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74 Horizontal Deflection Horizontal Deflection Basics This discussion will only touch on horizontal, (right-left, left-right) deflection of the electron beam across the face of the CRT. Vertical, (up/down, down/up), deflection will be covered in a later section. The horizontal output transistor and damper diode in the MM101 supplies horizontal deflection yoke current. Since the MM101 contains a separate hi-voltage supply, the horizontal output does not carry the traditional dual role of supplying both yoke current and high voltage (beam current). Although there is only one horizontal yoke winding, it is wound in such a fashion that current in one direction drives the beam away from the center to the left side of the screen, while current in the opposite direction drives the beam away from the center to the right side of the screen. The amplitude of the current determines how far from the center the beam is deflected. Deflection is accomplished by forcing current through the deflection yoke, creating an electromagnet of the yoke windings that either push the electron beam away from or allow it to drift back to the center of the screen. If there is no yoke current, the beam remains center screen creating a vertical line very close to the physical center of the CRT. Figure 6-1 and 6-2 show the electron beam position at various yoke current values, assuming a static DC current from a power supply is used. (These values are only for discussion and demonstration purposes. Actual yoke Center of Screen current and direction for exact beam positioning will be different.) Note +2A +4A +6A +8A +10A that as yoke current increases towards a higher positive value, the beam is driven farther towards the right side Electron Beam Position of the screen. As the positive yoke current approaches zero, the beam is Figure 6-1, Electron Beam Position with Positive Current closer and closer to center screen. As yoke current reverses, the beam is again driven away from center screen, but now in the opposite direction. The higher the negative current, the farther from center screen the beam is driven. As negative current decreases, the beam moves back towards center screen.
Center of Screen
-10A -8A -6A -4A -2A
Electron Beam Position
Figure 6-2, Electron Beam Position with Negative Current
Horizontal Deflection 75 Figure 6-3 shows how increasing positive current drives the electron beam towards the right side of the screen and increasing negative current drives the beam towards the left side. The amplitude of current drives the beam farther from center screen. (The scope captures are not in exact time alignment with the electron beam.) Again, the theory of positive and negative current flow is not important to this discussion. The concept of yoke current flow one way making the beam travel one direction, while yoke current flow in the opposite direction makes the beam reverse its travel is the point.
Center of Screen
Center of Screen
Electron Beam Travel
Electron Beam Travel
+Max Zero -Max
Increasing Yoke Current Drives Beam Away from Center To Right Side of Screen
Decreasing Yoke Current Now Allows Beam To Move Back To Center from Right
+Max Zero -Max
Center of Screen
Center of Screen
Electron Beam Travel +Max Zero -Max
Electron Beam Travel +Max Zero -Max
Yoke Current now Reverses and Begins Increasing, Driving Beam to Left Side of Screen
Decreasing Yoke Current Again Allows Beam To Move Back To Center from Left
Figure 6-3, Beam Travel Inductive Current Flow Among the many theories of deflection, yoke current versus yoke voltage is one of the most misunderstood by technicians. A yoke is simply an inductor constructed to induce its developed magnetic flux in a specific pattern around the bell of a CRT. The flux becomes stronger as current through the wire is increased, and weaker as it decreases. Figure 6-A compares voltage across a yoke winding with the resulting current through it and magnetic field developed by it. As voltage is first applied, the yoke tends to "limit" current flow. Even though maximum voltage is Flux immediately available, current builds slower as a result of Strength 0 inductive reactance. As the current builds, the magnetic Voltage flux field emanating from the yoke grows stronger. Current When voltage is removed, the yoke tends to continue Yoke 0 Winding current flow as the flux fields (with no current flow to sustain them) begin to collapse. As they collapse, current decreases and the magnetic field grows weaker. If voltage Volts is not reapplied, the current will fall to zero. The yoke is 0 not directional. If the opposite polarity voltage is Time applied, the same current pattern is observed, only in Figure 6-3A, Yoke Current versus Applied Voltage the opposite direction.
76 Horizontal Deflection Low Level Signal Generation Before explaining horizontal yoke drive, we will begin by developing the the horizontal drive waveform originating in the Deflection Processor, U14350. All control of the waveform is performed inside the IC via the IIC bus. The waveform is then current amplified to take the lower level signal from pin 20 and drive the horizontal drive transformer, T14300. T14300 current demands are different between 1H and the 2.xH scan rates. The horizontal output signal from U14350-20 is first inverted by Q14303, then current amplified by push-pull transistors, Q14301/02. They drive step down (~3:1) horizontal drive transformer, T14300. The horizontal drive waveform appearing on T14300-4 is shown.
