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192
CRT Management CRT Management Overview Controlling the beam with scan is one aspect of displaying a proper video on a CRT. Beam current must also be controlled in a defined fashion to provide video true to the original signal or to provide video closer to an "ideal" perception of the original signal. The MM101 uses several circuits after final video processing to properly set up the CRT to receive video data and peak CRT performance for the specific visual display. SVM (Scan Velocity Modulation) modulates scan to increase apparent contrast of high frequency luminance video. As with previous chassis' the MM101 employs an AKB (Automatic Kine Bias) system to track and compensate for the normal drift in beam current cutoff bias of a CRT. The MM101 uses a Dynamic Focus circuit to optimize the corner focus of CRTs larger than 27". Dynamic Focus modulates or varies the voltage to the CRT focus grids with a horizontal sawtooth and a vertical parabola signal. Scan Velocity Modulation Scan Velocity Modulation is used in the MM101 family chassis to produce a sharper picture without enhancing noise. Large amplitude transitions (black and white) with very fast rise/fall times are hard for most CRTs to display as the tube must go from cutoff, (or very nearly cutoff), to high beam current in a very short amount of time. To assist the CRT in the MM101 chassis, a separate yoke coil is placed around the neck of the tube in the vicinity of the electron guns. As the electron beam travels across the screen the SVM yoke modulates (accelerates/deccelerates) the beam to improve picture detail. SVM effectively reduces the size of the illuminated phosphor area while increasing the contrast. When the beam accelerates, fewer electrons will hit the phosphor, making the dark edges on the screen image appear darker. When the beam decelerates more electrons hit the phosphor, making bright edges on the screen image appear brighter. SVM circuitry is so effective, it can momentarily either double or completely stop beam scanning velocity as needed. SVM uses the first derivative of the luma video signal. The first derivative results in a slope detected output which peaks in amplitude during any high frequency black to white or white to black transition. During low level or low frequency video amplitudes, the SVM signal is zero. SVM is relatively unchanged with the exception of the addition of IIC bus control from the microprocessor to shut off SVM effects during OSD (On-Screen Display portions of scan. The entire horizontal line is shut off, not just the OSD portion of the line.
SVM Amplifiers
SVM Yoke
U22300 Video (Luma) SVM On/Off Switch
53
Delay
d/dt
25
48
Figure 14-1,
SVM Block Diagram
CRT Management SVM Operation SVM operation differs little from previous versions in the CTC179/189 and CTC195/197. The amplifier and yoke circuits are relatively unchanged, however the low level SVM signals are generated in the Video Processor IC, U22300. Only the luminance signal is sampled for SVM. This is because resolution of color detail by the human eye is not very acute. Edge detail is almost exclusively perceived by the black
+24Vr +9Vr +9Vr
R22346 100 R15215 10K
193
+24Vr
+24Vr
+24Vr
R15233 75
+24Vr
R15233 75
+210Vr CR15204 24V
Q15214
R15211 1000 R15203 1500 R15206 220 R15213 1500 R15205 1000 R15214 220 R15218 220 R15220 220
R15219 220
Q15216
R15233 75
Q22303 25 Clamp Key
SVM Gen
SVM Y Q22315
Q15215
Q15212 Q15210 R15223 360 R15228 10
R15234 5.6
1 4
L15203
6 5
R22329 270
Q15203 +24Vr R15207 39 R15241 9100
Q15204 R15208 39 Q15205 C15219 0.01 R15227 1000
R15226 220
Q15213
53
SEL Y2 IN
Q15212 R15236
R15237 5.6
Part of U22300 VIDEO PROC
/
SVM Yoke
Q15217
R15209 R15210 510 75
CR15201
+
C15217 10uF
Figure 14-2, SVM Circuit and white content of an image. The Video Processor, U22300, outputs an SVM processed signal based on this information. The video processor IC also provides phase, gain and parabola correction adjustments via the IIC bus. Parabola correction modifies the amount of SVM relative to the electron beam position on the screen. SVM is reduced as the beam moves away from center screen. SVM is turned off during blanking and also during OSD information. The IIC bus provides four levels of control over the SVM signal. They are: 2 bit phase adjustment to vary the SVM amplitude (0db, -6db, -9db and off) 2 bit delay adjustment (-40nS, -20nS, 0nS and +20nS) Parabola correction SVM defeat for OSD SVM Protection The SVM power supply has been increased to +180V resulting in an increase of maximum peak-to-peak output voltage. Q15205 has been added to protect the SVM output circuits from overheating during program material that may have an excessive amount of high amplitude black to white transitions. These transitions can produce high SVM yoke currents resulting in high dissipation in the SVM drivers and outputs. A DC bias is developed by C15217 representing power dissipation in Q15212/13. CR15201 acts as a switch to reduce current in bias transistor Q15205 when power dissipation voltage exceeds Q15205-B voltage. Q15205-B is set by a resistor divider network consisting of R15241 and R15209. This junction is about +1.2V. Normally the voltage at Q15205-E is +0.6V placing it in full conduction. If SVM yoke current increases, the power dissipation voltage developed at C15217 also increases. As Q15205-E is pulled higher, current through Q15205 decreases. This pinches off current in the differential amplifiers Q15203/04 and drive current decreases. When drive current decreases, the voltage at Q15205-E decreases, increasing the bias. More current is allowed to flow in the differential amplifiers and operation returns to normal.
