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Vertical Deflection Vertical Deflection Overview The vertical circuit in the MM101 is very similar to the CTC197 and the earlier CTC177/87 vertical circuits. Like earlier chassis, the output amplifier is DC coupled to the vertical yoke winding instead of using capacitive AC coupling. The input waveform from the deflection generator is also DC coupled. The DC coupled circuit has the advantages of fewer parts, lower cost and linearity becomes less dependent on electrolytic capacitor tolerance and aging. "S" correction, (the compression of the deflection current ramp at the beginning and end of scan), is accomplished from the deflection generator IC, U14350. Because of DC coupling, the DC level of the vertical reference ramp from U14350 pins 10&11 affects vertical centering. By moving the vertical ramp higher or lower around a DC current, vertical centering can be adjusted. This also compensates for variations in the reference ramp DC current. The MM101 uses a dual voltage power supply to the vertical output IC, instead of the traditional single ended supply. This allows the yoke current sense resistors to be grounded instead of using a "half-supply" as in previous TCE chassis. A "flyback" supply is also used to place a higher voltage on the yoke during retrace. This has the effect of speeding up retrace much the same as with horizontal deflection. Vertical scan becomes a little more complex due to the various scan rates the MM101 must display. All drive signals are generated from the deflection generator and applied directly to the vertical output IC. In the following discussions, scan time where video is present (normally when the electron beam is scanning down the screen) is referred to as trace. Scan time when the electron beam is being returned to the top of the screen is referred to as retrace. Low Level Vertical Signal Generation Vertical Size, Vertical Center and S-Correction are all adjusted via the IIC bus and software or the front panel service adjustments. The vertical circuit acts as a current to current converter. Figure 7-1 shows the low level signal chain from the deflection generator.
[R14504] 220 From System Control, U13101 12
H&V Detector Vertical Place Control & Generator
VOUT B To Vertical Output U14501-7 [R14514] 4700 220
10
Vertical Geometry
13 16
11
[R14503] 220 R14506 2430 1%
+8Vr +12Vr
Deflection Generator
V Amplitude S-Correction V Center
R14505 2430 1%
VOUT A To Vertical Output U14501-1
R14355 10
U14350
U14355 +8V REG Vertical 15 Offset R14508 1.8 1W R14511 Vsense from Vertical Output 1.8 1W
Deflection DAC
U24800
Figure 7-1, Vertical Signal Generation
Vertical Deflection Vertical Deflection Basics Much like the horizontal deflection yoke, current through the vertical yoke must travel in two directions. If there is no vertical deflection, the electron beam would settle in the middle of the screen. Yoke current in one direction drives the beam from center to the bottom of the screen. Current in the other direction drives the beam from center to the top of the screen. The active video (trace) is scanned down, while retrace moves the beam back to the top. This discussion will only touch on vertical, (top-bottom, bottom-top) deflection of the electron beam up and down the face of the CRT. Horizontal, (right-left, left-right), deflection has been covered in another section. The vertical output IC, U14501, supplies all vertical deflection yoke current. Similar to the horizontal yoke windings there is only one set of vertical windings. It is wound in such a fashion that current in one direction drives the beam away from center to the bottom of the screen, while current in the opposite direction drives the beam away from center to the top of the screen. The strength 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 electro-magnet of the yoke windings that either push the electron beam away from center as yoke current builds, or allow it to drift back to the center of the screen as yoke current decreases. If there is no yoke current, the beam remains center screen and assuming horizontal deflection is functioning, creates a horizontal line very close to the physical center of the CRT.
Center of Screen
93
+2A +4A Electron Beam Position +6A +8A +10A
Figure 7-2, Electron Beam Position
Figure 7-2 and 7-3 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 demonstration, actual yoke current for the exact positioning will be different.) Note that as yoke current increases towards a higher positive value, the beam is driven farther towards the bottom of the screen. As yoke current approaches zero, the beam is closer to center screen.
-10A
When yoke current reverses, the beam is again driven away from center screen, but now in the opposite direction. As negative current increases, the beam is driven farther from center screen. As the negative current decreases, the beam is allowed to drift back toward center screen.
-8A Electron Beam Position -6A -4A -1A
Center of Screen
Figure 7-3, Electron Beam Position
94
Vertical Deflection During the active portion of scan, current must flow in the yoke in such a direction to cause the beam to travel down the face of the CRT. During retrace, the yoke must stop the downward travel of the beam and return it to the top of the screen. While the beam travels down the screen relatively slowly (1/60th of second), it must return to the top in much less time. Retrace time is very consistant over the various scan rates. For that reason, the vertical circuitry must use some tricks to guard against "over" blanking active video during slower scan rates. These will be discussed later.