+12Vr
SCAN H_A PULSE FROM HORIZONTAL OUTPUT TRANSFORMER T14451-8
CR14301 R14308 1000
R14305 6.8
C14301 10uF 63V
Q14301 T14300 Q14303 1 HFB HOUT 20 R14358 10 R14319 47 Q14302 3 6 1 4 H Drive to Q14451-B
U14350 DEFLECTION CONTROL
C14305 470UF 16V
Figure 6-4, Low Level Horizontal Signal Generation There is an important feedback signal generated from the Horizontal Output Transformer and used by the Deflection Control IC to "fine tune" the low level horizontal signal. It is generated during the "flyback" portion of horizontal sweep and lasts the duration of retrace. In the MM101, retrace is held relatively constant regardless of scan rate. The two waveforms below are known as the "SCAN H_A" pulse and originate from pin 8 of T14451 (Figure 6-6). The pulse is generated during retrace time, while the waveform between the pulses is trace time. Notice that both pulses are very close to the same width in both 1H (NTSC) and 2.xH (SVGA), indicating a relatively constant retrace period. In SVGA mode, there are almost exactly twice as many retrace pulses also indicating less trace time. Remember that the electron beam must still travel the same distance across the screen, but it has much less time to do so at 2.xH.
Figure 6-5, SCAN H_A Pulse
Horizontal Deflection 77
+80Vp-p Horizontal Pulse to Parallelogram PCB
14
EY14467 1 + C14450 10uF 2000V
L14454 180UH SCAN B+
+20Vp-p Horizontal Pulse (SCAN H)
8
4 7
C14727 +1700Vr CR14710 + C14723 3300uF
+12Vr
12
CR14713 R14305 CR14301 6.8 R14308 1000 Q14301 T14300 Q14303 1 HFB HOUT 20 R14358 10 R14319 47 Q14302 3 6 C14305 470uF 16V 1 4 Q14451 R14304 10 C14456 470P 2000V C14453 9700P 1600V S-Cap Correction Circuits C14301 10uF 63V CR14452 Horizontal Yoke Winding
U14350 DEFLECTION CONTROL
Figure 6-6, Horizontal Feedback Loop One more reference waveform is generated from the flyback pulse. It comes from T14451-14 and is identical to the SCAN H_A pulse except for amplitude. It is normally about 80Vp-p and runs at the scan rate. It is used by the parallelogram circuits to correct specific raster distortions.
Figure 6-7, 80 Volt Pulse The horizontal drive waveform from T14300 is used to drive the horizontal output transistor, Q14451.
78 Horizontal Deflection Horizontal Output The low level horizontal signal is coupled to the horizontal output stage by transformer, T14300. The horizontal output transistor is a high power NPN transistor, Q14451. During normal operation, the horizontal retrace capacitor, C14453, is charged via the primary winding of T14451 and L14454 to an average voltage approximately equal to Scan B+. T14451 is connected as an autotransformer in this application. The output waveform from the low level generator is approximately 8V p-p at the selected horizontal rate. This is inverted by the driver transformer T14300 and applied to the base of Q14451. The resulting waveform at Q14451-C (shown in Figure 6-10) is approximately 1300 volts p-p. This voltage changes slightly depending upon the scan rate.
+80Vp-p Horizontal Pulse to Parallelogram PCB 14 EY14467 1 + C14450 10u 2000V 8 4 7 C14727 +1700Vr CR14710 + C14723 3300uF L14454 180UH SCAN B+
+20Vp-p Horizontal Pulse (SCAN H)
12
CR14713
CR14452
Horizontal Yoke Winding
Horizontal Drive Pulse From U14350-20
T14300 JW14301 1 4
Q14451 C14456 470P 2000V C14453 9700P 1600V S-Cap Correction Circuits
R14304 10 3 Horizontal Drive Transformer 6 C14305 470UF 16V
Figure 6-8, Horizontal Output
Figure 6-9, Horizontal Drive Waveforms at Q14451-B
Horizontal Deflection 79
Figure 6-10, Horizontal Output Waveforms at Q14451-C The retrace pulse is easy to see, but what happens to the waveform during trace is unlikely to be caught on a normal analog or digital scope. Figure 6-11 shows an expanded waveform that represents active trace. In figure 6-10, active trace appears to be a flat voltage, while figure 6-11 reveals the true waveform.
