sensitive microammeter. It isn’t sufficient to check this capacitor for leakage by measuring it with an ohmmeter, because the leakage may be several meg-ohms, or it may not begin to leak until it is subjected to the B+ voltage of the receiver.
Tone-control circuits can cause distortion when they are filtering out frequencies above or below the intended range.
A resistance measurement of the components is recommended in this case. When in doubt, substitute new parts.
Plate bypass capacitors cause distortion when they leak intermittently and should be checked by substituting new ones.
Also, the value of the plate-load resistor affects the audio quality, and this should also be measured. In like manner, the decoupling circuit shown in Fig. 5-4 should also be checked.
SLIGHTLY LEAKY CHANGED IN VALUE ± 50. OPEN CAPACITOR OR LOW-VALUE RESISTANCE
Fig. 5-4. Sources of distortion in an audio voltage amplifier.
Further Tests When the Injected Audio Is Still Distorted 5-8 The coupling capacitor to the grid of the next stage is mentioned again on the right side of SERVICING CHART III under Distorted Sound because certain defects in this unit will cause distortion even when audio is injected at the grid of the following stage.
This is also a good time to check the speaker. There are no good tests of speaker quality which are simple enough to be employed on a radio service bench, so substitution with a speaker of known quality is recommended.
TEST POINT 3, CHART Ill VOLTAGE AND RESISTANCE MEASUREMENTS AT THE PLATE AND SCREEN OF THE OUTPUT STAGE
5-9
Even when voltage measurements are normal, distortion can be caused in the plate circuit of the audio output stage if the bypass capacitor is intermittently leaky. When loud pas sages are reproduced, the voltage across this capacitor rises and may cause it to break down only on these peaks. The same thing is true of the windings in the transformer primary which may short between adjacent turns or to the core when the voltage is high. The best test is to substitute new parts, although the defects might be revealed by a sharp change in DC plate voltage when certain audio notes are present. This cause of distortion is more prevalent in auto radios and transistor portables, which are more often subjected to moisture and are operated at volume levels near the maximum.
In push-pull output stages, the resistance of the windings in the input-transformer secondary and the output primary should be checked to make sure that there is equal resistance from either end to the center tap. An off-balance condition here can result in distortion.
The cathode circuit of the audio output stage should be examined with the ohmmeter, and the technician should not hesitate to substitute a new cathode capacitor in cases where distortion has been isolated to the circuitry of this stage. The value of the grid resistor should not be overlooked.
TEST POINT 4, CHART Ill AUDIO INJECTED AT THE VOLUME CONTROL PRODUCES UNDISTORTED SOUND
5-10
The procedure is shown on the left side of SERVICING CHART III under TEST POINT 1, and all the tests are concerned with the IF or RF sections of the receiver. The mixer/osc and IF tubes should be substituted first, followed by an ohmmeter check of the resistance in the volume control, and from the A VC line to ground. An open here indicates a defective volume control or poor connections to it. This is a common failure in radios using printed-circuit boards. A very low resistance indicates that the A VC line is grounded, perhaps through a shorted A VC filter capacitor, or that the RF bypass capacitor in the detector is defective.
TEST POINT 4, CHART III OHMMETER CHECK OF MIXER AND IF CATHODES
5-11
It is unlikely that a cathode will be found open, since the receiver is still able to receive stations, but a shorted cathode capacitor in the IF stage can produce a muffled type of distortion in many receivers. In circuits where the mixer cathode is part of the oscillator circuit, it will not be necessary to make the resistance check at this point, since any failure would stop the oscillator and thus remove all signals from the audio output.
Further Tests When the Cathode Resistances Are Normal
The only remaining possibility is a defective IF trans former. A detailed discussion of some of the tests applied to IF transformers was given in Sections 4-10, 4-11, and 4-12; another test should be made in connection with the symptom of distortion.
Fig. 5-5 illustrates a way in which IF transformers can be checked for leakage between the primary and the secondary.
This defect will often be discovered by noticing that a slight positive voltage is present on the AVC line. But the test should be applied even when no change in A VC voltage can be noticed.
Fig. 5-5. Checking for a leaky IF transformer.
---------- Servicing Chart III: Distortion.
When the heaters are in parallel, the mixer and IF tubes are removed to stop all incoming signals and to increase the voltage between the transformer windings by eliminating any voltage drops caused by plate current flowing through resistors in series with the primary. When the heaters are in series, the tubes can be removed and B+ restored by soldering a silicon rectifier from the 120-volt AC line to the cathode pin of the rectifier-tube socket, making sure that the correct side of the AC line is used and that the cathode end of the silicon is on the cathode pin of the tube socket. Alternatively, the cathodes of the mixer and IF tubes can be opened.
A DC voltmeter set on a low scale is connected from the grid terminal of the secondary to B-. With the receiver turned on, a slight positive voltage indicates leakage. The reading may become more noticeable if the transformer is tapped with a small tool during the voltage measurement. A slight flicker of the voltmeter needle is enough indication to warrant changing an IF transformer. But it must be remembered that the secondaries of the IF transformers are tied together through the AVC line and that the leakage measured at one trans former could be caused by B+ current leaking through the other one. For this reason, once leakage has been found, the primary of each transformer is disconnected from the B+. The leakage will disappear when the defective transformer is disconnected.
When impedance coupling is used, there will be only a tuned plate coil coupled to the grid of the next stage by a capacitor.
The same method described in Section 5-7 can be used to check for leakage of the capacitor.
QUIZ
1. In the case of audio clipping or mushy sound, what other test could be used in place of the test recommended at TEST POINT 1?
2. Describe the difference between tunable hum and uncontrollable hum.
3. Suppose there were an intermittent short between adjacent turns in the audio output-transformer primary. Describe the series of tests that would be followed to find this failure.
4. A receiver has distorted sound when music from a phono crystal is injected either at the volume control or at the grid of the output stage. What parts should be checked before moving on to TEST POINT 3?
5. After the audio output transformer in a HI-FI audio amplifier is replaced with an exact replacement, the amplifier howls and squeals. What is probably wrong?
6. For what specific kinds of distortion would you suspect tubes?
7. Describe the series of tests necessary to discover that a defective IF transformer is the cause of distortion.
8. What component will cause the receiver to have distortion on very low volume but to play normally with normal set ting of the volume control?
9. A five-tube AC/DC receiver has a loud hum which can be controlled by the volume control but is not tunable. What parts would you suspect, and what tests would you use?
10. Draw a partial schematic of a tone-control circuit, showing a component which could cause distortion if it failed.