WHEN trouble is encountered that affects both the horizontal and vertical synchronization of the receiver, make sure to check symptoms on more than one station setting. Not infrequent ly, station trouble is present--i.e., the blanking and sync signals sent out at the transmitter do not have the correct amplitude relationship to the video portion of the composite signal. A check of reception on several channels will indicate whether it is so.
Unstable sync can be due to signal distortions set up outside the receiver. Undesired additions and cancellations of signal at the antenna site (caused by improper antenna orientation) may be producing such trouble. A test can be made by substituting an other antenna for the existing one (an indoor type will usually do). When tests point to the existence of trouble in the receiver proper, tube substitutions are in order.
A good preliminary check to make is an inspection of the sync pulse in the vertical and/or horizontal blanking bar (see Figs. 414 and 501). The brightness control should be turned up somewhat to permit the shading of the sync and blanking pulses to become visible. If there isn't much difference in contrast between the sync pulse and the blanking bar, as shown on the screen of the picture tube, sync compression is indicated.
The test cited will indicate whether the cause of trouble should be looked for in the sync or video signal stages. The presence of sync compression points to overloading in a video signal stage.
The various tube substitutions recommended in the section de-voted to overloading ( Section 4) should then be tried. If sync compression is not evident--that is, if the sync pulse is markedly blacker than the blanking signal and the darkest portions of the picture--trouble between the video amplifier and the sweep oscillator in the sync section of the receiver is indicated. Tubes in these stages should be substituted as a test for the trouble.
Fig. 501. Horizontal blanking bar is black when brightness and contrast control
settings are normal and no defect is present. The horizontal sync pulses are
not visible.
Poor immunity of the sync system to noise may be due to reduced agc voltage. When noise streaks, tearing of lines and/or vertical jitter is observed in the picture, note whether overload symptoms are present as well. If they are, a defective agc tube may be the source of the trouble.
Unstable sync due to noise may be due to a defective noise canceller or inverter tube. Substitute another anti-noise tube, if one is used in the receiver, and the symptoms just cited are present.
A defective video detector crystal may be responsible for poor or erratic sync. Before substituting another one, a scope may be used to monitor the output of the crystal. If the sync pulse output is insufficient, erratic, or improperly shaped at this point, and substitution tests of tubes in previous stages have revealed no trouble, replacement of the crystal may be tried. Substitution of a 1N34 for a 1N60 or 1N64 type crystal may be made for maximum improvement in synchronization.
In a receiver using stacked tubes, a bad audio output tube may cause an impairment of vertical and horizontal synchronization because sync tubes (as well as the video if and others) may derive their B supply voltages through it. When sync is poor and sound is distorted or absent in receivers of this type, try a new audio out put tube before making other tests.
Trouble in a sync tube (particularly in a relatively high-current unit such as a high-gain sync amplifier in a stacked-tube receiver) may affect the sound as well as the synchronization. The bias of the audio output tube in a circuit like the one shown in Fig. 302 will be affected by a change of current in the tubes in series with it. If sound and sync symptoms are both present, then substitution of sync as well as sound tubes should be tried. Study the set schematic to determine which tube substitutions are most logical.
Trouble in video if tubes may affect voltages in sync and sound stages; video if tube substitutions are therefore in order in stacked tube sets when sync trouble exists.
A condition known as sync lockout is a possibility in receivers employing keyed agc circuits. It manifests itself as a loss of horizontal and vertical synchronization. Various sources may be responsible so let us consider the general nature of the trouble to understand better which remedies are logical.
Fig. 502-a,-b. Clipping of sync pulses due to overloading of a video if tube.
The condition is most likely to occur in the last video if amplifier where
the if input signal is highest.
Normally, when a receiver is switched from an unoccupied to an assigned channel, the agc voltage goes from zero to some substantial value fairly rapidly. When a weak agc tube (or other circuit trouble) is present, however, the agc voltage may build up too slowly, possibly causing it to be smaller than it should be in the first few seconds after the station switch. As a result, the bias of agc-controlled stages is too low and clipping of the sync pulses tends to occur. See Fig. 502.
