IN this section, interference line and bar effects will be considered. The lines and bars are those created by Barkhausen oscillation, microphonics, radiation from various tubes and other troubles. Picture and raster hum bar symptoms, vertical bars in the picture and microphonics will also be described. TVI due to missing or improperly grounded tube shields The absence or improper grounding of shields on the following tubes can cause interference: last sound if, last video if, sound and video detectors. Harmonic signals are generated in these tubes that may be radiated back into the TV front end. When the undesired signals fall into the passband of an assigned TV channel, the harmonics may beat with the rf picture or sound carrier, producing herringbone or other interference patterns on the screen. These patterns will vary in step with the sound being received.
To determine whether TVI is due to radiation from a sound tube, remove one of the tubes in the sound if section. If the interference disappears from the screen, a harmonic of the sound if is most probably getting back into the tuner. (Use a dummy tube for series string receivers.)
[ Since, in this guide we are primarily concerned with tube troubles, outside sources of TVI will not be discussed. ] To check on whether interference is due to radiation from an unshielded last video if or video detector, connect one end of a short length of shielded wire (Fig. 601) to one of the antenna input terminals. Hold the other end close to the last video if or detector tube. Approximately 1 inch of wire should protrude from the shield at each end and the shield itself should be firmly attached to the receiver chassis. If the interference is intensified by the in creased coupling, radiation from the last video if or video detector is indicated. When the source of interference is a sound or video tube, add a shield if one is missing or check the ground connection of any shield present.
Fig. 601. Technique for checking the source of radiation interference.
Radiation from horizontal output tube
In fringe areas the reduction in voltage generally associated with weak incoming signals increases the sensitivity of the receiver. The increased contrast setting also brings up receiver gain.
For both reasons, formerly invisible, internally caused TVI may be amplified to a point where it becomes troublesome.
One such condition is due to radiation of harmonics of the 15; 750-cycle sweep signal from the horizontal output tube. The interference manifests itself as a floating disturbance whose shape and form vary from set to set. The area of interference is on the right side of the picture-tube screen. The symptom is, in many cases, seen only when the receiver is switched to an unused channel (the agc voltage is then least and receiver sensitivity is maximum). If the interference persists when a station is tuned in and readjustment of the horizontal drive control does not eliminate it, try another horizontal output tube.
Fig. 602. Appearance of Barkhausen oscillation in the raster.
"Tweets" due to defective video detector crystal
A herringbone type interference pattern that varies with changes in picture modulation and is caused by the entrance of harmonics from the video detector into the front end, is often referred to as "tweet" interference. A defective crystal may be its cause. The symptoms may occur on some channels as the fine-tuning control is varied. Replace the crystal to test for as well as eliminate this source of trouble.
Barkhausen oscillation
The most notorious interference produced by the horizontal output tube is Barkhausen oscillation. It causes radiation from the horizontal amplifier to take place; the radiated signal is picked up by the front end or video if section and amplified in succeeding stages. The condition generally manifests itself as one or more vertical black lines on the left-hand side of the screen (see Fig. 602). The lines may appear white if the oscillation is relatively weak.
In some cases, the line will be visible only in the raster; in others, they will be apparent, to a greater or less degree, in the picture as well. They will often be most evident on weak channels since the gain of the rf amplifier is maximum at such settings.
Barkhausen lines will generally change in width and intensity if channels are switched or the brightness setting is altered. They are most noticeable, as a rule, at the higher-frequency channel settings and when the brightness setting is low. When Barkhausen oscillations are severe, synchronization may be upset.
Barkhausen oscillation generally gives trouble when the receiver uses a built-in antenna. A built in antenna often necessitates a higher contrast setting in the receiver since its signal pickup is poor; furthermore, the built-in antenna is positioned close to the horizontal output circuit. For both reasons, pickup of Barkhausen oscillation tends to be maximum in such sets.
One way to determine whether Barkhausen oscillation is present is to loop a section of the lead-in (see Fig. 603) and hold it near the horizontal output tube. If the intensity of the lines increases or if they change from white to black, Barkhausen oscillation is causing them. Another test consists of putting a magnet (ion magnet or a pm speaker core) near the horizontal amplifier tube. If symptoms are diminished or disappear, they are due to Barkhausen oscillation.
In some instances, a slight change in the setting of the horizontal drive control will make the symptoms unnoticeable. When such a resetting provides no solution, an ion magnet or a commercial anti-Barkhausen magnet (Fig. 604), may be placed around the horizontal output tube in such a position that the symptoms are eliminated. The output tube can also be replaced to cure the condition. Don't discard the original tube-it will probably function normally in some other set.
In a few instances, a condition closely resembling Barkhausen oscillation has been produced by a defective plate-cap connection on a 1B3-GT high-voltage rectifier tube.
