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(source: Electronics World, Mar. 1966)
By WALTER H. BUCHSBAUM
A new approach to small screen color CRT's, coupled with greatly simplified chroma, convergence, and high voltage circuits, produces a different type of color-TV receiver.
The current flood of new color-TV receivers contains many variations of receiver design, all based on the same type of shadow-mask picture tube. With the introduction of a new type of picture tube, the 11SP22, the G-E "Porta-Color" is the smallest of all color-TV sets and differs in many important respects from conventional color sets. Much of the circuitry is quite unusual and contains many novel features. Even the method of mounting components, the accessibility of the chassis components, and the over-all construction are quite different from previous designs. As will be illustrated in this article, the major innovations are in the mechanical construction; the 11-inch picture tube; and in the new and simplified circuits used in the chroma section, the color sync, and the convergence circuits. While the G-E "Porta-Color" receiver still uses a shadow-mask tube with three electron guns and a phosphor dot screen, demodulates the B-Y and R-Y chroma signal, and still uses horizontal and vertical convergence coils, the detailed circuits and components are quite different from any previous receiver.
As shown in Fig. 1, the entire receiver, with the picture tube, becomes accessible by removing the one-piece cabinet body and back. The bottom of the cabinet consists of cross-mounted channels which directly support the printed-circuit assembly. Louvered panels protect the bottom of the printed circuits and can be removed for testing. A single etched circuit board contains the majority of the receiver circuitry and the high-voltage compartment. A small sub-chassis, containing the power-supply components and the tuner assembly, is mounted separately, as shown at the left of the photograph.
The picture tube is clamped directly against the front panel and mounts the deflection yoke and convergence assembly on its neck. In addition to the picture tube, the set uses 13 tubes, one transistor, and 13 diodes. The tubes are of the multi-element Compactron type used in many previous G-E receivers and actually represent many more functional stages.
The Picture Tube
The 11SP22 is a shadow-mask tube, differing primarily from its predecessors in that the electron guns are horizontally in line and their beams hit the phosphor dot screen in a horizontal rather than a triangular configuration. This is illustrated on the front cover of this issue. In the conventional tri angular electron-beam configuration, each electron beam must be aligned in a number of different directions, while in the horizontal in-line arrangement used in this new tube, the center electron beam is used as a reference with the other two being aligned with respect to it. This is apparent from the convergence assembly itself which contains convergence coils and magnets only for two electron beams. The separate vertical and horizontal convergence coils are mounted on a plastic assembly as shown in Fig. 2. A purity magnet assembly is used, but its action is not as critical as in other color tubes.
To illustrate the operation of the horizontal and vertical convergence adjustment, refer to Fig. 3 which shows the front and rear view of the convergence assembly. The green electron gun ( at the center) is not affected by the convergence fields of the other guns. The red and blue guns contain two separate pole pieces to interact with the vertical and horizontal coils respectively. Note that in the case of the horizontal convergence, shown in the front view, the magnetic flux passes vertically across the path of the electron beam, causing it to move in a horizontal direction. For the vertical case, the horizontal magnetic field does not have any effect because the coil and internal pole piece are displaced longitudinally. For vertical convergence, the magnetic flux is oriented horizon tally, causing vertical beam movement. One result of the simplified convergence design is that there are no adjustments and waveshaping circuits for vertical and horizontal convergence amplitude and tilt. The only adjustment that is required is setting the magnet on each of the horizontal and vertical coils for optimum convergence. Each set of convergence coils is connected in series with its respective deflection-yoke winding, as shown in the basic circuit of Fig. 4.
The various circuit innovations of this set can be illustrated by the simplified diagram of Fig. 4. Both the v.h.f. and the u.h.f. tuner are conventional, with the latter using a transistor as the local oscillator and a diode as the mixer. The i.f. amplifier section is also conventional and uses three stages, followed by two separate detectors, as shown. A 41.25-mc. trap is connected before the video detector to reduce the amplitude of any 4.5-mc. component in the video. To further re duce the 4.5-mc. inter-carrier signal, a series trap is located in the output of the video detector. A single video amplifier is used for the brightness signal, with a delay line in its output to assure proper coincidence between the Y (brightness) and the chroma signals at the picture tube. As in many black-and white receivers, the contrast control is part of the cathode circuit of the video amplifier, which also contains a bifilar trans former tuned to 3.58 mc., the color subcarrier frequency. By this arrangement, the contrast control also affects the chroma signal amplitude, and this reduces the viewer's problem of setting contrast and chroma gain to the right balance.
The 4.5-me. audio detector is tuned primarily for the inter carrier sound signal, but enough video information is amplified in the 4.5-me. sound i.f. to drive the sync separator and a.g.c. stages. The audio section itself is conventional. Neither the sync separator nor the vertical sweep section contain any radically new circuitry.
While the horizontal a.f.c. oscillator and flyback appear conventional, the horizontal output amplifier uses a novel arrangement to provide some regulation of the high voltage and the horizontal sweep signal. In practically all color sets, the high-voltage section contains a shunt regulator because variations in high voltage can show up as color errors; however, no regulator as such appears in this model. The regulation scheme in this set is shown in simplified form in Fig. 5 and consists of resistor R1, capacitor C1, diode D1, and the blanking winding of the flyback transformer connected to the screen grid of the output amplifier. Without the pulse from the transformer winding, diode DI is reverse-biased by the voltage drop across R1. The transformer pulse (which is also the blanking pulse) is normally about 150 volts, causing the diode to conduct and charge screen-grid capacitor C1. Since the pulse amplitude varies with the transformer loading, it controls the screen-grid voltage and thereby the gain of the out put amplifier. Transformer loading depends on both the sweep amplitude and the high voltage, but, under normal operation, only the high voltage varies with beam current. While this scheme may not provide as close a regulation as the usual shunt regulator, it permits elimination of the separate high voltage regulator and the usual width-control coil as well.