Such clipping can remove the sync pulses from the composite video signal. Under these circumstances the agc keyer tube, instead of having flyback pulses applied to its plate and sync pulses to its grid, will be fed only by flyback pulses. Its conduction will there fore drop and the agc voltage output will either be very low or absent. Overloading and sync pulse clipping will result, and the receiver will be "locked out" of synchronization.
The lockout may sometimes lift if, just after a switch from an unoccupied to an occupied channel, the sync and flyback pulses happen to be in phase. Under such circumstances, the agc voltage at the output of the keyer tube will build up more rapidly and may reach a level large enough to prevent lockout in the short time it has to do the job.
The logical tube substitutions to make in the event of intermit tent or continuous sync lockout include the agc tube as well as ones that can affect the agc voltage, such as rf, video if, video detector and video amplifier tubes.
Sync lockout is particularly likely to occur in a receiver using a noise inverter or noise canceller. A tube of this type is normally biased to be nonconducting in the presence of sync pulses. If the bias applied to the inverter is not correct, it may conduct on sync pulses as well as noise, eliminating (in some circuits) or inverting (in others) the sync pulses normally applied to the sync stages.
Synchronization is prevented in consequence and lockout occurs.
Since the bias of the anti-noise tube depends in part on the in coming signal level, any condition that greatly changes the signal level may produce lockout of this type. Defects in the rf and video if detector or amplifier or agc tube may thus cause the symptoms described.
A defective anti-noise tube or a bad sync tube can be the cause of lockout. If the anti-noise tube becomes gassy, its resultant reduction in bias may cause it to conduct on sync pulses as well as noise, inverting or eliminating the sync pulses.
In some instances an excessive length of time is required for vertical and horizontal synchronization to become normal. The trouble may lie in a slow-heating sync tube. If sound, picture or raster also take a long time to become normal, a defective low voltage rectifier is indicated. Substitute another rectifier or try sync tube substitutions to test for such sources of trouble.
Line-voltage changes may be the cause of unstable synchronization--i.e., tendency of the picture to lose horizontal and vertical sync at intervals, coupled with increased difficulty in setting the hold controls properly. A test of set operation at various line-volt age levels (permitted by the use of an autotransformer with a variable output) will determine whether this is the case. When the presence of such trouble at low line voltage has been verified, try replacing the deflection oscillators with several other tubes of the same type but of different make. It is often possible to eliminate symptoms by using as an oscillator a tube with enough reserve emission to operate properly even in the presence of a substantial drop in line voltage.
Vertical sync trouble
When vertical synchronization is absent or unstable (i.e., when continuous or intermittent vertical roll or jitter is present) one of several stages may be the problem. Clues provided by inspection of the picture or test pattern or by various localization tests will often help to narrow down the area in which to substitute tubes.
A helpful preliminary test when continuous roll is present is to remove a sync amplifier tube and note whether the roll can be stopped, even momentarily, by manipulation of the vertical hold control. (A "dummy" tube of the proper filament voltage rating, with prongs other than filaments cut off, may be used in place of the sync tube in series-filament circuits to simulate sync tube removal.) If roll is stopped even for an instant, the vertical oscillator can be eliminated as a possible source of trouble. If it cannot, substitution of another vertical oscillator tube is called for.
An exception must be noted at this point: If the vertical hold control in the test just described can be set to produce momentary cessation of roll only at the extreme end of its range, the vertical oscillator cannot be absolved of blame.
If trouble in the vertical oscillator is not indicated by the fore going test, quickly inspect the vertical blanking bar for evidence of sync compression. If such compression is apparent, trouble in any of the stages through which the video signal passes or the agc stage may be the source of the symptoms. Try the tube substitutions in the sync stages that pass the vertical, as well as the vertical and horizontal, sync signals.
Some readers may wonder at this point why a tube which passes both vertical and horizontal sync signals should affect one but not the other when it becomes defective. Part of the answer lies in the fact that a sync tube may amplify one type of signal more than the other due to the frequency response characteristics of the stage involved. The presence of a very stable afc circuit may also promote adequate horizontal synchronization even when the horizontal sync pulses are reduced in amplitude, whereas nothing comparable may exist to aid vertical synchronization when the vertical sync pulses become smaller than normal.