Active Trace
Figure 6-11, Expanded Horizontal Output Waveforms at Q14451-C Horizontal Scan Operation In any discussion of deflection circuits, actual current flow and voltage diagrams prove to be of little use. It is more important to understand energy flow during trace and retrace periods. By understanding how energy is transferred and when, troubleshooting becomes more routine. The MM101 has a greatly simplified horizontal scan generator, however, without understanding the energy transfer, troubleshooting will be reduced to "shotgunning" parts. The following pages are meant to familiarize the technician with the energy flow of the high level horizontal scan generator.
80 Horizontal Deflection Horizontal Output Operation Prior to the first full scan cycle, Q14451 is off, and current is drawn from ground through the S-Cap(s), yoke winding, T14451 primary and L14454 to Scan B+. Current flow through C14453 charges it to around Scan B+. When all devices reach full charge, current flow in the yoke stops. If nothing else affects this quiescent state, (no yoke current) the electron beam will remain in the center of the screen. At this point, there is no energy in the yoke, however, the retrace cap, C14453 is at its maximum energy.
+80Vp-p Horizontal Pulse to Parallelogram PCB 14 EY14467 1 + C14450 10u 2000V 8 4 7 C14727 +1700Vr + C14723 3300uF L14454 180UH SCAN B+
+20Vp-p Horizontal Pulse (SCAN H)
12
CR14710
CR14713
CR14452
Horizontal Yoke Winding
Horizontal Drive Pulse From U14350-20
T14300 JW14301 1 4
Q14451
R14304 10 3 Horizontal Drive Transformer 6 C14305 470UF 16V
C14456 470P 2000V
C14453 9700P 1600V S-Cap Correction Circuits
Figure 6-12, Horizontal Output Block Diagram At the beginning of retrace, the horizontal output transistor, Q14451 is shut off. The high voltage that appears is generated from the flyback conduction of T14451. This very high voltage returns the electron beam back to the left side of the screen very quickly...less than 7 microseconds. Trace (in this example, 1H) occurs for about 53.5 microseconds. Notice the drive waveform on Q14451-B does not turn the output transistor on until almost 1/2 of trace is complete, yet the voltage on Q14451-C is less than zero. Let's follow the yoke current path, energy transfer and timing.
Q14451-B
Q14451-C
Figure 6-13, Horizontal Waveforms at Q14451
Horizontal Deflection 81 Scan from Center to Right (Trace) To begin, it must be understood the resonant circuits which are involved in horizontal scan. T14451 primary and the yoke are effectively in parallel. The retrace cap, C14453 forms a resonant circuit with both the yoke and T14451 primary determining the energy transfer period of retrace. (The S-Cap serves to block DC from the yoke and provides S-correction and energy storage.) The entire trace-retrace period may be thought of as transforming, transferring and controlling the flow of energy in the circuit.
Center of Screen
+80Vp-p Horizontal Pulse to Parallelogram PCB
14
EY14467 1 + C14450 10u 2000V
L14454 180UH SCAN B+
+20Vp-p Horizontal Pulse (SCAN H)
8
4 7 C14727
Energy transfer from Scan B+ to T14451
+1700Vr C14723 3300uF
12
Electron Beam Travel
CR14710
+
CR14713
Energy transfer back to yoke
Horizontal Yoke Winding
CR14452
Horizontal Drive Pulse From U14350-20
T14300 JW14301 1 4
Q14451
R14304 10 3 Horizontal Drive Transformer 6 C14305 470UF 16V
C14456 470P 2000V
C14453 9700P 1600V S-Cap Correction Circuits
Figure 6-14, Yoke Current and Q14451-C Waveform during Horizontal Output Conduction (Trace)
When horizontal drive turns Q14451 on, current begins increasing through Q14451 and the primary of T14451. This also allows increasing current through the yoke from the S-Cap. The retrace cap, C14453 is not involved in trace. As current increases, yoke energy increases, moving the electron beam farther and farther to the right side of the screen. T14451 and the yoke are now storing energy.