Fig. 603. Bringing the loop near the horizontal-output tube will increase
the intensity of Barkhausen lines.
Other vertical-line troubles
Radiations from the horizontal amplifier may produce a vertical black beady line or lines at the left side of the picture or a dotted line at the left or right side of the picture. Vertical lines at the right side can also be due to spurious oscillations or other defects in the horizontal oscillator and damper tubes. Substitute tubes to test for the trouble in each case.
A defective horizontal output tube may produce a condition in which the right edge of the raster begins to pull toward the center of the screen several minutes after the receiver has been turned on.
This right edge takes on the appearance of a steadily broadening milky line, due to a loss in focus toward the end of the horizontal trace. Nonlinear operation of the horizontal amplifier is probably the cause of the trouble.
Fig. 604. Barkhausen
oscillation eliminator. The magnet is placed over the horizontal-output
tube.
One or more faint white stripes at the left-hand side of the raster are the normal products of a slight amount of ringing in the yoke; they are unnoticeable in the picture under normal circum stances. When the stripes are intense enough to be noticeable and a resetting of the horizontal drive control does not eliminate them, try changing the damper, horizontal amplifier and other horizontal circuit tubes. When two horizontal amplifier tubes are present, change both at the same time and then readjust the horizontal drive trimmer for best results.
The appearance of one or more vertical lines at the left side of the picture may be due to spurious oscillation in the horizontal oscillator or damper tube; try tube substitutions as a test. Lines or bars produced by damper trouble are most often white. In some cases, trouble in the damper will produce light and dark vertical bars at the left-hand side of the raster. Linearity and width are often impaired when damper trouble is present.
Vertical lines due to improper lead dress
Vertical lines in the picture may be introduced by accidental changes in lead dress during tube tests. The service technician should be particularly careful to avoid bringing picture-tube cathode or grid leads close to the horizontal output tube.
Vertical white line due to failure of horizontal retrace blanking tube
In some TV receivers a horizontal retrace blanking circuit (Fig. 605) is employed. A circuit of this type eliminates the effects of sync overshoot at the transmitter. Sync overshoot refers to a narrow spike that is created at the transmitter by the sync pulse and directly follows it. The spike is opposite in polarity to the sync pulse and extends into the white region. Since it effectively unblanks the picture tube during the blanking interval, it becomes visible as a vertical white line at the left-hand side of the picture.
The line generally has a jagged or irregular appearance due to the fact that noise is associated with the sync pulse. It may become readily visible if the horizontal retrace blanking tube becomes defective.
The symptom may be differentiated from similar ones produced by damper trouble by manipulating the horizontal hold control.
If the line is caused by sync overshoot, it will shift its position when the horizontal hold control is rotated. Another clue to the source of such trouble lies in the fact that it will generally manifest itself on only one particular channel.
Change the horizontal blanking tube (if one is used in the receiver) when the symptoms just described exist.
"Spook" interference
"Spook" interference is likely to be generated when the en closure in which the damper tube is often located, is not adequately shielded from the rest of the receiver. Radiation of high-frequency harmonic signals from the damper to rf and video if stages is responsible for this condition. The interference shows itself in the picture as a narrow vertical line that is positioned very close to the left-hand margin of the picture or raster. The line resembles the one produced by Barkhausen oscillation. In the presence of a weak signal, the line is black and its edges are ragged. When the signal is relatively strong, the line tends to break up into small, crawling diagonal lines. The line is not usually visible because it falls beyond the mask or in the blanked-out portion of the picture.
Fig. 605. Horizontal retrace blanking circuit. A sample of the fly back pulse is fed to the grid of the horizontal-blanking tube. A positive voltage is sent to the picture tube cathode through a capacitor and cuts off the picture tube during horizontal blanking time.
Spook TVI may cause sync instability as well as the symptoms just described.
To avoid introducing such trouble during tube replacement tests, make sure that the high-voltage section in which the damper is generally found is properly shielded at all times.
Radiation from audio output tube
Symptoms similar to those produced by auto ignition noise may be due to radiation from the audio output tube. Since the symptoms are continuous rather than intermittent as in the case of ignition TVI, the two conditions may readily be differentiated. Re placement of the audio output tube will, in some cases, eliminate the trouble.
"Snivets"
"Snivets" are black vertical lines that appear in the raster and sometimes in the picture as well on channel settings. The condition is caused by signal radiation from the horizontal output tube.
The undesired signals are picked up by the tuner. Snivets in the raster are often masked by the incoming picture signal. When the incoming signal is not strong enough to override the interference, try changing the horizontal output tube. A different tube may either eliminate them or else move them to a portion of the spectrum to which no station is assigned.
Microphonics
Microphonic effects are disturbances in the picture and/or sound produced by undesired mechanical vibrations of tubes or other components. Since this guide deals primarily with tubes, we will restrict ourselves to a consideration of tube-caused micro phonics.
