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Fundamentals of Color TV - Tri-Gun Receiver Circuits
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 expressly verboten. In fact, going into
a friend's house for any reason was rare. The privileged appointments were strictly
adhered to and monitored by my friend's mother. After a seeming eternity of
days, it was finally my turn to watch color television. The Flintstones were
on that particular afternoon. Discovering that Wilma had red hair was unexpected
enough, but a purple Dino the dinosaur nearly rocked me back on my 10-year-old
heels. Of what other essential details in the age of 'Living Color' had I been
deprived, I wondered?Fundamentals of Color TV Tri-Gun Receiver CircuitsBy Milton S. Kiver Part 4. Tuner, video i.f., video amplifier, and sound circuits of typical color TV sets described in detail.
R.F. Tuner. The introduction of color in no way alters or modifies the r.f. section of the television receiver. Thus, the r.f. amplifier should still possess high gain and low noise; the oscillator still provides a signal which, when mixed with the incoming signal, will produce the desired difference or video i.f. frequencies. For the reception of v.h.f. signals, either a turret tuner or a continuous arrangement is employed. For u.h.f. reception, continuous tuning is the most common method although there is also available an 82-channel turret tuner. A typical v.h.f. turret tuner circuit is shown in Fig. 1. Cascode amplifiers are common in the r.f. stage, although some manufacturers favor single high-frequency miniature pentodes. The oscillator tube is invariably a triode, usually half of the mixer tube. The latter may be another triode (i.e., 1/2 of a 6J6) or pentode (1/2 of a 6U8). This arrangement requires only two tubes for the entire tuner section. In the tuner shown in Fig. 1, the cascode r.f. amplifier uses a 6BZ7 duo-triode. One section of a 6J6 serves as the mixer while the other section functions as the oscillator. Balanced 300-ohm and unbalanced 75-ohm (coaxial line) input impedances are provided by a center-tapped primary winding, L101A. All signals must pass through a high-pass filter designed to attenuate all signals below channel 2. The secondary winding, L101B, is tuned by the input capacity (of the first triode unit) in series with alignment trimmer C105 Loading of L101B by R101 provides the required bandpass, particularly on the lower v.h.f. channels. The a.g.c. bias is applied to the first triode of V101 through decoupling resistor R102. Direct coupling is used between the first triode plate and the second triode cathode. This is normal in cascode circuits. With cathode feed to the second triode, C103 is used to place the grid at r.f. ground potential. Since the two triode sections of V101 are in series across a common plate supply, the cathode of the second triode is positive with respect to chassis ground. A divider across the "B+," consisting of R103 and R111, places the grid of the second triode at a sufficiently positive potential (with respect to its cathode) for proper operating bias. The signal at the plate of the second triode of V101 is inductively coupled into the grid circuit of the mixer. At the same time, a voltage from the oscillator is similarly brought into the mixer circuit. The mixer combines both signals to produce the desired i.f. and then transfers this signal to the following i.f, stages. The oscillator is of the ultraudion variety with a front panel fine-tuning control. Video I.F. Section. The video i.f. system follows the r.f. tuner. This will consist, usually, of four and sometimes five separate stages. See Fig. 2. In the conventional black-and-white television receiver, three i.f, stages was the number most frequently used, although four stages were found in some sets. The increased number of i.f, stages in a color receiver stems, in part, from the wider bandpass required (4.2 mc.) and from the greater precautions that must be taken to insure that the response curve will possess the right form. The desired response curve for the video i.f. section is shown in Fig. 3. Of particular interest is the care with which the low frequency end of the curve must be shaped so that it provides the proper amplification for the color subcarrier and its sidebands. Note that the curve is flat down to approximately 41.65 mc, and then the "roll-off" is quite steep. The steep decline is needed to prevent the sound carrier from receiving too much amplification, producing a 920-kc. beat note at the video second detector which would appear on the screen as an interference pattern. Furthermore, too much sound voltage at the detector will produce a fine-grained 4.5-mc. pattern on the screen and/or sound bars. The latter effect, of course, can occur in all television receivers, whether they be of the black-and-white or color variety. The 920-kc. interference, however, arises only when a color signal is being received.
Fig. 1. - Typical r.f. tuner used with color TV receiver. This is a turret-type unit for v.h.f. only, however combination v.h.f., u.h.f, models are also used.