82 Horizontal Deflection Scan From Right to Center (Beginning of Retrace) When horizontal drive stops, Q14451 shuts off, stopping current flow. Noting the equivalent parallel resonant circuit, C14453 and the yoke, the first half of retrace occurs as T14451 and yoke inductive energy transfer to the retrace cap, C14453. The frequency of the resonance is set up by the parallel combination of C14453, T14451 and the Yoke equivalant inductance. Equivalent yoke inductance actually appears lower due to the tuning effect of the series S-shaping capacitor. If something upsets this resonance, the beam may take a longer or shorter time to return to center screen.
Center of Screen
+80Vp-p Horizontal Pulse to Parallelogram PCB 14 EY14467 1 + C14450 10u 2000V 8 4 7 C14727 L14454 180UH SCAN B+
+20Vp-p Horizontal Pulse (SCAN H)
Energy transfer from T14451 to C14453
12
CR14710
+1700Vr + C14723 3300uF
Electron Beam Travel
CR14713
Energy transfer back to C14453
CR14452 Horizontal Yoke Winding
Horizontal Drive Pulse From U14350-20
T14300 JW14301 1 4
Q14451
R14304 10 3 Horizontal Drive Transformer 6 C14305 470UF 16V
C14456 470P 2000V
C14453 9700P 1600V S-Cap Correction Circuits
Figure 6-15, Yoke Current During Beginning of Retrace As yoke current (energy) decreases to zero, the electron beam moves back to center screen. Flyback voltage from T14451 reaches a very high peak, charging C14453 which begins storing energy from the flyback pulse. NOTE: A secondary voltage is generated from pin 7 during flyback. It is used to generate the supply for the Dynamic Focus circuit described later.
Figure 6-16, Q14451-C Voltage Waveform
Horizontal Deflection 83 Retrace from Center Screen to Left The energy stored in T14451 is now decreasing rapidly, allowing a voltage drop at Q14451-C. This is shown in Figure 6-16. Continuing with parallel resonance, the second half of retrace occurs when C14453 capacitive energy just stored from the beginning cycle of retrace, transfers back to T14451 and the yoke. The only difference is yoke current is now driving in the opposite direction. Since yoke current is moving the opposite direction, the electron beam is driven from center screen toward the left as the current increases. It should be noted that anytime the electron beam is moving left, (retrace) the CRT is biased off (blanked), preventing beam current so that retrace lines, which contain no video information, will not be visible on the screen. If something happens to upset the timing between blanking video and scan, retrace lines will be visible.
Center of Screen
+80Vp-p Horizontal Pulse to Parallelogram PCB 14 EY14467 1 + C14450 10u 2000V 8 4 7 C14727 +1700Vr + C14723 3300uF L14454 180UH SCAN B+
+20Vp-p Horizontal Pulse (SCAN H)
12
CR14710
Electron Beam Travel
CR14713
CR14452
Horizontal Yoke Winding
Energy transfer back to yoke
Horizontal Drive Pulse From U14350-20
T14300 JW14301 1 4
Q14451
R14304 10 3 Horizontal Drive Transformer 6 C14305 470UF 16V
C14456 470P 2000V
C14453 9700P 1600V S-Cap Correction Circuits
Figure 6-17, Yoke Current, Center Screen to Left The beam reaches the left side of the screen at maximum yoke current.
Figure 6-18, Horizontal Waveforms at Q14451 (Repeated)
84 Horizontal Deflection
Damper Diode Operation Following the positive cycle for retrace, at the moment of zero voltage across C14453 (all energy transferred i.e. maximum negative yoke current), the damper diode, CR14452 conducts shunting T14451 and Yoke current, preventing what would otherwise become a negative retrace cycle. If the damper diode fails, induced current from the yoke will buck induced current from T14451 and at the very least distort the raster horizontally. Catastrophic failure of other components due to excessive voltage differences may also occur.
+80Vp-p Horizontal Pulse to Parallelogram PCB
14
EY14467 1 + C14450 10u 2000V
L14454 180UH SCAN B+
+20Vp-p Horizontal Pulse (SCAN H)
8
4 7 C14727 +1700Vr + C14723 3300uF
12
CR14710
CR14713
CR14452
Horizontal Yoke Winding
Horizontal Drive Pulse From U14350-20
T14300 JW14301 1 4
Q14451
R14304 10 3 Horizontal Drive Transformer 6 C14305 470UF 16V
C14456 470P 2000V
C14453 9700P 1600V S-Cap Correction Circuits
Figure 6-19, Damper Diode Operation
Horizontal Deflection 85
Beginning of Trace With yoke energy now maximum in the negative retrace cycle as a result of earlier transfer from the retrace capacitor, C14453, trace begins from the left side as yoke current flows through the damper diode, CR14452 to the S-Capacitor. Yoke energy is now decreasing and the beam is scanning from left to center screen. The S-Cap capacitance is now controlling how fast the inductive field from the yoke collapses. Similar collapse of T14451 energy (energy returned to the B+ supply) occurs during this time. Once retrace is complete and trace starts, the CRT is "unblanked" allowing display of video information.