Cathode-to-heater or filament-to-grid leakage in an rf or video if stage may be responsible for an impairment of vertical sync.
Such leakage decreases the agc voltage and thus tends to introduce sync clipping. Tube substitutions in the stages referred to will test for this source of trouble. In some instances the presence of a rather faint horizontal hum bar in the picture (but not in the raster) will point to such leakage as the culprit. Roll produced by cathode-to-heater leakage may be intermittent or continuous; it is more likely to be noted on network programs than on local ones.
A weak rf or video if tube may, also be the source of vertical sync trouble; insufficient agc voltage is developed in such a case, promoting sync clipping due to overloading.
A weak video amplifier may, by reducing the amplitude of the sync pulse fed to the vertical oscillator, be the cause of roll or vertical jitter.
A loss of emission in an audio output tube can promote roll.
In a conventional receiver, unstable vertical synchronization may be caused by overloading, resulting indirectly from the increase in B voltage that occurs when the audio output tube loses emission.
When a receiver with stacked tubes is producing roll, the fault may be due to the change in the supply voltage of one or more sync tubes. Such a change is produced when the audio tube is in series with the sync tube(s) and acts as a voltage-dropping resistor; a decrease in its plate current is equivalent to an increase in its resistance.
Low output from a crystal used as a video detector may be the cause of roll. A scope test at the output of the crystal will indicate whether grounds exist for trying a crystal substitution. It is assumed that preceding tubes will be eliminated as a possible cause of low output before the more time-consuming crystal substitution is resorted to.
Rolling due to 6BL7-GT characteristics
Early-make 6BL7-GT type tubes used as vertical oscillators tend to introduce rolling because of design imperfections. Symptoms may be described as follows: the picture is steady for some time after the receiver has been turned on, then starts to roll. The roll can generally be eliminated by resetting the vertical hold control.
A short time later, rolling will begin again, generally disappearing when another resetting of the vertical hold control has been made.
Stable operation will now follow unless the set is turned off and then on again.
In other cases, the symptom may be slow stabilization of vertical synchronization. A warmup time of 25 to 60 seconds may be necessary before roll stops and stable vertical synchronization is obtained.
The reason why 6BL7-GT tubes sometimes give trouble when used as vertical oscillators lies in their tendency to develop grid emission. Such emission changes the frequency at which they operate, due to the change in bias produced by the emission.
Improved versions of the 6BL7-GT are currently being manufactured; substitution of an improved for an early type tube will in most cases solve the problem. Replacement of the 6BL7-GT with a 6BX7-GT may also be resorted to as a means of eliminating the symptoms. A wiring change will not be necessary when such a substitution is made.
Tube substitutions to improve vertical stability in weak-signal areas In weak-signal areas, vertical stability can often be improved by substituting higher-gain tubes for the ones present in sync (as well as in video signal-carrying) stages. When a 12AU7 is being used as a sync amplifier, for example, a 12AT7 may be substituted.
Vertical jitter due to lead dress
Sometimes the technician during the course of a tube substitution may move a horizontal yoke or flyback lead too close to the vertical oscillator tube. The resultant coupling of horizontal signals into the vertical circuit may cause the oscillator to trigger at the wrong times, causing a rapid vertical jitter to be produced.
The remedy, of course, lies in redressing the improperly placed lead.
Vertical blanking bar in pix
When a steady picture is received in which the vertical blanking bar is visible, the condition may be due to cathode-to-heater leakage in a sync tube. (It's assumed, of course, that the condition cannot be permanently eliminated by suitable adjustment of the vertical hold control.) The vertical oscillator may be prematurely triggering on the hum voltage instead of on the vertical sync signal, causing the blanking bar to appear at the wrong place. To check on the source of trouble, try substituting other tubes for the ones present in the sync stages.
Single vertical pix unobtainable
When lock-in can be obtained only for two or more frames (and not for one), a defective vertical oscillator tube may be present.
Try substituting another tube as a test.