Microphonics are usually produced by the feedback of sound energy from the speaker to one of the receiver tubes. An undesired mechanical vibration in a tube is reinforced by this sound energy, causing the vibration to become self-sustaining in many cases. The signal in the tube affected is modulated by the microphonic, due to changes in interelectrode capacitance, gain or other factors.
Sound energy from the speaker may get to the microphonic tube indirectly via mechanical coupling through the chassis and cabinet or directly through the impingement on the tube of the sound waves created by the movement of the speaker cone.
All tubes are microphonic to some extent; it is the job of the set designer to isolate from the speaker those tubes most likely to give trouble. In rare instances, where the tube layout is not satisfactory in this respect, remedial steps will have to be taken by the service technician.
Split-carrier receivers are generally more inclined to produce troublesome microphonics than intercarrier sets. The likeliest source of microphonics in the split-carrier set is the rf oscillator.
The symptom generally produced is a howl or whistle when the volume control is at a moderate to advanced setting; noise may also be heard in the speaker when channels are changed. Symptoms stop when the volume control setting is reduced.
To test for as well as remedy the microphonic condition, try substituting tubes in the oscillator stage. Several tubes should be tried since even new tubes may be microphonic to an extent undesirable in any particular application. Test the operation of the new tube on the station at which microphonics have been most troublesome, and with the volume control setting well advanced.
The test should be made with the chassis in its cabinet tinder conditions of normal operation.
In both intercarrier and split-carrier sets, a microphonic front end, video if or video amplifier tube may produce black horizontal bars in the picture. Similar bars can be produced by a micro phonic vertical output, horizontal oscillator, afc tube or even the picture tube. To determine whether the symptoms are due to a microphonic, turn down the volume control. If the symptoms disappear, the trouble is probably (though not always) of microphonic origin.
The condition just described is intermittent and should not be confused with the continuously present sound bars introduced into the picture when the fine-tuning control is not properly adjusted (it is assumed an audio signal is being received). Horizontal bars due to microphonics will appear in the picture when the receiver chassis is tapped or when sounds of loud volume are heard.
The bars will usually vary in width and their intensity will fluctuate in step with the amplitude of the audio signal.
When a microphonic vertical amplifier is the source of the trouble, alternate light and dark horizontal bars, approximately 1/4 inch thick, may be seen in the picture. Other symptoms that can be produced by microphonics are quiver in the picture, as if the image were about to be pulled out of horizontal synchronization, and excessive "grain" in the picture. Microphonic tubes in the front end or video if stages are particularly capable of producing such symptoms.
To determine which tube is at fault, try tapping each suspect lightly with the rubber end of a lead pencil or a light screwdriver.
When tapping one tube-and one tube only- causes the condition to appear or become worse, replace the tube and note results.
If no microphonic tube can be found by these tests, the possibility that inadequate filtering in the audio stages is the cause of the trouble may be quickly checked by opening one voice coil lead.
If the bars previously seen in the picture remain, filtering trouble is probably to blame; if the bars disappear, however, microphonics are definitely present and should be checked further. Don't permit the receiver to operate with the voice coil disconnected for more than several seconds to avoid damaging the audio output transformer.
There are a number of remedies that can be tried when a micro phonic condition involving a tube is not eliminated by tube substitution (or proper seating of the tubes in their sockets). When an inspection reveals that the socket pins may be loose, try tightening them with a thin pointed tool. Loose contacts pro mote microphonics.
In some instances, placing a tight-fitting lead weight over the rf oscillator will clear up a microphonic; in others such a weight may prove more of a liability than an asset. Try removing it (leaving the sheet metal shield around the oscillator in place, however) and note results.
Reversal of the leads going to the speaker voice coil may eliminate a tendency to microphonics in certain receivers.
An effective way of getting rid of low-frequency microphonics is to mount the speaker on rubber or felt. One practical way to do this is to insert rubber washers between the speaker rim and the cabinet. There should be no solid connection between the speaker and the cabinet at any point. For best results, leave a gap no more than 0.25 inch wide between the edge of the speaker frame and the cabinet section it formerly contacted. (The slight change in the audio response that results will hardly be noticeable.) Before re sorting to this expedient, the service technician should first test set reception with the speaker out of the cabinet and at some distance from the chassis. If such a placement of the speaker eliminates the microphonics, shock-mounting of the speaker in the manner described becomes a logical succeeding step. Relocation of the speaker, when this is practical, will cure many stubborn cases of microphonics.
Fig. 606. Sound bars in the picture.
A socket shock mounting (floating a tube socket on rubber spacers) can be used in those instances when speaker relocation is not feasible and a microphonic-prone tube is positioned too close to the speaker. One set manufacturer, for example, recommends shock-mounting of the 6BL7-GT, a tube used in his receivers as a combination vertical oscillator and amplifier, when a micro phonic condition is traced to this tube.