Center of Screen
+80Vp-p Horizontal Pulse to Parallelogram PCB
14
EY14467 1 + C14450 10u 2000V
L14454 180UH SCAN B+
+20Vp-p Horizontal Pulse (SCAN H)
8
4 7 C14727 +1700Vr + C14723 3300uF
12
CR14710
Electron Beam Travel
CR14713
CR14452
Horizontal Yoke Winding
Horizontal Drive Pulse From U14350-20
T14300 JW14301 1 4
Q14451
R14304 10 3 Horizontal Drive Transformer 6 C14305 470UF 16V
C14456 470P 2000V
C14453 9700P 1600V S-Cap Correction Circuits
Figure 6-20, Beginning of Trace
86 Horizontal Deflection Before yoke current has reached zero, the horizontal output transistor, Q14451, is turned on and current builds in the yoke as a result of voltage supplied by the S-Cap. Likewise, current builds in T14451 due to voltage supplied by Scan B+. The electron beam continues travelling from center screen to right as current in the yoke increases, beginning the scan cycle again. Note that any transfer of inductive energy from the horizontal output transformer, T14451, to the yoke/S-Cap can occur only during retrace action with C14453.
Center of Screen
+80Vp-p Horizontal Pulse to Parallelogram PCB 14 EY14467 1 + C14450 10u 2000V 8 4 7 C14727 +1700Vr + C14723 3300uF L14454 180UH SCAN B+
+20Vp-p Horizontal Pulse (SCAN H)
12
Electron Beam Travel
CR14710
CR14713
CR14452
Horizontal Yoke Winding
Horizontal Drive Pulse From U14350-20
T14300 JW14301 1 4
Q14451
R14304 10 3 Horizontal Drive Transformer 6 C14305 470UF 16V
C14456 470P 2000V
C14453 9700P 1600V S-Cap Correction Circuits
Figure 6-21, Continuing Trace Scan Rate and Scan B+ To maintain appropriate scan current for the different scan rates, Scan B+ is varied. Figure 6-22 shows Scan B+ values for each scan rate available in the MM101. This method of changing Scan B+ and S-shaping capacitors is desirable over switching retrace capacitors due to circuit simplicity. Therefore, the retrace time of scan remains relatively constant in all frequency modes. To begin, if all other component interaction is considered constant, changing the drive frequency at the horizontal output transistor, Q14451-B, varies the output voltage at a fixed rate. As frequency rises, Q14451 "on-time" increases, increasing current in the output transformer, T14451.
Scan Rate 1H 2H 2.14H 2.4H Scan B+ 67 Volts 130 Volts 145 Volts 165 Volts 1250 Q14451-C 1300
Figure 6-22, Nominal Retrace Capacitor Voltages
Horizontal Deflection 87 Remember that Scan B+ determines trace current for a given trace time, while retrace is determined primarily by T14451/Yoke and C14453 resonance. (See the Tech Tip below.) The premise is to place a smaller voltage on the yoke during trace, and a much larger voltage during retrace. The voltage across the yoke winding during trace is very close to Scan B+. Figure 622 shows lower voltages at lower scan rates. The electron beam must scan quicker at higher scan rates. If yoke impedance is constant, to cover the same distance in less time, more voltage must be applied to the yoke. Retrace voltage levels are a little less obvious. The same rule, higher voltages move the beam faster over the same distance, applies. However, in the MM101 chassis, retrace amplitude is somewhat lower in monitor modes due to reduced overscan thus less width. The velocity of retrace, even at >2H frequencies, is more than enough to return the beam in the required time. Slew Rate The electron beam must be moved back and forth across the screen at very different velocities in order to provide a proper raster. In the MM101 chassis, the fastest trace occurs in about 26 microseconds, while retrace must happen in about 5 microseconds. The beam must cover the same linear distance during both times. To make the trace go faster (not travel farther), yoke current must increase in a shorter time span. To increase yoke current, yoke voltage must increase. Most deflection systems place a higher voltage on the yoke during retrace to build yoke current more rapidly. Figure 6-23 shows the effect. In a traditional system that maintains a fixed B+, scan would not decrease but retrace amplitude would increase, placing possible catastrophic requirements on the output transistor. The MM101 regulates the pulse amplitude to maintain constant width assuming retrace remains fixed for the respective mode. If retrace changes due to decreases in the retrace capacitor, retrace amplitude will remain constant and width will decrease. Note that while the time constants of a circuit may remain the same, by using a higher supply voltage, the current ramp is much steeper. The current ramp is known as the slew rate of the circuit. It means the time to reach a specified current is much quicker for higher voltages.