Low Level Vertical Deflection Generator The deflection generator U14350, located on the deflection SIP board, provides a current generating a 1.0 volt p-p vertical sawtooth to pin 7 of U14501. The ramp is derived from the vertical sync pulse arriving from System Control at pin 12. It can be adjusted via the IIC data bus using either the front panel service menu or Chipper Check. Vertical height (size), vertical centering and S-correction are adjusted by varying the amplitude and DC component of the output waveform. The average DC level at R14508 is approximately zero volts. The vertical ramp (with correction) from pin 11 is superimposed on sampled yoke current from the current sense resistor R14505 and applied to pin 1 of the vertical output IC, U14501. The output ramp from pin 10 is applied to pin 7 of the vertical output IC, U14501. If there were no vertical ramp, the small DC bias from the deflection generator and perhaps a small DC offset on the inputs of U14501, would shift the output of the vertical IC negative or positive. Without vertical scan, only a single horizontal line near the center of the screen would be seen. The small DC offset would be amplified by the vertical IC, U14501 shifting the horizontal line up or down by a distance proportional to the DC offset.
VOUT B To Vertical Output U14501-7 [R14514] 4700 220
[R14504] 220 From System Control, U13101 12
H&V Detector Vertical Place Control & Generator
10
Vertical Geometry
13 16
11
[R14503] 220 R14506 2430 1%
+8Vr +12Vr
Deflection Generator
V Amplitude S-Correction V Center
R14505 2430 1%
VOUT A To Vertical Output U14501-1
R14355 10
U14350
U14355 +8V REG Vertical 15 Offset R14508 1.8 1W R14511 Vsense from Vertical Output 1.8 1W
Deflection DAC
U24800
Figure 7-4, Low Level Vertical Waveform Generator
Vertical Deflection Vertical Circuit Operation The vertical outputs of the deflection processor are both current outputs having a common DC bias (Idc) with electron current flow into pins 11 (VOUT A) and 10 (VOUT B). The ramp current superimposed on Idc at pin 10 adds to Idc at the start of scan and subtracts from it at the end of scan. The ramp current superimposed on Idc at pin 11 subtracts from Idc at the start of scan and adds to it at the end of scan. The current into VOUT A flows through R14506 and causes a ramp voltage to appear at U14501-1 on a positive DC bias voltage as shown in Figure 7-8. Because the power operational amplifiers have a very high voltage gain, the output, U14501-5 is forced to a voltage causing the voltage feedback to pin 1 to match the pin 7 voltage. If the current in R14503 were zero, voltage Vsense would match the voltage at pin 1 which would match the voltage at pin 7 within a very small error. Thus a 1Vp-p ramp at pin 7 would cause a 1Vp-p ramp at pin 1 and a 1Vp-p ramp at Vsense. Yoke current would be (1Vp-p)/(900 ohms) or about 1.1Ap-p. Since a ramp current flows to VOUT A (of opposite polarity to that of VOUT B), it causes a ramp voltage drop across R14505 which forces Vsense to have twice the ramp amplitude that appears at U14501-7. The high gain power op amp with the negative feedback loop still forces the feedback voltage at pin 1 to match that at pin 7 within a small error. The only time voltages at U14501-1 and U14501-7 do not match is when the loop is open. The loop is in normal operation open during retrace when the pin 5 voltage is limited at the positive boost supply rail. The loop would also be open if any feedback component, such as the yoke, R14513 or R14505 is open.
Idc Iac 411 uA
+15Vr CR14505 CR14509
95
I10
0
CR14501
CR14510
CR14502
Idc
Iac 411 uA
I11
6 2
C14503 220UF
Vertical Sync Pulse to Micro, U13101-34
3
0
VOUT A From U14350-11
[R14503] 220 1 [R14504] 220 VOUT B From U14350-10 R14505 2430 1% R14506 2430 1% -15Vr Positive current flow 7 POWER AMP 4 5 R14513 .75 1W
U14501
VERTICAL YOKE WINDING
Vsense
R14508 1.8 1W R14511 1.8 1W
Figure 7-5, Vertical Output
96
Vertical Deflection I/O Comparision The relationship between the input and output of the vertical IC, is important. Pin 1 is the inverted input. This means any signal present will be inverted in the output stage. Pin 7 is the non-inverted input. Any waveform input to this pin will be reproduced faithfully on the output pin 5. Remember that it is the difference in voltage between these two pins that is important. Very small input currents are translated into very large currents in the output. Figure 7-6 shows correct input waveforms to pin 1 (trace 1) and pin 7 (trace 2) of U14501. Notice pin 1 contains a barely perceptable overshoot at the bottom of its ramp. This is because it is connected to the feedback loop from the current limiting resistors, R14508 & R14511. It will reflect the output waveform at a lower amplitude.