A simple cure for some cases of microphony is to slip a rubber sleeve (the rubber cushion from an old auto-radio vibrator will serve in many instances) around the microphonic tube.
There are cases where a tendency to excessive microphonism is strengthened by the absence of a good connection between the shield envelope of a metal tube used in a video if stage and ground.
Make sure that the socket contact going to the tube shield pin is grounded to chassis when a microphonic metal tube of this kind is located. An above-chassis check can be made to verify whether this ground exists by scraping away the paint at the top of the metal tube and connecting a wire between this scraped-off area and chassis. It the signal strength increases, either there is no connection or the tube construction is defective.
Sound in pix
Possible causes of sound bars in the picture (Fig. 606) other than microphonic tubes are few if we exclude circuit troubles. A defective video detector crystal may be one such source-the correct tuning point may no longer coincide with the point where sound ripple is absent. The crystal used as video detector should be checked by substitution if sound appears in the picture when the fine-tuning control is adjusted for optimum resolution and likelier sources of trouble have been eliminated.
Fig. 607. Bar patterns that may be seen when a 60-cycle hum signal reaches
the picture tube. Which pattern will be seen depends on the phase of the hum
with respect to the line voltage. Drawings a and c show a 90° phase difference.
The drawings in b and d show the effects of reversing the line plug.
It is worth noting that a crystal that is just beginning to deteriorate will, in this early stage, often produce sound bars in the picture. Further deterioration will result in weak or no picture and/or loss of synchronization.
Improper dress of speaker leads that brings them too close to the video detector may in some instances be the cause of sound bars in the picture. Avoid introducing such a condition during tube replacements. The source of trouble can be readily determined-as well as cured-simply by rerouting the leads and noting results.
Hum bars in picture and raster
Tube-caused hum is often due to heater-to-cathode leakage and less frequently to leakage between filament and grid. In either case, a 60-cycle modulating frequency will affect sweep, sync or video signals. Symptoms produced include: horizontal bars (shading) in the picture, wavy raster edges, horizontal picture pulling, vertical roll, hum in sound and complete loss of picture.
Horizontal bars due to 60-cycle hum may cause the picture to have a dark bar over approximately half of its vertical area and a light bar over the other half. See Fig. 607. Or else a single prominent hum bar-either light or dark, depending on the polarity with which the line plug is inserted into the outlet-will be visible.
Closer inspection will reveal that the shading of the picture sections not covered by the bar is improper (too light when the bar is dark; too dark when the bar is light). To verify whether cathode-to-heater leakage rather than some other trouble is responsible for the symptoms present, reverse the line plug. If the bar shading reverses--i.e., goes from light to dark or vice versa-the presence of cathode-to-heater leakage in a tube becomes practically certain.
The section containing the defective tube may be located as follows: Remove the last video if tube from its socket, leaving the contrast control at its normal setting. If the symptoms are still seen in the raster, the trouble lies in the video detector, video amplifier or picture tube. (In older receivers, if a dc restorer is present, it must be included among the possible suspects.) Pull out the video amplifier tube next (unless it is direct-coupled to the picture tube). If the hum symptoms persist, the picture tube is most probably the villain. If the symptoms disappear, try a new video amplifier. If this does not restore conditions to normal, withdraw the video detector from its socket. If the hum effects are no longer visible, the trouble probably originates in the video detector; try a new tube, to verify this.
When the video amplifier is direct-coupled to the picture tube withdrawing it may upset picture-tube bias to an extent where it proves difficult to interpret test results. Simply substitute tubes in the video stages in such a case to check for the source of hum.
If hum symptoms disappeared when the last video if tube was withdrawn, cathode-to-heater leakage in an rf or video if tube is indicated. Try new tubes in these stages as a check. If the receiver is out of its cabinet, the following bottom-chassis tests may be performed to localize the trouble further before tube substitutions are made.
Momentarily short the cathode resistor of each tube in the video if and rf stages. (In the case of a front-end tube whose socket contacts are not readily accessible, either use an adapter that makes them available or else withdraw the tube being tested slightly from its socket and short the cathode prong to chassis with a wire.) When a stage is reached in which such a short eliminates the hum symptoms, cathode-to-heater leakage probably exists in the tube of that stage.
The procedure just outlined cannot, of course, be used in those cases where shorting a cathode to ground impairs receiver operation to a point where the test results cannot readily be interpreted.
Some service technicians may conclude that the presence of hum symptoms in the raster means that the rf and video if stages cannot be the source of the trouble.