Fig. 2. - The video i.f. circuits of one color TV receiver. Four stages are used here 10 assure a wider and more uniform bandpass than for black-and-white sets. Video i.f. systems in color receivers follow the same practice as for black-and-white receivers in so far as interstage coupling is concerned. Most common types of coupling are bifilar coils and/or single wound coils. For example, the circuit of Fig. 2 uses bifilar coils predominantly (T201, T202, T203 and T204), but two of the tuned circuits have single-wound coils (L108 and L201) The interstage coils are stagger-tuned, ranging from a low frequency of 41.4 mc. to a high frequency of 45.5 mc. Also present are five shunt traps, three tuned to the sound i.f. signal of 41.25 mc., one to the video carrier frequency (39.75 mc.) of the adjacent higher channel, and one to the sound carrier frequency (47.25 mc.) of the adjacent lower channel. A number of sets resort to complex coupling circuits in one or more i.f. stages in order to obtain the desired attenuation at certain trap frequencies, such as the adjacent-channel video carrier, adjacent-channel sound carrier, and the sound carrier of the channel being received.
Fig. 3. - Video i.f. response curve of a color TV receiver. Note the steep slope of the curve between 41.25 and 41.65 mc.
Fig. 4. - Five stage video i.f. system employed by RCA in its color TV sets.
Fig. 5. - Sound i.f. and audio circuits of a typical color television receiver. In one RCA color receiver, a bridged-T circuit is inserted between the tuner and the first video i.f. amplifier. See Fig. 4. The network contains a trap tuned to the accompanying sound carrier, 41.25 mc. In order to reduce interference from this source (i.e., cross modulation), the sound carrier is attenuated as soon as possible in the i.f, amplifier. (The signal is not removed completely, however, since enough must be available for the sound system. The latter ties into the video system at a subsequent point.) A more elaborate bridged-T network, combined with an m-derived bandpass circuit, is employed between the first and second i.f. stages. This contains two rejection traps, one tuned to 39.75 mc. (video carrier of adjacent higher channel), the other tuned to 47.25 mc. (sound carrier of adjacent lower channel). A second such complex coupling network is found between the final i.f. stage and the video second detector. This, too, contains two traps, one for the accompanying sound carrier at 41.25 mc. and one for 47.25 mc. It will be noted from Fig. 4 that the sound take-off occurs in the plate circuit of the final video i.f. amplifier. This does not necessarily denote a split-sound type of receiver, as mentioned earlier, but stems from a desire on the part of the set designer to avoid any interaction between the color sub-carrier and the sound carrier that could produce (by mixing) a 920 kc. beat note. The sound carrier is permitted to travel with the video signal up to the plate of the final video i.f. amplifier and then it is diverted to a germanium crystal where it mixes with the video carrier to produce a 4.5 mc. signal. In the meantime, the monochrome and color subcarrier signals proceed to the video second detector for their demodulation. By this arrangement, the sound signal can be strongly attenuated in the video detector thereby minimizing the development of a 920 kc. beat signal. Automatic gain control is applied to the first two or three video i.f. stages in the same manner, and for the same reason, that it is applied in monochrome receivers. The r.f. amplifier also receives all or a portion of the same a.g.c. voltage. Sound Channel. As indicated previously, the sound signal is diverted from the video path in the plate circuit of the final video i.f. amplifier. This signal and a portion of the video carrier are then mixed in a germanium diode to produce the desired 4.5 mc. intercarrier sound signal. See Fig. 5. This is followed by several 4.5-mc. i.f. amplifiers and then the signal is applied to a ratio detector. Here the audio intelligence is recovered from the FM signal. Further amplification by audio voltage and power amplifiers raise the signal to the proper level for operating a loudspeaker. Just how extensive this portion of the audio system is will be governed by the price range of the receiver. If a high-fidelity system is desired, then the audio stages can be elaborated, perhaps by the addition of push-pull output, phase inversion, feedback networks, etc. The system shown in Fig. 5 is commonly found in most TV receivers where economy and good sound is desired. Luminance Channel. The video signal is demodulated in the video detector (Fig. 7), providing an output 0 to 4 mc. monochrome signal plus the I and Q color sidebands. (The color subcarrier, it will be remembered, was deleted at the transmitter.) The detector itself may be either a germanium diode (1N60 or its equivalent) or one section of a vacuum tube. There appears to be a definite swing toward the germanium crystal but vacuum tubes are still widely used. Beyond the detector, both the monochrome and color sideband signals are applied to at least one stage of amplification before they are separated. In the circuit of Fig. 8, the output from the video second detector is applied first to the triode section of a 6U8, then to the pentode section. Both signals remain together only in the triode because at the grid of the pentode, a portion of the signal is fed to the bandpass amplifier, which is the input stage to the chrominance section of the receiver. Hence, separation of the monochrome and color signals might be said to occur at the output of the triode video amplifier. The second video amplifier in Fig. 8 deals solely with the monochrome portion of the total color signal. This fact is further accentuated by the 3.58 mc. series trap which is present in the plate circuit of this stage. The trap attenuates any 3.58 mc. color subcarrier voltage which may be present here in order to prevent it from reaching the picture-tube screen and producing a visible interference pattern. The presence of the 3.58 mc. trap limits the response of the luminance or monochrome channel to a somewhat lower value, usually 3.0 or 3.2 mc. Since most present monochrome receivers operate within this bandwidth, both in their i.f. and video amplifier systems, any loss of detail will be no more apparent on color sets than on black-and-white sets. At this point the reader may wonder why a special 3.58 mc. trap is required when, in fact, no 3.58 mc. color subcarrier is being sent with the signal The answer rests in the fact that while it is true that at no time is there any voltage at precisely the 3.58 mc. frequency, the phase excursions of the color signal cause the carrier to move back and forth from frequencies above 3.58 mc. to frequencies below 3.58 mc.