Figure 7-6, U14501 Input Waveforms
+15Vr CR14505 CR14509 CR14510 CR14501 CR14502
C14503 220UF
Vertical Sync Pulse to Micro, U13101-34
6 2 [R14503] 220 VOUT A From U14350-11 [R14504] 220 VOUT B From U14350-10 R14505 2430 1% R14506 2430 1% -15Vr Positive current flow 7 1 POWER AMP 4
3
5 R14513 .75 1W
U14501
VERTICAL YOKE WINDING
Vsense
R14508 1.8 1W R14511 1.8 1W
Figure 7-7, Vertical Output
Vertical Deflection Waveform Comparisions, Output to Input The low level signal from the deflection generator is all that is required for vertical deflection. Figure 7-8 compares U14501 input pin 7 (trace 1) with the output on pin 5 (trace 2). Here it can be seen how the output ramp follows the input ramp exactly. Notice the output ramp falls through zero and into negative voltages, while during retrace, the positive flyback supply allows the input waveform to drive the output to a much higher voltage than the normal supply rails.
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Figure 7-8, Vertical IC Waveforms In this case, the peak flyback voltage is slightly greater than +40V and occurs with the positive going slope of the input ramp. Figure 7-9 shows the actual supply voltage on pin 6 (Trace 1) of U14501 in relation to the output on pin 5 (Trace 2). Pin 6 rides at the normal supply of +15V until retrace, when the flyback supply is placed in series with it and the +15V supply. This adds the two, reaching approximately +40V for the duration of retrace. As soon as retrace finishes, the flyback supply drops out and the normal (+15V) supply remains.
Figure 7-9, Vertical Flyback Waveform
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Vertical Deflection Vertical Scan, Center to Bottom During trace, input current to U14501 is increasing. Internally, the amplifier sources current in direct proportion to the input. Current flows from the IC (using the negative supply) and through the yoke to ground. The beam is now moving from center screen to the bottom of the screen.
Center of Screen Increasing Yoke Current Drives Beam Away from Center To Bottom of Screen
+Max Zero -Max
Electron Beam Travel
Figure 7-10, Electron Beam Movement, Center to Bottom
Current flow is from the -15V supply through the yoke winding and to ground. With little voltage difference between its leads, retrace capacitor C14505 charges only slightly.
+15Vr
~25V
CR14502
C14503 220UF 6 3
2
Input Ramp From Deflection Generator
[R14514] 4700
1 7 POWER AMP 4 5
U14501
-15Vr
C14505 .22UF
VERTICAL YOKE WINDING
Figure 7-11, Yoke Current, Center to Bottom
Vertical Deflection Vertical Scan, Bottom to Center As the beam reaches the bottom of the screen, trace is completed. The input current of U14501 is now increasing, which causes the output at pin 5 to begin increasing towards zero and retrace begins. (Remember, the output has just reached maximum negative supply potential.)
Center of Screen
99
Decreasing Yoke Current Allows Beam to Drift from Bottom to Center of Screen
+Max Zero -Max
Electron Beam Travel
Figure 7-12, Electron Beam Movement, Bottom to Center
As the current supply from U14501 stops, retrace capacitor C14505 begins to charge through the yoke. The current path is now from the -15V supply through C14505 and the yoke to ground. As the capacitor charges, current flow drops off, yoke current decreases and the beam begins to settle back towards center screen.
+15Vr
~25V
CR14502
C14503 220UF 6 3
2
Input Ramp From Deflection Generator
[R14514] 4700
1 7 POWER AMP 4 5
U14501
-15Vr
C14505 .22UF
VERTICAL YOKE WINDING
Figure 7-13, Yoke Current, Bottom to Center
100
Vertical Deflection Vertical Scan, Center to Top When the input ramp switches pin 5 of the output amplifier from the -15V supply to the +15V supply, the positive supply has switched to its "flyback" mode to increase the supply voltage available for retrace. (More on the flyback supply later.) The vertical input ramp is now increasing, and pin 5 of U14501 supplies current from ground, through the yoke to flyback supply, deflecting the electron beam to the top of the screen.
Center of Screen Yoke Current now Reverses and Begins Increasing, Driving Beam from the Center to Top of Screen
+Max Zero -Max
Electron Beam Travel
Figure 7-14, Electron Beam Movement, Center to Top
C14505 now charges towards the higher flyback supply voltage.