194
CRT Management Dynamic Focus Dynamic Focus in the MM101 is used to correct the focus of the electron beam as the distance between the electron gun and the phosphor surface changes. In other words, without dynamic focus or dual focus, if the beam was adjusted to provide optimal focus of the horizontal scan lines vertical lines may look slightly soft. If the beam was adjusted for optimal vertical lines, horizontal detail may look soft. Some compromise was required. Dual focus grids are used to optimize focus in the vertical and horizontal direction. Fixed voltages on the two focus grids control the electron beam shape. One optimizes vertical picture elements, the other optimizes horizontal picture elements. This is used in the traditional "Dual Focus" CRT's, however it still results in some compromise. Dynamic focus modulates the voltage difference between the two focus grids such that focus uniformity is achieved over the entire picture. Dynamic Focus Operation Dynamic Focus uses the vertical E/W correction waveform and a sample of the horizontal S-Cap waveform to modulate the outer focus grid of the CRT. This grid is labeled "FOC2" on Service Data. Dynamic Focus in the MM101 may be divided into four areas. First is a +1700V high voltage supply generated from the horizontal output transformer. Dynamic focus must modulate the focus voltage nearly 1400Vp-p. This required a higher supply than was available from the main supplies. An extra winding boosting the flyback waveform of the horizontal output transformer is used. Second is an adaptive filter used to control the gain of the incoming horizontal S-cap waveform. This achieves a horizontal focus parabola amplitude consistency for the different scan rates. Third is the focus voltage amplification circuitry. It provides the voltage used to vary focus. Fourth is a dynamic focus blanking circuit required to prevent interference with AKB operation. +1700V
E/W Correction Parabola from U14732-7 C14736 R14733 SUPPLY CR14713 C14727 CR14710 C14732 300Vpp VERT 1000Vpp HORIZ
R14739 ATTENUATOR Q14732, Q14734, Q14707 FREQ. COMP. FILTER R14783 C14724 P-P DET C14725 CR14711 C14740 R14732 FOCUS SUBSTRATE
S-CAP Parabola From Horizontal Deflection
+ R14729 HV AMPLIFIER Q14738 Q14739 Q14731 Q14730
DYNAMIC FOCUS BLANKING SWITCHES U14702 Q14708
VERTICAL BLANKING
Q14735 Q14736 +
2.88V REFERENCE R14784 R14786
Figure 14-3, Dynamic Focus Block
POWER EQUALIZER SWITCH Q14710
N T S C IH MODE
CRT Management Dynamic Focus Power Supply Focus voltages must vary about 1300V in order to maintain correct beam focus from the center portion of the screen to the outer edges. Pin 4 of T14451 , the horizontal
14 1 EY14467 L14454 180UH
195
SCAN B+ + To Horizontal Yoke
C14450 10u 2000V
8
4
+1700Vr
12 7 C14727 3300pF CR14710 C14723 3300pF 3KV
CR14713
Horizontal Output Circuit
From HOT Q14451-C
Figure 14-4, Dynamic Focus Power Supply output transformer, reaches about +1300V during the flyback portion of horizontal scan. In this circuit T14451 acts like an autotransformer. Pin 7 has more turns and develops about +1700V. Most of this is available to the dynamic focus circuits. C14727 protects the deflection transformer from overcurrent damage in the event of component failure in the focus circuits.