Their reasoning is that these stages can pass a low-frequency signal such as hum only when it modulates the high-frequency signals which these stages are designed to accept. This is accurate only if the input signal has been killed by reducing the contrast setting to zero or by shorting the antenna input. If this has not been done, however-if the raster is, let us say, inspected on an unassigned channel, and the contrast setting is up-hum may get through by riding in on top of noise or spurious signals, thus causing raster shading. Incorrect deductions regarding the section causing the hum may result. Reduce the contrast setting to zero and/or short the antenna input to avoid making an error of this sort during a raster inspection.
It is worth noting that a hum bar caused by 60-cycle modulation of the transmitted signal (the condition present when cathode-to-heater leakage exists in an rf or video if amplifier) will be stationary if the station tuned to is being operated from the same line-voltage source as the receiver. In some cases, however--a net work telecast, for instance--this condition may not be present. The hum bar will roll up or down the picture under these circum stances.
Intermittent or continuous vertical roll, horizontal pulling and/or hum in the sound may be associated with the hum-caused bars previously described.
In some instances, hum bars of the type just considered may be due to cathode-to-heater leakage in a sync tube. This can occur when the circuit permits a feedback of the hum signal to the video detector or amplifier. Many service technicians may overlook such a possibility because hum-caused raster shading is generally associated with cathode-to-heater leakage in the rf, video if, detector, video amplifier and picture tubes only.
Another possibility that may be overlooked is cathode-heater leakage in an agc tube. The 60-cycle hum signal will be fed to the grids of the agc-controlled tubes and will receive passage through the various stages by modulating any rf or if signals present. Tube substitution (rather than a tube-tester check) should be used in this case, as well as the others cited, to determine the source of the trouble. In some instances, tube testers may not reveal small leak ages that are nevertheless large enough to cause trouble.
When cathode-to-heater leakage in a video or picture tube is severe, a complete or almost complete loss of video information will result, leaving only the hum bars visible In less severe cases, leakage between cathode and filament of a C-R tube can produce a light, faded picture overlaid by retrace lines. The brightness control may have no noticeable effect on the picture in such cases, since the cathode-to-ground hum overrides the normal bias of the picture tube.
If tube substitutions have eliminated other sources of trouble and the C-R tube seems to be the culprit, a preliminary check may be made before trying a time-consuming replacement. The test is performed with a scope and consists of measuring the hum voltage present between the picture-tube cathode and ground. If a substantial 60-cycle ripple is noted at this circuit point and this ripple disappears when the picture-tube cathode is disconnected from the circuit, a defective GR tube is indicated.
Similar scope tests may be made at the cathode of any suspected tube. No appreciable amounts of 60-cycle voltage should be measured at the cathode of a tube if cathode-to-heater leakage is absent.
More specifically, the 60-cycle ac voltage present between cathode and ground should not exceed the ripple amplitude that exists at the B-plus source feeding the tube in question.
When horizontal pulling is present in the picture and causes it to be bent into the shape of an S; no shading is seen in the picture or raster, and the raster sides are straight, cathode-to-heater leakage in a sync tube is indicated. The setting of the contrast control will, in many cases, have little effect on pulling of this type.
Horizontal synchronization will often be extremely critical and horizontal jitter or weaving may be seen most of the time. In other instances, cathode-to-heater leakage in a sync tube will cause the picture to lose horizontal phase; the horizontal blanking bar will become visible in the picture instead of remaining beyond the edge of the mask where it normally is positioned.
A slight amount of leakage in an afc or horizontal oscillator tube may produce no noticeable symptoms when a local station is coming in; in a network program a slow horizontal weaving may develop. Pulling so severe that it makes the picture virtually unrecognizable can occur when considerable cathode-to-heater leakage has developed in an afc circuit tube.
Some cases of corona and arcing are so closely associated with tubes that they deserve consideration in a guide of this kind. Corona and arcing may be unintentionally introduced during tube replacements and merit attention for this reason as well.
These troubles are important not only service-wise but also from a safety standpoint. A bad case of corona, for instance, can start a fire in the television receiver. At the time this was written, newspapers carried a report of the death of two children as the result of a fire that started in a TV set. To remain at peace with his conscience-and sometimes with his pocketbook-the service technician should know all he can about preventing such dangerous conditions and remedying them when they exist.
Causes and effects of corona and arcing
When high-voltage wiring is terminated at a relatively sharp point, the air immediately surrounding this point is exposed to a very high potential. The sharper the terminal, the more concentrated is the charge with respect to the surrounding air. Air ex posed to this high potential tends to ionize--i.e., to break up into negative and positive ions. The movement of ions in the air surrounding the high-voltage terminal is called corona; it causes a characteristic bluish or purplish luminescent glow. Ozone, formed as a result of the ionization process, has a destructive effect on many kinds of insulation.
Arcing is the name given to a condition where a discharge takes place between two conductors through the air between them. Most cases of external arcing occur between high-voltage points and adjacent ground.