Poor interlace
Interlace refers to the meshing of the two fields in every picture frame. Slight defects in vertical synchronization may upset inter lace and detail will be lost since the lines of one field will overlap, in part or in full, those of the other, masking out picture information.
A loss in interlace can be readily detected when a test pattern is present by inspecting the horizontal wedges. When a moire (see Fig. 503) is noticeable--i.e., circular movement in the horizontal wedges-interlace is being lost.
In the absence of a test pattern, a loss in interlace may be detected by inspecting the scanning lines of the picture or raster.
If the dark spaces between scanning lines are nonuniform in width (most obvious when two successive spaces are compared) or the spaces are too wide, interlace is imperfect. See Fig. 504-a,-b. The condition will also affect the vertical retrace lines, causing these to pair off. Advancement of the brightness control to the point where the retrace lines become visible will permit their inspection.
Fig. 503. Mottling of the horizontal wedges (moire effect) due to poor interlace.
The first thing to check when interlace is imperfect is whether the setting of the vertical hold control is correct; improper adjustment may be responsible for impaired interlace. If resetting of the control does not clear the trouble, the corrections referred to in the paragraphs following may be made.
Sync compression due to slight overloading in an rf or video if amplifier can be the reason for poor interlace. When the compression present reduces the vertical sync pulse amplitude to the point where proper sync separator action is prevented, video signals will get through the separator from time to time, causing a correspondingly intermittent loss in interlace. Defective tubes in rf or video if or amplifier stages may be the cause of overloading and loss of interlace; a defective agc tube can also create these symptoms. Check these tubes by substitution if inspection of the blanking interval in the picture indicates that overloading exists.
Fig. 504. Good interlace is shown in the photo at the left, but in the photo
at the right the scanning lines of one field fall over those of the second
field. The dark spaces between lines become wider, more visible.
A loss of emission in a sync tube can reduce the vertical sync pulse to a level where it will provide proper oscillator triggering only over a smaller-than-normal vertical hold control range. Loss of interlace due to an improper setting of this control therefore becomes more likely under such circumstances. Insufficient output from a sync tub, due to reduced emission may also lead to improper sync separation. (Proper sync separator action depends on the feeding of an adequately large sync pulse to the input of the separator.) Video signals may get into the vertical oscillator as a result, causing intermittent losses in interlace. The presence of undesired video signals in the input to the vertical oscillator is particularly likely if interlace becomes improper only when a large area at the bottom of the picture suddenly becomes dark due to a change in scene. Check sync tubes by substitution to determine if such temporary losses in interlace are due to a weak or other wise defective sync tube.
An intermittent loss in interlace may cause a slight vertical jitter in the picture. Or it may produce a rather rapid pairing and un pairing of the horizontal scanning lines.
Intermittent losses of interlace may occur when noise is present in excessive amounts. Holes may be punched into the sync signals by the noise. If a hole occurs during vertical sync pulse time, proper triggering of the vertical oscillator will not take place at this time and interlace may thus become improper. Check for noisy tubes or a defective anti-noise tube when interlace is not normal, particularly when waveform tests in sync stages indicate that excessive noise is present. (See Fig. 505).
Fig. 505. (a) Normal sync waveform. (b) Sync waveform with holes punched
in it.
Fig. 506. Complete loss of horizontal synchronization.
A continuous impairment of interlace may be due to the en trance of horizontal sweep signals into the vertical oscillator. The condition may be the result of improper lead dress or the absence of a shield on the vertical oscillator tube, among other things.
Leads most likely to cause trouble are the plate leads of the horizontal amplifier, the high-voltage rectifier and the "hot" lead of the horizontal yoke. Be careful not to change the original placement of such leads during a tube substitution. A missing shield is--most likely to produce symptoms when a metal picture tube is used. This type of tube is an effective radiator of horizontal sweep signals.
Horizontal sync trouble
When horizontal synchronization is completely absent and can not be brought in by manipulation of the horizontal hold control (see Fig. 506), trouble in the afc or horizontal oscillator section is indicated. A defect in one of the sync stages through which both vertical and horizontal sync pulses pass may also be the cause. Substitute tubes in these sections to check on this possibility.