SCREEN 1/3 TAP FOC2 FOC1
The +1700V supply is switched off the focus grid to prevent interaction with AKB. This is accomplished by shutting off the +1700V switch, Q14730 during vertical blanking.
R14688 100
R14768 +12Vr +1700Vr
3
R14747 820 CR14709 R14736 1Meg CR14705 Q14730 R14753 100 C14721 39
2
1
EY14416 +1700Vr
R14735 10K
CR14750
DYNAMIC FOCUS BLANKING
Q14740
R14790 5600
CR14747 +12Vr U14702 R14798 1000 Q14731 R14694 1000 R14799 5600 Q14710 +12Vr Q14738 R14732 C14740 3800 0.1uF R14781 270 Dynamic Focus Parabola R14794 100 Q14708 R14733 C14736 7500 47uF From E/W Correction IC, U14732-7 From Horizontal Adaptive Circuit R14739 1Meg CR14746
1H VCC BUFFER 1H: +12V 2.xH: 0V
R147695 10K
R147696 4700
+2.88V Reference
Q14739
Figure 14-5, Dynamic Focus Amplifier
196
CRT Management Dynamic Focus Adaptive Filter The adaptive filter automatically gain adjusts the S-cap voltage to standardize the dynamic focus signal in all modes (see Tech Tip on next page). A voltage to current converting clamp consisting of C14735, CR14748 and R14731 AC couples the S-cap parabola voltage and negative peak clamps it to +12Vr. R14731 then supplies a parabola shaped single polarity current to the emitters of attenuator transistors Q14732/34. Most of the current from R14731 flows through Q14734 and produces a parabola shaped voltage across R14730 which is buffered by emitter follower Q14707 and passed to the dynamic focus high voltage amplifier input. If this voltage is too high, Q14732 is turned on by an attenuation control signal to divert the excess current to ground and attenuate the R14730 signal. The control signal is formed by a sample waveform from the output of buffer Q14707. It is low pass
+12Vr
From Horizontal S-Cap
C14735 0.01 R14691 100 R14734 8200
CR14748 R14731 43K
+12Vr CR14712 +11.3V
+12Vr
R14782 6.2K
Q14732
Q14734 R14730 8200
R14784 8200 R14783 5100
Horizontal Dynamic Focus
Q14707
+2.88V C14725 1uF R14786 CR14711 2200
Q14736
Q14735
R14740 10K
+ C14731 10uF
C14783 390
Figure 14-6, Dynamic Focus Adaptive Filter filtered by R14783 and C14783, peak to peak detected by Q14735 and CR14711, and compared to a 2.88V reference voltage by differential amplifier Q14735/36. The low pass filter attenuates the gain control feedback at the higher frequencies. The overall effect causes the buffer output Q14707-E to be greater at high frequencies. This is required to correct a response roll off of the dynamic focus high voltage amplifier.
CRT Management Dynamic Focus Theory Dynamic Focus corrects the electron beam focus for the differences in distance from the electron gun to the phosphor surface. At the screen center this distance is short and in the corners the distance is greater. The distance varies in a parabolic manner so a parabolic correction voltage is added to the DC focus. A greater amplitude parabola is added for the horizontal direction than the vertical direction because the picture is wider than it is tall. Dynamic Focus may be used with single electrode focus as in projection CRTs or with dual electrode focus. Dual electrode focus uses 2 DC focus voltages and 2 adjustments. One optimizes the beam spot vertically and the other optimizes the spot horizontally. The Dynamic Focus signal is capacitor coupled to one of the two electrodes and the other is capacitor bypassed to ground. The Dynamic Focus amplitudes are chosen to make the focus effect uniform across the entire picture. One important point of Dynamic Focus is that the same grid voltage will provide focus at the same physical location regardless of scan rates. In other words, if 6.25KV provides sharp focus at about the midway point between center screen and the outer edge, that voltage will provide sharp focus at that point whether the scan rate is 1H, 2H or higher. The same applies to the vertical position. If 5.75KV provides sharp focus midway between center and upper edge, it will always provide the same focus regardless of the vertical refresh rate.