+15Vr
~25V
CR14502
C14503 220UF 6 3
2
Input Ramp From Deflection Generator
[R14514] 4700
1 7 POWER AMP 4 5
U14501
_
-15Vr C14505 .22UF VERTICAL YOKE WINDING
Figure 7-15, Yoke Current, Center to Top
Vertical Deflection 101 Vertical Scan, Top to Center When the input ramp reaches its peak and begins to drop, trace begins. The input ramp voltage is now decreasing and the output voltage at pin 5 proportionally decreases. Current through the yoke decreases, allowing the beam to descend towards the center of the screen. When the input current is about 1/2 way through its descending ramp, output current is zero and the beam is at or near center screen.
Center of Screen
Decreasing Yoke Current Again Allows Beam To Drift Back To Center
+Max Zero -Max
Electron Beam Travel
Figure 7-16, Electron Beam Movement, Top to Center
+15Vr
~25V
CR14502
C14503 220UF 6 3
2
Input Ramp From Deflection Generator
[R14514] 4700
1 7 POWER AMP 4 5
U14501
-15Vr
C14505 .22UF
VERTICAL YOKE WINDING
Figure 7-17, Yoke Current, Top to Center
102
Vertical Deflection
Center of Screen
Increasing Yoke Current Drives Beam Away from Center To Bottom of Screen
+Max Zero -Max
Electron Beam Travel
Figure 7-18, Electron Beam Movement, Center to Bottom Vertical Scan, Center to Bottom At this point, the cycle is complete and begins again. As the input ramp crosses over the zero current point, the -15V supply again supplies current to the output of U14501. Current is drawn from ground through the yoke, U14501 and to the -15V supply. C14505 again begins to charge from the -15V supply, through the yoke and to ground and the vertical scan cycle begins again.
+15Vr
~25V
CR14502
C14503 220UF 6 3
2
Input Ramp From Deflection Generator
[R14514] 4700
1 7 POWER AMP 4 5
U14501
-15Vr
C14505 .22UF
VERTICAL YOKE WINDING
Figure 7-19, Yoke Current, Center to Bottom
Vertical Deflection 103 Vertical Flyback Supply During retrace, the beam must be returned to the top of the screen awaiting the next frame of video to begin. Similar to horizontal scan, if a higher voltage is applied to the yoke during retrace the beam will return to the top much quicker than it scanned down. During most of vertical scan, dual matching supplies are desirable so a differental input may be used and output linearity is achieved. However, because retrace occurs in such a short time compared to trace, a much higher positive supply may be used to supply U14501 and not greatly upset the symmetry set +15V Supply up by the +15V supplies.
CR14502
Flyback Cap Charge Path
~25V
C14503 220UF
~+30V
6 2 3
During scan time (trace), the negative lead of C14503 on pin 3 is connected to the -15V supply inside U14501. Since the positive lead is connected to the +15V supply, there is now about 30 volts across the cap. During trace, the cap charges to about 25 volts from the negative supply to the positive supply.
1 7 POWER AMP 4 5
U14501
-15V Supply
Figure 7-20, Retrace Capacitor Charge Circuit At retrace, the flyback generator switch inside U14501 connects pin 3 to pin 2 applying the +15V supply to the negative side of C14503. CR14502 now blocks current flow between C14503 & the +15V supply. The charge stored on C14503 and the +15V supply on the negative terminal produce about +40 volts to ground on pin 6, which now begins to supply output current for the yoke. The increased supply voltage quickly retraces the beam to the top of the screen.
U14501-6
~25V
+15Vr ~25V
C14503 220UF
+15V Supply
CR14502
C14503 220UF
~+40V
6 2
3
Input Ramp From Deflection Generator
[R14514] 4700
1 7 POWER AMP 4 5
U14501
-15Vr
C14505 .22UF
VERTICAL YOKE WINDING
Figure 7-21, Retrace Capacitor Supply Circuit
104
Vertical Deflection Vertical Scan Rates Like horizontal scan, vertical scan must adapt to different rates in order to display the various inputs properly. At slower scan rates, compensation is provided to delay the start of trace until video information is available. The Deflection Processor, U14350, contains a register called STSC, which causes a delay from the start of the vertical scan leading edge. It may be set from 1 to 64 lines. The number of lines in vertical scan may also be programmed and selected by software between 200 and 900. Figure 7-24 shows the input ramps of the Vertical Output IC, U14601. Notice the horizontal settings for the oscilloscope. During the waveform capture they were set identically to assist in comparing the different scan rates. Supported resolutions and vertical refresh rates are shown in the table below.
Resolution Horizontal 720 720 640 640 800 800 Vertical 350 400 480 480 600 600
Vertical Refresh 70 70 60 72 56 60
Figure 7-22, Vertical Refresh Rates
Vertical Deflection 105
Standard Video
Computer VGA
DIGITAL
Component Video
Figure 7-23, Vertical Input Waveforms for Different Scan Rates