Fig. 6. - Video amplifier circuit using two pentodes and a 3.58 mc. trap in the first video plate circuit for recovering the 3.58 mc. burst signal for color synchronizing. Furthermore, most of the color energy is concentrated in the sidebands around the 3.58 mc. frequency and if we remove the bulk of this energy with a trap, we minimize any tendency of the color signal to produce interference patterns on the screen. Another fact to note is this: The frequency of the color subcarrier (and hence, the frequency of its sidebands as well) was purposely chosen so that all this energy would fall midway between the clusters of energy of the monochrome signal. Any color signal reaching the screen of a monochrome receiver will tend to at least partially cancel itself out on successive frames so that its visibility is reduced. The same action occurs in a color set when the color signal reaches the screen via the luminance channel. Hence, the combination of the 3.58 mc. trap with the frequency interlace principle act to reduce the visibility of any interference pattern from this source to a considerable degree.
Returning to the circuit of Fig. 8, the luminance signal is finally applied to the matrix section where it combines with suitable I and Q signals to provide the original red, green, and blue voltages. Two additional representative video amplifier systems are shown in Figs. 6 and 9. The circuit in Fig. 6 is taken from an RCA schematic and employs a 1N60 crystal diode as the video second detector. The output of this stage is fed to a 6CL6 video amplifier. Here both chroma and monochrome signals are amplified. The monochrome signal is then transferred to a second video amplifier and from this stage to the matrix network. The chroma signal is taken from the cathode circuit of the 1st video amplifier and transferred to the bandpass amplifier which stands at the head of the chrominance section.
Fig. 8. - Video amplifier circuit using a triode-pentode tube. There are a number of things to note about Fig. 6. A 3.58 mc. resonant circuit in the plate circuit of the 1st video amplifier transfers the 3.58 mc. signal to a burst amplifier for use in the color sync section of the receiver. The same arrangement also attenuates the amount of 3.58 mc. voltage reaching the second video amplifier. The response of this latter amplifier extends to approximately 3.2 mc., enabling it to impose additional attenuation on the color subcarrier.
Fig. 9. - Cathode follower video amplifier circuit for color TV. Connection to the sync and .g.c. circuits is made at the plate of the 1st video amplifier. Also, a 1.0 microsecond delay line is inserted in the path of the luminance signal between the 1st and 2nd video amplifiers. The delay line is terminated in a 1500-ohm potentiometer which serves as a contrast control for the luminance signal. A contrast control for the chrominance portion of the signal is mechanically ganged to the luminance contrast control, thereby insuring that both signals will be varied in equal amounts. This is required to maintain the proper voltage relationship between the two signals. A 4.5 mc. trap in the cathode leg of the 1st video amplifier attenuates any 4.5 mc. voltage that may develop , in the video detector through the beating of the video and sound carriers. For the color TV video amplifier circuit shown in Fig. 9, the detector stage is formed by using one-half of a 6BK7 duo-triode. The grid and plate are tied together so the triode function as a diode. The second triode section of the 6BK7 is operated as a cathode follower, thereby permitting a number of circuits to obtain their signals from the detector without imposing any capacitive loading on this stage. The plate circuit of the cathode follower provides signal voltages for the sync separator, a.g.c., and burst amplifiers. The cathode of the same tube contains a 500-ohm potentiometer which provides the signal for both a luminance amplifier and a bandpass amplifier and controls the contrast for both channels simultaneously. The brightness or luminance signal is amplified by a single triode stage and then passed through a 1.0 microsecond delay line that is terminated in the matrix network. There are no special traps in this circuit, but response falls off rapidly beyond 3.2 mc. attenuating any color subcarrier and 4.5-mc. voltages that might be present. Color Television Articles
Posted September 1, 2013 |
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