197
TECH TIP
198
CRT Management Dynamic Focus Waveform Amplification Now that the dynamic focus waveform has been generated it must be amplified to the correct amplitude. The dynamic focus amplifier is a very high voltage inverting feedback amplifier similar to an operational amplifier or a kine driver. The summing junction is at Q14738-B. The non inverting input Q14739-B is connected to a 2.88V reference voltage. The horizontal and vertical parabola signals are AC coupled and gain adjusted via C14736, R14733, C14740 and R14732. R14729 sets the DC operating point. Q14738 is a current generator that is cascode connected with Q14708, the blanking switch and high voltage transistor Q14731. The feedback resistor is R14739. Q14730 is a bootstrap pull up. Q14731 and Q14730 operate as a class B push pull circuit. A boost supply consisting of CR14709, C14721 and R14736 provides base drive for Q14730 so that it can drive the output all the way up to the 1700V supply. C14721 charges during the half cycle while Q14731 is conducting and discharges through R14736 into the base of Q14730 during the half cycle that Q14730 is conducting. It is necessary to eliminate Dynamic Focus voltage variations during the time that AKB is measuring the CRT current to prevent measurement error. This is done with 2 switches, U14702 and Q14708. U14702 turns off during vertical blanking and causes Q14730 to be continually on. Q14708 turns off during vertical blanking and replaces the normal amplifier current from Q14738 with a constant current determined by R14799. This current is chosen to be equal to the average current of normal amplifier operation. This even loads the power supply to prevent transient picture distortion. In 1H mode the average power is less, so Q14710 turns on and reduces the current during blanking.
SCREEN
1/3 TAP
FOC2
R14768 +12Vr +1700Vr
FOC1
3
R14747 820 CR14709 R14736 1Meg CR14705 R14688 100 Q14730 R14753 100 C14721 39
2
1
EY14416 +1700Vr
R14735 10K
CR14750
DYNAMIC FOCUS BLANKING
Q14740
R14790 5600
CR14747 +12Vr U14702 R14798 1000 Q14731 R14694 1000 R14799 5600 Q14710 +12Vr Q14738 R14732 C14740 3800 0.1uF R14781 270 Dynamic Focus Parabola R14794 100 Q14708 R14733 C14736 47uF 7500 From E/W Correction IC, U14732-7 From Horizontal Adaptive Circuit R14739 1Meg CR14746
1H VCC BUFFER 1H: +12V 2.xH: 0V
R147695 10K
R147696 4700
+2.88V Reference
Q14739
Figure 14-7, Dynamic Focus Amplifier (Repeated)
CRT Management Dynamic Focus Blanking Dynamic Focus must be stopped or blanked during the three AKB test lines that occur in 3 of the 4 lines that immediately follow the vertical blanking interval. The vertical blanking pulse is stripped from composite sync using a peak detector and low pass filter, then input to U14733-4. It is then digitally delayed at the horizontal rate by clocking it through 4 D flip flops. The vertical pulse delayed 1 line from U14733-15
Vertical Blanking Interval
199
Figure 14-8, Exploded View: Vertical Blanking Portion of DSC and the vertical pulse delayed 4 lines at U14733-3/5 are "OR'ed" to drive Dynamic Focus blanking switch Q14740-B. This switch blanks Dynamic Focus and maintains average power supply power as previously described.
Composite Blanking from U14301-3
Vert Pulse Stipper
4
U14733 D Flip Flip Delay Circuit
15 3 5
R14792 10K
R14688 Q14740 100
DYNAMIC FOCUS BLANKING
R14796 10K
R14790 5600
Horizontal Pulse
9
Figure 14-9, Dynamic Focus Blanking Stripper & I/O Waveforms
200
CRT Management The dynamic focus blanking signal is input to the blanking switch Q14708-B and also an opto-isolator, U14702-1. When vertical blanking occurs, the Dynamic Focus Blanking signal is pulled low, shutting off U14702. At the same time, Q14708-B is also pulled low, shutting off the dynamic focus drive signal to Q14731. A current path now exists from the +1700V supply through CR14709, R14736, Q14730-B/E, CR14746, Q14731-E/C (biased on by the +12V supply and Q14710) and R14799. The Dynamic Focus Blanking signal is at ground completing the current path. Q14730-E is now at or very near the +1700V supply. It is also undesirable to have the +1700V supply switch from full on to full off. This might add interference, (causing horizontal scan distortion) to the horizontal scan transformer where the +1700V supply is generated. To equalize horizontal output transformer loading during dynamic focus and dynamic focus blanking, Q14731 is biased to continue drawing current with or without the DF parabola driving it. Finally, the second focus grid should have adequate voltage to maintain sharpness when scan begins. The configuration of the dynamic focus circuit allows the start of dynamic focus after blanking during the maximum peak of the DF parabola.