TECH TIP
1000 Volt Supply 100 Volt Supply 100A Horizontal Yoke Winding
10A
Trace voltages on the yoke winding are less than 165 0A volts, depending upon the scan rate. Retrace voltages TIME are on the order of 12501300 volts. Figure 6-23, Retrace Voltage Effect If the retrace capacitor does not maintain value, or fails, the resonant circuit between the cap and the yoke is detuned and transfer of power from the horizontal output transformer decreases. The raster will "pull in" from the sides with retrace visible on each side of the raster.
88 Horizontal Deflection
To Horizontal Output, Q14451-C Horizontal Yoke
Linearity Coil Ckt
C14463 1.54UF 250V +12Vr Q14455 SCAP SW From Deflection DAC, U24800-9 Scan Rate Voltage 1 H 0 V 2 H 5 V 2.xH 10V R14449 2200 [R14470] 470
C14457 .22UF 400V
C14454 .36UF 400V
R14465 4700
[R14466] 2000
Q14456
[R14450] 22K
CR14455 5.1V R14467 4700 Q14457 [R14468] 2000
+12Vr R14454 2200 Q14454 [R14469] 470
[R14453] 22K
JW14403
Figure 6-25, S-Cap Correction Circuit
S-Correction Capacitor Switching Geometric distortions relating to horizontal drive, resulting from a fairly flat CRT face are corrected by S-correction and Pincushion correction for the MM101. The "S" capacitor in series with the yoke is resonant at approximately 1/3 of the horizontal scan frequency. The resulting voltage in the yoke path is greatest at the center of the screen and minimum at the edges. The appropriate minimum to maximum voltage variation, determined by the capacitance of the S-Capacitor, varies yoke current to avoid stretching at the sides. What is new to the TCE line is the need to switch different values of S-Correction components in and out of the yoke circuit depending upon the scan rate. The MM101, with its various scan rates, cannot satisfy each with a single or even multiple capacitors. However, various values of capacitors may be switched into and out of the S-Cap circuit to change the amount of S-Correction depending upon the requirements of the selected scan rate. S-Correction switching control comes from the deflection DAC, U24800-9. During 1H scan, the output is low and Q14456 & Q14457 are biased off. This allows a high from the two divider networks of R14459 & R14450 and R14454 & R14453 to be placed on the gates of S-Cap switches Q14454 & Q14455, turning them on.
Horizontal Deflection 89 During 2H scan rates, the output voltage from U24800-9 is 5 volts. This turns on Q14456, shutting off Q14455 and removing C14463. 5.1 volt zener diode, CR14455 blocks the 5 volt signal, preventing Q14457 from also turning on. At higher scan rates, U24800-9 outputs 10 volts, turning both Q14456 & Q14457 on. This shuts off Q14454 & Q14455 removing C14457 & C14463 from circuit, leaving only C14454.
Scan Rate 1H 2H 2.1H 2.4H
Q14454 ON ON OFF OFF
Q14455 ON OFF OFF OFF
C14454 IN IN IN IN
C14457 IN IN OUT OUT
C14463 IN OUT OUT OUT
Figure 6-26, S-Cap Switching Diagram Figure 6-26 shows the switching chart to place the different capacitors in and out of the circuit. The S-Cap switches, Q14454 and Q14455 are in series with two of the S-Caps, C14457 and C14463, respectively. S-Cap C14454 is always in series with the horizontal yoke winding. C14457 and C14463 are placed in parallel with C14454 to change the time constant of the S-Correction. During 1H scan, both Q14454 and Q14455 are turned on, which places C14457 and C14463 in parallel with C14454. During 2H scan Q14455 is turned off, removing C14463 from the circuit. This leaves C14457 and C14454 in parallel with each other, and both in series with the yoke winding. During higher scan rates (2.14H & 2.4H) Q14454 and Q14455 are both off placing only C14454 in series with the horizontal winding. Generally, S-Cap problems will affect both right and left sides of the raster equally. Incorrect switching could be placing incorrect values in the S-correction circuits. Value changes will affect the raster. Too low a value will result in raster distortion that appears to be squashed on the edges. Too high and the raster will appear stretched at the edges. An open S-Cap would result in no horizontal deflection because no yoke current could flow. A shorted S-Cap would probably place enough extra load on the Scan Supply that it would go into protection.