Both corona and arcing reduce high voltage. Arcing will do this to a much greater extent than corona. These troubles also tend to introduce receiver noise whose effects are especially noticeable in the presence of weak incoming signals. Snow in the picture, crackles and buzzing in the sound, noise streaks, bright intermittent flashes on the screen, horizontal tearing and other impairment of horizontal sync can result from corona and arcing. Reduced brilliance due to high-voltage losses, intermittent or continuous in nature, may also be produced.
In areas where humidity is high, the tendency for corona to develop becomes greater because the dielectric constant of the air around high-voltage points is reduced, causing it to break down more readily.
Grit and dust particles are great promoters of corona. When such particles attach themselves to high-voltage conductive surfaces, they provide areas small in surface and high in potential-conditions very favorable to the development of corona.
Arcing occurs only when the insulation between two points whose potential difference is high is less than the minimum necessary to prevent arcing. Arcing may be intermittent or sustained in character.
Corona may be identified by the blue or violet discharge visible at its site, the hissing or sizzling sound it produces and the odor of the ozone created by the ionization of air. Arcing generally produces a snapping, crackling or popping sound.
Localizing corona
The detection of corona is not always simple since the fizzing or hissing sound it produces is barely audible under the best of circumstances and the bluish or purplish glow it creates is quite dim. To locate the source of trouble more readily, turn off all external lighting and reduce the brightness control setting. The resultant decrease in picture-tube current will reduce the loading on the high-voltage power supply, increasing the high voltage and intensifying the corona effects. If the area affected still remains invisible, a piece of fiber or bakelite tubing, about 1 1/2 feet long and 1 inch or so in diameter, may be used as a stethoscope of sorts.
Put one end of the tubing close to your ear; move the other end around in the area where corona is suspected or is likely. Look for corona at the point where the loudest sound (hiss) is heard.
Localizing arcing
Visual inspection is generally adequate to locate the site of an arc. Look in the high-voltage section of the receiver; when the arc is small, it will help to darken the room. When the arc is too small or infrequent to be readily observed, using a Variac type trans former to apply a larger line voltage to the set will increase the high voltage and promote a larger or more continuous arc.
Another method of locating an arc in the high-voltage is to place a sheet of some good insulating material into different positions in the high-voltage cage. When arcing stops, examination of the area will readily locate the two points between which arcing is occurring.
Arcing and corona associated with tubes
In locations where humidity is high, a breakdown may occur between a high-voltage rectifier and a tube adjacent to it. A polystyrene corona shield for the rectifier tube and socket assembly may be added to eliminate the condition. Zenith makes such a shield; possibly other manufacturers do too.
Corona may also develop on the outer surface of the glass envelope of the high-voltage rectifier. One method used to eliminate it is to wrap a strip of polyvinyl electric tape about 7 or 8 inches long and 0.25 inch wide around the tube. Beginning at the point where the prongs emerge from the tube, wind the tape upward diagonally to a distance of about 3 / 4 inch above the base. Go three times around the glass. Make sure the surface of the tape is perfectly smooth-wrinkles or sharp edges in the tape may nullify the beneficial effects of this procedure.
In some receivers, arcing between the 1B3-GT high-voltage rectifier plate lead and the glass envelope of the damper may destroy the damper. To avoid such arcing, the lead to the plate cap of the rectifier must be dressed sufficiently far away from the damper (and other tubes) so that arcing does not occur. A damper with a shorter glass envelope may be substituted for the one present in some cases. Similar arcing conditions may occur in other sets. Dress flyback and other high-voltage leads away from tubes to remedy or prevent these troubles.
Arcing between the glass envelope of a 6BG6-G horizontal out put tube and some terminals on the horizontal output transformer is possible. Proper insertion of the tube in its socket may prevent it.
Arcing due to improper placement of the plate clip on the high voltage rectifier may occur. If the clip is placed on the plate cap in an inverted position, the plate lead will be brought closer to the grounded walls of the high-voltage cage, tending to promote arcing. The tendency toward arcing in this case is further increased when the underside of the clip has sharp edges.
When corona or arcing between the high-voltage rectifier and the top of the high-voltage cage occurs and no lead dress changes can remedy it, waxing-or re-waxing-the lead going to the cap of the rectifier at the point where it enters the tube cap may eliminate the trouble.
Arcing between the plate lead of the horizontal output tube and the glass envelope of a nearby damper tube may occur. When adequate separation of the lead and envelope cannot be assured, add several inches of Fiberglass sleeving over the plate lead of the horizontal output tube.
There is an internal connection between pins 7 and 8 in most 1B3-GT rectifiers. If there is none, it is externally made at the sockets contacts. Absence of such an external or internal connection will cause arcing between the two tube prongs through the air or through the socket. When a 1B3-GT is to be replaced, make a resistance check on prongs 7 and 8 on the new tube. If the reading is infinite, make another ohmmeter check between the socket contacts that go to prongs 7 and 8. If a similarly open reading is obtained, either get a 1B3-GT type in which prongs 7 and 8 are internally connected or add a short wire at the socket, connecting the appropriate socket contacts to each other.