If it is possible to remove the afc phase-control tube without disabling the horizontal oscillator or eliminating the picture, a preliminary localization test may be made. To make such a test, withdraw the afc tube referred to and see if a single stationary or almost stationary picture can be brought in. If it can, the horizontal oscillator stage is normal and trouble in the afc or sync is likely.
Horizontal sync instability
Fig. 507. A case of severe picture "hook" in the test pattern.
A slight horizontal movement of the entire picture, or a section of it, may be due to overloading. Overloading tends to cause sync compression which reduces the height of the sync pulse with respect to the rest of the composite signal. The vertical or horizontal blanking interval may be inspected on the screen for evidence of compression as described previously. If compression is noted, overloading due to an agc tube defect or a fault in any of the video signal-carrying stages may be responsible. Substitute tubes in these stages as a check. If compression is not noted, substitute tubes in sync stages as a test for trouble here.
Horizontal instability is particularly likely to be caused by a defective reactance tube. Some 12AT7 tubes used in such applications tend to develop grid emission, causing the horizontal oscillator to drift off frequency, thus introducing unstable synchronization. Presence of a positive-going grid voltage with the coupling capacitor disconnected will indicate whether the tube is causing such trouble.
A microphonic tube in the horizontal section may cause horizontal instability. To test, gently tap each of the horizontal-section tubes in turn. If the trouble is intensified when one of these tubes is struck, substitute a new tube and note results.
Horizontal pulling
A certain type of horizontal movement in the picture (see Fig. 507) is referred to as pulling, snaking, bending, hooking or S distortion. Straight vertical sides of objects in the picture are bent when such a defect is present. This should not be confused with tearing, wobble or jitter.
Loss of emission in an afc or other sync tube is a likely cause of pulling and should be checked for by tube substitution.
Cathode-to-heater leakage in an rf amplifier or video if tube may be the source of horizontal pulling. Try replacing such tubes as a test, particularly when hum bars are present on the screen. Cathode-to-heater leakage in sync or horizontal sweep tubes may also be responsible for the trouble. Cathode-to-heater leakage in the damper can very easily pull the raster into the shape of an S. Pulling of the upper third of the picture is not infrequently due to cathode-to-heater leakage in the horizontal output tube. A similar bending or curling at the top of the picture may also be due to cathode-to-heater leakage in the horizontal oscillator.
Grid emission or the formation of gas in a tuner, video if or sync tube can cause pulling. This problem generally occurs after the receiver has been in operation for some time. If bottom-chassis checks can be made readily, the presence of such a defect may be verified by testing (1) the voltage drop across the agc resistor and (2) the grid voltages of suspect tubes not controlled by the agc system. The voltage drop measured across the agc resistor should not appreciably exceed 0.5 if it has no dc return to ground. A drop exceeding this points to the presence of gas in one of the tubes fed by the agc line. The grid voltages of suspect tubes outside the agc system should be adequately negative with respect to the cathode.
Pulling or picture hook may be due to a defective crystal that introduces sync compression. A dark picture will often be an associated symptom. If a scope waveform check of a crystal detector reveals that the sync portion of the composite video waveform is compressed, try a new crystal in place of the original one.
When the area in which pulling occurs varies with changes in scene, improper sync separation is indicated. Change the sync separator tube to determine if the trouble lies here.
Some 12AU7 tubes used as horizontal oscillators have a marked tendency to introduce a bend in the picture. The bend may be either stationary or move slowly up or down. Leakage between the filament and another element in the tube is responsible for the symptoms. Some 12AU7 tubes are much more prone to develop this condition than others; it is therefore desirable that (1) several makes of tube be tried in making substitutions and (2) several samples of the best make be tested in the set to obtain a tube with the smallest possible leakage.
Elimination of pulling in cases where the strength of incoming signals is excessive may sometimes be obtained by judicious tube substitutions. Use of lower-gain tube types in sync stages is a logical remedy when overloading of such stages is causing the trouble. Tubes with less "hop" may also be tried in video if stages when overloading of these stages is not due to a tube or circuit fault. Such substitutions are, of course, not recommended in locations where weak as well as strong signals are received.