The deflection yoke contains toroidally wound coils, and, as mentioned before, the convergence coils are simply connected in series with their respective deflection coils. Because toroidal deflection yokes require a larger current at lower volt ages, it is possible to use this current directly in the convergence coils without the usual arrangements of separate trans former windings, waveshaping circuits, and amplitude and tilt controls.
Most previous color receivers have used a 3.58-mc. oscillator controlled by a phase detector that compares the phase of the color oscillator with the incoming color sync burst. In this new set, an entirely different scheme using a crystal ringing circuit is employed. As shown in Fig. 4, the chroma signal is picked off the video amplifier cathode and gated in the burst gate by the horizontal pulse so that only the 3.58-mc. color sync burst passes through to the crystal ringing circuit. This circuit consists of a crystal and an impedance-matching, tuned LC circuit and is based on the principle that a crystal, excited by a signal of its own resonant frequency, will continue to oscillate for a short period of time. Fig. 6 illustrates the wave forms for the circuit. The 8-cycle burst (top) transmitted from the TV station excites the crystal to ring. This ringing, after the first 8 cycles are over, exponentially decreases in amplitude but starts again at a high amplitude when the next burst comes in ( center). The 3.58-mc. amplifier following the ringing crystal amplifies and limits the signal so that a relatively constant amplitude 3.58-mc. sine wave reaches the output transformer (bottom) ). The variable capacitor connected across the primary of the color sync output transformer acts as the tint control because it varies, to some extent, the phase of the color sync signal in the primary. Each of the two secondaries is tuned to provide the desired phase shift, thus generating the color sync signals necessary for demodulation. In addition to the obvious simplicity of this circuit, it also has the advantage of greatly simplified alignment.
The chroma signal is taken from the cathode of the burst gate through the color gain control and is applied to two balanced-diode demodulators. This type of demodulator has been used previously only in military applications and requires a relatively high amplitude of sync signal as well as a low-impedance chroma signal to operate properly. Detailed circuit operation of the diode demodulator circuit can be understood from the simplified circuit and its waveforms shown in Fig. 7. The color subcarrier is applied through capacitor C3 to diodes D1 and D2 and appears across the three series resistors, R1, R2, and R3. Potentiometer R2 is set to produce zero volts d.c. from the output of the detectors regardless of the presence or absence of the color sync signal. This feature overcomes any amplitude variations of the color sync signals and also eliminates the need for a color killer because, in the absence of both chroma and color sync (during monochrome transmission), the output of the balanced detector will be zero. Each of the two diodes functions as a peak detector.
When no chroma signal is present, C1 and C2 are charged up to their peak value by the reference signal, and, with the diodes cut off, both capacitors discharge in series across the three resistors. Diode conduction then occurs only at the peak of each reference signal. Diode D1 conducts when a negative signal is present at its cathode, but, at the same instant, a positive signal is presented at the other end of the transformer, and diode D2 conducts in the opposite direction.
These opposite and equal currents produce zero output at the center of the balanced potentiometer.
During the period when the diodes are conducting, the chroma signal is detected. D1 will develop a positive voltage proportional to the sum of the chroma and its respective reference signal, while D2 will develop a negative voltage proportional to the sum of the chroma and the 180° out-of-phase reference signal. This is the same effect as the operation of the gating elements in a synchronous demodulator. When the reference signal causes conduction of the diodes, they become a low-impedance path and allow the chroma information to pass. in the voltage waveforms of Fig. 7, the E, signal indicates the chroma signal and the bottom line is a vector presentation, showing the reference signal 180° out of phase.
Two identical diode demodulators are used to produce a signal 8.1° out of phase with the R-Y and 4.6° out of phase with the B-Y chroma signals respectively.
The phase angle is determined by the electrical position of the two secondaries with respect to the primary. These phase-angle relations cannot be changed without changes in the transformer, and any variation of the primary tuning capacitor (tint control) will affect both demodulated signals to the same extent.
The blue and red color difference signals are applied to the grids of three tri ode amplifiers that drive the respective grids in the picture tube. The G-Y signal is obtained, as in many conventional color receivers, by combining the cathodes of the three amplifiers and by adding a small video component from the resistor network common to the two de modulators, as illustrated in Fig. 4. The outputs of the three color difference amplifiers are d.c.-coupled to their respective picture-tube grids, and, in the case of the blue and green, a d.c. bias setting determines the blue and green amplitudes.
The picture tube is not too different from other shadow-mask picture tubes in its over-all operation. Three separate screen-grid adjustments are available to obtain proper black-and-white balance.
Focusing is accomplished by a common low-voltage electrostatic focus element which can be jumped either to + 280 volts or to the horizontal boost voltage.
A degaussing coil is permanently mounted around the screen of the 11SP22 and is actuated by a manual switch which discharges a normally charged capacitor through the degaussing coil. The rapidly decaying current through the coil performs the de-magnetization.
The receiver uses a transformerless voltage-doubler silicon rectifier circuit to generate +280 volts and a half-wave rectifier to provide +135 volts. All the tube filaments, including those of the color picture tube, are in series. In servicing this receiver, it is essential that an isolating transformer be used to avoid accidental shock.
Editor's Note: The latest version of the G-E 21-inch color set uses a similar approach to chroma demodulation. In these sets, tint control is via a voltage variable capacitor in the subcarrier amplifier and a third secondary (for G-Y, including its own diodes) is added to the demodulator transformer. Also, a separate shunt regulator is used.