SCREEN 1/3 TAP FOC2
R14768 +12Vr +1700Vr
FOC1
3
R14747 820 CR14709 R14736 1Meg CR14705 R14688 100 Q14730 R14753 100 C14721 39
2
1
EY14416 +1700Vr
R14735 10K
CR14750
DYNAMIC FOCUS BLANKING
Q14740
R14790 5600
CR14747 +12Vr U14702 R14798 1000 Q14731 R14694 1000 R14799 5600 Q14710 +12Vr Q14738 R14732 C14740 3800 0.1uF R14781 270 Dynamic Focus Parabola R14794 100 Q14708 R14733 C14736 7500 47uF From E/W Correction IC, U14732-7 From Horizontal Adaptive Circuit R14739 1Meg CR14746
1H VCC BUFFER 1H: +12V 2.xH: 0V
R147695 10K
R147696 4700
+2.88V Reference
Q14739
Figure 14-10, Dynamic Focus Amplifier (Repeated)
CRT Management Dynamic Focus Troubleshooting The most common failures in the Dynamic Focus section will be diodes, the High Voltage transistors and the opto-isolator. These are SAFETY DEVICES. DO NOT USE SUBSTITUTES. Consult the latest TCE Service Data for part numbers. Look for the proper response at R14735 (EY14416) with a X100 High Voltage Probe. This should be a 1000V horizontal rate parabola summed with a 300Vp-p vertical rate parabola with a vertical blanking line at about 1400V and a minimum voltage of about 100V. If the dynamic focus signal is clipped at the positive peak near vertical retrace, check the bias boost components around Q14730.
201
The 1700V power supply normally has 200V p-p of vertical rate modulation due to E/W correction. It should measure between 1500V to 1700V. Check different modes to assure the horizontal adaptive filter gain control circuit is working. Switching between modes should not affect the amplitude of the horizontal dynamic focus waveform at Q14707-E greatly, although some degree of change is normal. A low 2H transient pulse due to the switching power supply located near the negative peak of the 1H waveform is normal. Loss of power matching (Power Equalizer Block)during vertical retrace will cause a width modulation ring at the top of the picture. Loss of dynamic focus blanking will cause blue color temperature errors. If Dynamic Focus Blanking is lost, AKB may be affected. Further discussions of AKB follow. If a failure resulting in the loss of current draw from the +1700V supply during blanking occurs, raster hooking will result. The hooking will draw inward due to increased loading on one side of the horizontal output transformer rather than an equalized current draw. Focus Adjustment 1. Apply a crosshatch signal to the appropriate input and adjust the set for a normal picture. Use the factory resets in the customer menus for "Bright Lighting". 2. Adjust FOC1 for good horizontal line focus. 3. Adjust FOC2 for good vertical line focus. 4. Check raster for overall focus, touching up as needed. Small increments should be used at this point. NOTE: There is some interaction between the two grids. Some compromise may be required to provide the best overall focus.
202
CRT Management CRT Drivers The CRT driver circuit consists of three single IC video amplifiers and supporting components. Each IC is a single 30MHz bandwidth, 125Vp-p video output amplifier contained in a 13 pin single in-line (SIL) package. The device was specifically designed to drive CRT cathodes in high-end and HDTV capable television applications. The device is suited for operation in the MM101 due to its wide bandwidth, high power output, low static power draw (app. 2.5W), excellent protection against kine arcs and power supply glitches and a cathode current measurement output, labeled Ik, used for Automatic Kinescope Bias (AKB). The Ik output level does not match the video processor IC AKB sense input level and requires some level shifting to be useful. The IC requires two supplies. A 12V DC supply for low signal amplifiers and a +205V supply for the final output amplifiers. An input voltage approximately 2Vp-p will drive the output to about 140Vp-p. CR15111/21/31 protect the IC's from kine arcs.