Horizontal tearing
Fig. 508. Tearing out of lines at the top of the picture. The condition may
be intermittent or practically continuous.
When regular portions of picture information are displaced at the left side of the image, try substituting a new horizontal oscillator tube. If the picture tears horizontally toward the right side of the screen, look for a defective horizontal output tube. A defective horizontal amplifier may also be the source of trouble when entire sections of the picture are displaced. Displacement of regularly spaced segments of picture information from the picture proper is likely to be caused by a damper tube defect.
Tearing out of lines at the top of the picture (Fig. 508) is due to insufficient agc voltage. If the contrast and agc control settings are correct, substitute a new agc tube or tubes.
Tearing, particularly intermittent tearing, may be due to a noisy signal-carrying tube--i.e., an rf or video if, detector or amplifier tube. Tapping and/or replacement tests of tubes in such stages may be employed to locate the source of trouble. Horizontal tearing may also be produced by a defect in a sync or horizontal circuit tube.
Filament-to-cathode or filament-to-grid leakage in an rf or video if tube may cause tearing out of several horizontal lines.
Hum shading may not be apparent in the picture when such leakage is present, particularly if the leak is a high-resistance one.
Filament-to-grid leakage in 6CB6 tubes used in video if stages can be the cause of line tearing and should be checked for by tube substitution.
When line tearing is present in a receiver using an anti-noise tube, such as a noise canceller or noise inverter, replace the tube and note results. In operation of such tubes sensitizes the receiver to noise.
Tearing may sometimes be due to grid-to-cathode leakage in the picture tube. When no other sources of trouble can be un covered, substitute a good tube and note results.
Tearing out of several lines may be due to a corona discharge taking place at the picture tube or elsewhere. To determine whether corona is responsible for the tearing, inspect waveforms in sync stages with the high voltage first connected and then disconnected. If noise pulses are associated with the sync pulses only when the high voltage is connected, corona is indicated. Localization of corona trouble is described in a separate section.
Corona due to improper spacing of the high-voltage rectifier's socket lugs may produce lines in the picture (Fig. 509) similar to those produced by auto ignition interference. Inspect the tube socket terminals of the high-voltage rectifier to check on this source of trouble.
Horizontal jitter
When sideways picture jitter is encountered, a preliminary check should be made of the adjustment of the contrast, horizontal hold and afc controls. If the jitter is associated with displacement of sections of the picture, a new horizontal amplifier should be tried. Horizontal jitter may be due to a defective horizontal oscillator or a noisy or microphonic tube in the rf, video if, video or sweep sections of the receiver. Horizontal jitter (Fig. 510) may be due to cathode-to-heater leakage in a sync tube. Yoke leads may accidentally be dressed too close to the picture-tube grid lead during a tube replacement, introducing jitter. Separate the leads. Symptoms associated with controls When horizontal lock-in of the picture is obtained only at the mid-setting of the horizontal hold control and a loss in sync occurs Fig. 509. Lines in picture due to auto-ignition interference. The picture has been retouched to emphasize the effect. Similar lines may be caused by corona trouble at the high-voltage rectifier socket.
at either side of this setting, trouble in the afc section is indicated.
Try new tubes in this section, starting with the sync discriminator.
When poor horizontal synchronization is associated with an appreciable decrease in the lock-in range of the horizontal hold control, trouble in the sync separator, amplifier or limiter is likely.
A defective afc tube can also be responsible for these symptoms.
If the horizontal hold adjustment is very critical, a defective agc or afc tube may be to blame. Critical hold and unstable sync may also be the result of a drop-even a small drop-in line voltage. Testing the set on a variable source of line voltage will help the service technician determine if this is actually the trouble.
Replacement of the horizontal oscillator with a tube having re serve emission--i.e., greater emission capabilities in the presence of reduced line voltage-will often remedy such a condition.
Miscellaneous horizontal sync symptoms When horizontal synchronization becomes normal only after a long warmup period, low emission in the afc or horizontal oscillator tube may be present. This is especially likely when the two are dual sections of one tube. A faulty selenium rectifier in the set's low-voltage power supply may also be the culprit. A voltage check will reveal whether the selenium rectifier output is below normal during the long warmup interval.