Arcing associated with tube plate-cap leads
The lead going to the plate cap of the horizontal output tube can be troublesome if it is not properly dressed. When it is placed too close to one of the walls of the high voltage cage, arcing will occur right through the insulation of the wire. When such arcing has been noted, replace the lead--its insulation characteristic probably been impaired--and dress the wire away from the cage walls, as well as from any nearby tube or other component.
To prevent breakdown between the horizontal tube plate cap lead and chassis (at the hole where the lead enters the chassis) rubber grommets are often inserted into the hole. A protective sleeving may also be present on the lead as well. Check for these safeguards, when arcing occurs at this point. If such safeguards aren't present, add them.
When snapping sounds are heard at intervals, and the trouble has been localized to intermittent breakdowns between the plate cap wire and chassis, high-tension auto-ignition type cable may be substituted for the wiring present. (It is assumed that re-orientation of the wiring has not completely eliminated the trouble.) Cables of this type will generally not arc even if they touch the chassis.
When a tube is replaced in the high-voltage section, the service technician should be extremely careful not to change the lead dress present, especially the dress of the horizontal amplifier and high voltage rectifier plate leads. Not only should hv leads be kept away from ground points-leads at or near ground potential should also be kept away from high-voltage wiring. If this precaution is not observed, corona spray (in mild cases) or arcing (in severe ones) can start the insulation on the wires burning, and cause a fire. Arcing from plate-cap clips that yields to no simpler solutions may be eliminated by substituting ceramic caps for the clips.
Corona and arcing at tube sockets
Corona at the socket of the high-voltage rectifier tube is sometimes due to a poor connection between the hv terminal at the socket and the corona ring. Resolder the connection, to test for as well as remedy the trouble. The corona ring-a round loop made of metal-is generally part of the socket assembly. Its purpose is to prevent corona from developing at the tube socket connection by keeping the air in the vicinity of the socket contacts at a high potential. The surface of this ring should be perfectly smooth; no bumps, scratches, etc. should be visible.
Arcing within a damper tube socket may occur when the boost voltage created by the tube becomes excessive. Such arcing is particularly likely to occur in receivers where the vertical deflection circuits are fed from the B boost supply. When the vertical output tube becomes defective, or when the height control is set at a low point, loading of the B boost supply may be decreased to a point where the boost voltage becomes too high; arcing may develop in consequence.
In severe cases of socket arcing, where no other defect can be found, the socket itself is no doubt the source of the trouble; re place it to eliminate the arcing. For maximum assurance that the trouble will not recur, change a molded bakelite type socket to a mica-filled unit. The insulating properties of the latter make it better suited to withstand high voltages.
Carbonization of a socket is, of course, a sign that it needs re placement. A check of the high-voltage filter capacitor is in order at such a time, to make sure that a leak in this capacitor is not responsible for the trouble.
Covering socket contacts with a high-melting point wax (avail able from Crosley, to cite one source) will often be helpful in eliminating socket corona or arcing. The wax is melted by heating it, then applied freely with a brush. The wax should not be heated to a temperature above 350° F. If no suitable thermometer is avail able, heat the wax just enough to melt it, and no further. A good grade of corona dope may be used, instead of the wax, to cover socket contacts.
Arcing inside the crt
Electrostatic-focus C-R tubes have a tendency to arc internally, during the first few days after they have been put into operation.
The condition is due to the presence of dust or other particles in side the tube; these particles work themselves into the gun structure and cause intermittent arc-overs. The arcs usually occur be tween the high-voltage anode and the focus electrode. The effect produced on the screen is a momentary blinking in the picture.
Secondary arcs can possibly be stimulated in other high-voltage areas, especially around the corona ring, by the original arcs. The high-amplitude current that momentarily flows through the B supply may burn out the fuse in the low-voltage section.
Insertion of a resistor of about 47,000 ohms in series with the focus electrode will eliminate the secondary arcs, and protect the fuse. The primary arcing may, however continue for some time- often several days-until the loose particles in the crt gun are all burned away.
Intermittent arcing may occur in any type of c-r tube, particularly when it is first placed in service. The condition does not necessarily indicate a non-remediable defect in the crt. In many instances, the short will burn itself out. In more persistent cases, the trouble can often be eliminated by removing the tube, holding it face down and lightly tapping the bell, either with the wooden end of a screwdriver, or the fingers (keeping the fingernails out of action). If the short cannot be cleared in this way, replacement of the crt will generally be necessary.