+205Vr
+12Vr R15112 100 2 6
R15141 100
CR15111
R15142 470 10 Driver Vcc Out R15115 330 12
U15101 Red Driver Ik Sense 7 FROM VIDEO PROCESSOR U22300-43 FROM VIDEO PROCESSOR U22300-42 FROM VIDEO PROCESSOR U22300-41 RED DRIVE 8
+205Vr FILAMENT
GREEN DRIVE R15122 100 2 BLUE DRIVE
+12Vr
R15151 100
CR15121 G1 +10V 12 Out BLUE 12 R15125 330 RED 9 GREEN 7
R15152 470 6 10 Driver Vcc
U15102 Green Driver Ik Sense IK Sense TO AKB Circuits 7 8
+205Vr
R15135 330
+12Vr R15132 100 2 6
R15161 100
CR15131 FOC2 R15162 470 10 12 Out R15106 2200 1/2W 10% FOC1 FOCUS
BC15103 SCREEN
U15103 Blue Driver Ik Sense 7 8
Driver Vcc
Figure 14-11, CRT Drivers
CRT Management
+205Vr +12Vr +12Vr
203
R15177 330K
R15176 330K
R15175 20K
CR15171
CR15172
R15175 330 C15173 4.7uF 350V Q15172
Q15171 R15173 20K
C15172 470uF
+12Vr
R15178 100K CR15174 Q15173 CR15173
R15170 2000
+9V
R15172 5100 R15179 220 1/2W 10%
R15174 20K
C15174 2200 500V
FILAMENT RED 9 GREEN 7
G1 +10V
Grid Kick
Figure 14-12, Grid Kick Circuit
New for direct view sets is a "grid kick" circuit to prevent after glow. When the +12V or +205V supply is removed, the cathode output at pin 12 is clamped to ground which may cause after glow. Beam current may still flow, but with no scan and diminishing focus voltage, it appears as an out of focus "blob" in the center of the screen. In severe cases, if focus voltages hold, it may damage the phosphor. Projection CRT's have used some form of beam current cutoff for several generations. The MM101 drives the CRT harder than previous chassis and therefore utilizes the grid kick circuit to ensure beam current cutoff at shut down or with the loss of run supplies. During normal operation, Q15173 is biased on and uses the +205V supply to provide about +10V of control grid bias. When the +12Vr supply is up, C15172 charges to about +12V through CR15171. Q15171 and Q15172 are biased off. C15173 charges through Q15173 and CR15173 to about 180V. When the chassis is shut down, Q15173 turns off, removing the +205V supply. C15172 maintains +11.5V for a short time forward biasing Q15171 from the decaying +12V supply on its base to the emitter. Q15171 and Q15172 now turn on. This grounds the positive terminal of C15173. Since it is normally charged to +180V and the negative terminal is connected to the control grid, the grid is now at about 180V. With the control grid now at this negative potential and the cathode at around zero volts, beam current is severely limited or completely stopped, preventing after glow.
204
CRT Management AKB Theory Basic CRT theory states beam current cutoff is dependant upon a bias voltage between the cathode and screen grid. If an incoming video signal black level is not matched to the actual beam cutoff level of the CRT, the color temperature, or apparent color tint, of the low level light output of the CRT will change. Maintaining this match allows the original color temperature setup established at the time of manufacture to provide the same color renditions over the lifetime of the CRT. AKB will require setup only in the event of CRT replacement or G1 (Control) and G2 (Screen) voltage errors that require component replacement. More CRT background is available in TCE publication #T-CRT95-1, Cathode Ray Tubes. Hardware based AKB is discussed in detail in the CTC 179/189 Technical Training Manual, T-CTC179/189-1. CRT beam current is varied by changing the voltage on the cathodes. In short, the cathode voltage required to bias a CRT to zero beam current may drift over time. AKB tracks the drift, providing a correction bias allowing incoming video to always know exactly where beam current cutoff is. The MM101 AKB performs a sample and compare of each electron gun every vertical field and makes CRT cathode bias adjustments accordingly. Changes in bias are minimal preventing any rapid change in color temperature by AKB. AKB response time is limited to an update rate which adequately corrects long term cutoff bias drift without causing noticeable instantaneous changes in color temperature. The MM101 AKB system is hardware based and completely contained in the video processor IC, U22300 using feedback from the CRT driver IC's, U15101/02/03. AKB pulses of known amplitude are placed on the outgoing RGB drive signals from the video processor to the CRT drivers. This pulse is high enough to draw beam current. The pulse is then fed back to the AKB circuits in the video processor and compared to a threshold level to determine if proper beam current has been drawn. If it is not acceptable, the bias level of the RGB outputs of the video processor are adjusted until proper beam current is drawn.