90 Horizontal Deflection Horizontal Pincushion Horizontal pincushion correction is used to compensate for horizontal scanning distortion that occurs as a function of vertical position. Pincushion correction is applied via the Scan Power Supply and results in greater amplitude at center screen than at the top and bottom. Related service adjustments are Parabola and Corner Correction. An E/W parabola is generated by the Deflection Processor, U14350-6. This waveform contains the signal necessary to correct the horizontal scan distortions. The waveform is passed through an op amp to establish a DC bias for the waveform to ride on. In this case it is -4V. The correction waveform is about 4Vp-p and runs at the vertical scan frequency. It is used to modulate Scan B+ at the vertical rate. As Scan B+ varies, horizontal width varies. Also, by varying Scan B+, beam velocity may be varied. This allows compensation of each horizontal line by width and by position versus time. A sample waveform is shown. Since SVGA, NTSC and DVD inputs are all very close to a vertical scan rate of 60 Hz, the waveform will not deviate much from this basic shape, taken in NTSC mode.
TECH TIP
Typical pincushion problems would be either not enough correction or too much. This would be seen as the correction waveform amplitude too low or too high. If pin correction is inadequate, the picture will have pin distortion. Too much pin correction results in barrel distortion.
Keep in mind that the pincushion circuits are modulating Scan B+. Any other circuit failures that modulate Scan B+ at close to the vertical rate can cause defects in the raster that may appear to be pincushion circuit related.
+8Vr R14853 15K +12Vr Deflection Processor U14350 R14856 8200 EW 6 OUT R14851 10K 8 -15Vr
U14352
Vcc Vdd
4
2
+
1 R14850 100
Correction Parabola to Q14808-B, Series Pass(Low B+) Scan B+ Regulator WIDTH REF (SIP Pin 11) Correction Parabola to Q14811-B, ZVS (High B+) Scan B+ Regulator
3 R14451 10K Deflection DAC U24800 Width Align 11 R14852 10K
Figure 6-27, Pincushion Correction and Sample Waveform
Horizontal Deflection 91 Horizontal Linearity Now that the beam has been corrected for S distortions, it must be corrected for linear distortions caused by power loss factors in the yoke and other components. If uncorrected, the right side would be compressed as compared to the left. The inductance of the linearity coil (in series with the yoke) varies depending on the direction and amplitude of current passing through it. Linearity coil inductance is less on the right side of the screen. The deflection DAC, U24800-16 controls which inductors are placed in the linearity circuit. The three linearity correction inductors are L14456, L14455 and L14451. L14456 is always in the circuit, while L14455 is placed in series with it during 1H scan, and L14451 is placed in series during higher scan rates. In this case, there are only two voltages from the DAC. During 1H, pin 16 is 0V. Q14452 is off, preventing current flow in the inductor switching relay, K14401. Pins 3 & 4 are normally closed, placing L14451, along with L14456 in series with the yoke winding. During higher scan rates (>2H), U24800-16 outputs +10 volts, turning on Q14452 providing current for relay K14401. The relay is now activated, and contacts 2 & 4 close, removing L14451 and placing L14455 in series with L14456 and the yoke.
To Horizontal Output, Q14451
Horizontal Yoke +12Vr
R14452 100
1 CR14451
2 K14401 4
L14451
5 16 IIC DATA Run 2 3 U24800 Deflection DAC 15 14 13 12 11 10 9 R24802 1K R24803 1K R24806 100K R24805 1K VCO FREQ 1H SWITCH WIDTH ALIGN FREQ OFFSET S CAP SWITCH R24807 1K LINEARITY SWITCH R14458 47K Q14452
3
L14455
L14456
R14456 150 3W C14452 0.1uF 250V
IIC CLOCK Run 2
4
S-Cap Ckt
Figure 6-28, Linearity Switching Circuit