Eliminating corona and arcing at bell of crt
Corona or arcing may develop at the bell of the crt in different ways, and for different reasons. Arcing from the front of the picture tube to the picture window is possible. The existence of this condition is very likely if the arcing stops when the chassis is re moved from its cabinet.
In some receivers, the picture window is electrically grounded by means of a braided shield and spring connection to chassis.
Check for an improper connection here, if the type of arcing just described seems to be present. In other sets, arcing between tube and window may be due to the presence of dust. Clean the tube and window, to remedy the trouble in this case.
Defects in the polyethylene sheet which is used in some sets to insulate the picture tube from the window in front of it may be the cause of arcing. Check this unit visually. Clean it with carbon tet if it is dirty; replace it, if it seems imperfect in any way (i.e., has scratches, holes, tears, etc.) When replacing a cabinet-mounted sheet of this type, use short mounting staples. These staples should not be inserted too close to the edges of the sheet.
If any uncertainty exists as to whether the picture tube is the site of the arcing present, disconnect the 2nd anode lead from the tube. If the arcing (or corona) now stops, the trouble is definitely related to the crt. Corona or arcing at the anode button of an all-glass c-r tube is a common trouble. The trouble is generally due to the effects of dirt, or deterioration of the rubber suction cover on the 2nd anode connector. Certain types of rubber used in anode caps tend to decompose as a result of the high electrostatic stresses present at the anode button. The action increases their conductivity, permitting high voltage to leak onto a normally insulated area. A corona discharge between the button connection on the hv anode lead and the surrounding glass area tends to result, particularly when the air is damp or humid.
One remedy consists in cutting off the suction cap, leaving only the sleeve portion which supports the lead to the button connector. The rubber will no longer touch the glass, eliminating leak age onto the latter. The remedy just described is also helpful in cases where the suction cup has not deteriorated, but is promoting arcing (during humid weather) because of its accumulation of moisture.
To clean off the deposit on the glass made by the defective rubber, use a rag, water, and a mild cleaning material, such as Bon Ami. (Water alone may not remove the chemical deposit). Be particularly sure to clean the well or receptacle into which the hv connector fits. Drying, then polishing the area that has been scrubbed clean will insure that no residue is left to cause future trouble.
Carbon tet may be used to clean off the deposit, instead of scouring powder; or both may be used, the scouring powder first, then the carbon tet (after the area has dried). The addition of an anti-corona preparation to the area will serve as additional insurance against a recurrence of trouble here. A similar cleaning and coating process will eliminate corona and arcing at the crt anode receptacle (due to accumulations of dirt). The bell of the tube should, when necessary, be cleaned with a damp rag, then dried.
Elimination of the suction cover (as previously described) is not always desirable, since dust cannot be as readily kept out of the sensitive area around the anode button with this cover missing.
In cases where excessive dust accumulation is likely, the best service procedure would be to replace the anode connector and lead (when the suction cup on the hv lead has deteriorated). Use of a connector and lead assembly that provides a Neoprene, corrosion resistant suction cover is desirable under the circumstances.
In cases where connector and lead are replaced (as well as in instances where sources of corona are being looked for) make sure that the soldered joint on the anode button terminal to which the anode lead connects is smooth, and has no sharp points.
When the service technician wishes to retain the dust-proofing benefits of a worn or cracked suction cup, several short layers of hv tape should be used to press the cup firmly in place against the bell of the crt.
Arcing may be due to a loose connection at the anode receptacle of an all-glass crt; spreading the prongs on the anode connector slightly will eliminate such trouble in many cases.
Re-waxing the high-voltage cable at the point where it enters the rubber suction cup will help prevent corona spray in humid weather.
In some instances, visible evidence of arcing (streaks in picture, or other symptoms) may be present, but the site of the trouble cannot readily be located. Push back the rubber suction cup, in such cases, turn the set on and look, as well as listen for, arcing at the anode receptacle on the crt. Sometimes arcing occurs between the anode terminal in the c-r tube and the internal graphite coating. Replacement of the crt is the only cure for such trouble.
Arcing at metal C - R tubes
Metal-cone picture tubes tend to develop arcing conditions in the area of their glass bell, due to the presence of dust and moisture here. A method of dealing with this problem used by RCA service people consists of removing the entire coating present on the glass bell, through the application of methanol or acetone. The area is then washed with a good detergent and dried thoroughly; a liberal coating of Johnson's Car-Plate is then applied to it (with a brush). When the coating has dried, the white residue is wiped off.
Arcing due to improper crt grounding
An outer Aquadag coating is used on many glass c-r tubes as a shield. Two hair-pin type springs are often used to connect this Aquadag coating to ground. When the contact between the springs and the coating is poor, arcing in this area (as well as excessive sync buzz, or other symptoms) can be produced. Correct such trouble by pushing the c-r tube into better contact with the springs, or replacing the springs, if the latter are defective.