An erratic picture that shifts from left to right calls for substitution of the afc tube (s) present.
The presence of vertically moving notches in the picture and raster may be due to a defective damper, particularly when this tube is a 6AS7-G.
Fig. 510. Horizontal jitter or weaving. The trouble in this case was due
to a short between cathode and heater of a sync clipper-amplifier tube.
If the picture is stable to begin with but loses phase and finally tears out completely in the horizontal direction, a loose grid cap on a 6BG6-G type of horizontal amplifier may be the source of trouble. Reheating the cap will generally cure the defect. Try this only if a tube substitution has definitely localized the fault to this tube.
Poor or missing horizontal synchronization may be due to a gassy picture tube-try another tube as a test if no other source of trouble can be located.
A defective horizontal output tube may cause circles in the test pattern to resemble cogwheels and be responsible for vertical or diagonal lines looking jagged.
A horizontally out-of-phase picture (Fig. 501) that cannot be made normal by suitable manipulation of the horizontal hold or afc control points to a defective afc section. Substitute tubes in this section to check on this possibility.
Improving horizontal sync stability by tube substitutions In fringe locations, an improvement in horizontal synchronization will be effected if higher-gain tubes are substituted in video signal-carrying stages. A 12AU7 video amplifier tube may, for in stance, be replaced by a 12AT7.
Flashes in picture
When flashes in the picture are observed in receivers using a cascode type tuner, the rf amplifier (generally a 6BK7-A, 6BQ7-A, 6BZ7 or similar tube) is often responsible. Less frequently, a defect in the converter tube can introduce the symptoms. A tuner tube fault (intermittent short) is particularly likely if the flashes or white streaks seen in the picture decrease in intensity as the set warms up.
To determine quickly whether the tuner or some other section is the site of the defect, switch the receiver to a blank channel. If flashing is no longer seen, a tube in the front end is most probably the cause of the trouble. When flashing persists at the blank channel setting (as well as on every other channel) there may be a bad horizontal output, damper or front-end tube. Defects in the damper tube may, in certain receivers, cause popping sounds in the speaker as well as flashes in the raster and picture.
"Christmas-Tree" effect
In the presence of certain defects, the picture and raster may break up into several sections of varying width. Their faint re semblance to a Christmas tree has led to the symptoms being labeled "Christmas tree" effect. When the breakup is more severe, the raster may disintegrate into a series of bright horizontal lines that cover the central portion of the raster (Fig. 511) and occupy about 50% of the normal raster area. A loud singing or frying noise coming from the horizontal output transformer often ac companies the symptoms just described. These symptoms, incidentally, are most often seen (in certain cases of trouble) when the station selector is set at an unoccupied channel or when channels are switched.
Since the symptoms effectively represent a loss of synchronization we are grouping Christmas-tree effect with other sync troubles. One possible cause of the condition is regeneration in the video if amplifier section. In such cases, when the station selector is set to an assigned channel, a vertical band may be seen at the left edge of the picture. This band or bar contains diagonal stripes of varying width; the stripes are alternately light and dark and cause the bar to resemble a barber pole.
Try replacing the tubes in the video if and agc sections to determine whether a defective tube here is responsible. A defective tube in the afc section may also produce a Christmas-tree effect and should be checked. The author knows of one case where a weak video amplifier was responsible for the condition, apparently because the sync pulse input to the afc section was inadequate. A bad vertical oscillator or amplifier tube may be the source of the trouble. So can arcing in the high-voltage rectifier socket.
Fig. 511. "Christmas-tree" effect.
Placement of horizontal yoke leads too close to the picture-tube may cause horizontal sweep signals to be coupled into its input (which is effectively part of the output circuit of the video amplifier); from there, these signals may get into the sync stages and produce a Christmas-tree condition. Proper lead dress will remedy such trouble.
Raster sides rough
A raster that looks very rough at both edges may be produced by parasitic oscillation in the horizontal amplifier. We are grouping this symptom with other sync troubles because it may be in correctly diagnosed as a sign of sync noise.