AKB Pulse
R IN
Red 43 Drive
AKB Ref
Red CRT Driver G1 +10V
FILAMENT
U22300 Video Processor
Green 42 Drive Green CRT Driver
G IN
AKB Ref
B IN
Blue 41 Drive
AKB Ref
Blue CRT Driver
AKB Sense 56
FOC2 Voltage Level Shift and Pulse Shaping AKB Reference AKB Sum FOCUS FOC1 SCREEN (G2)
AKB Pulse Vertical Sync Pulse from U13101-34
Figure 14-13, AKB Block Diagram
CRT Management AKB Operation AKB is active during all alignment and setup periods except screen control adjustment and updates continuously on every vertical field. Fixed amplitude pulses are generated by the video processor IC, U22300 and placed on the red, green and blue drive outputs. These pulses are put on progressive horizontal lines and occur immediately after the start of scan, but still prior to active video. This prevents the pulses from being seen by a viewer. They are placed on different lines so the video processor can recognize the correct R, G or B signal. The AKB pulses may be noticed in 2.xH modes that do not require overscan. In this case, the pulses occur on the first three full lines of scan and if that portion of screen information is low brightness, the pulses may be visible. Since all beam current is supplied by the CRT drivers, by measuring current from those drivers, beam current may also be measured. A voltage representative of beam current is fed back to the video processor and used by AKB to adjust the DC bias level of the RGB outputs from pins 41, 42 and 43. Changing the DC output results in varying the CRT driver output current and consequently beam current. In essence, AKB acts like a servo system varying CRT cathode bias voltage to hold beam current cutoff at a known fixed amount over the usable life of the CRT. Zero Beam Current Before AKB can judge what cathode bias voltage is required to provide exact beam current cutoff, it must know what voltages are present in the CRT drivers when there is no beam current. The CRT drivers are measured during blanking (zero beam current) to remove unwanted or erroneous current from the measurement prior to the AKB adjustment period. Any voltage measured during this period is the result of CRT driver current, not beam current. It is nulled from the AKB circuitry by the voltage level shift and pulse shaping network. Then, during the AKB sample period, the drive current from U22300 is adjusted until the desired voltage level from the CRT drivers is reached.
Blanking Level (0Vdc)
R IN
205
TECH TIP
Red 43 Drive
AKB Ref
Red CRT Driver CRT Driver Noise (+4.6Vdc) G1 +10V
FILAMENT
U22300 Video Processor
Green 42 Drive Green CRT Driver
G IN
AKB Ref
B IN
Blue 41 Drive
AKB Ref
Blue CRT Driver
AKB Sense 56
FOC2 Voltage Level Shift and Pulse Shaping AKB Sum FOCUS FOC1 SCREEN (G2)
AKB Level (0Vdc)
Vertical Sync Pulse from U13101-34
Figure 14-14, AKB Block Diagram (Repeated)
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CRT Management AKB Sample Pulse The AKB sample pulse is approximately a 10 IRE signal output from the final video amplifier of the video processor, then measured at the output of the CRT drivers. The bias current, when set properly, is the amount required to drive the CRT drivers enough to draw a minimum current from the electron gun. This becomes the AKB reference current output from the RGB drives of the video processor and is CRT cutoff. The reference voltage is then locked and made available on the video processor RGB output amplifiers. Video information is added to the bias current, matching black video information with the bias and applied to the cathode. There is an AKB reference threshold inside U22300 fixed at +1.6V used for AKB operation. The 10 IRE signal is output from the RGB outputs of the video processor. It then drives the CRT drivers and may or may not draw beam current. If the AKB pulse returns from the cathodes, the AKB bias is instantly decreased, lowering CRT cathode drive to decrease beam current. This becomes the new AKB pulse. AKB bias is continuously lowered until no feedback is returned. AKB then assumes the bias is too low and now begins to incrementally increase the bias by increasing the AKB pulse. This servo process is now similar to the previous description only the AKB bias control is now increasing instead of decreasing. It is important to note that the adjustable bias and subsequent AKB feedback may be any level, although they normally will be somewhere around +1.6V. AKB Pulse Level Shift As previously stated, the AKB pulse is generated internally by U22300, the video processor and applied to the CRT drivers. However, it is not possible to return it back from the CRT drivers without some level shifting and wave shaping. The cathode current sense output (Ik) from the CRT driver IC, and the AKB sense input of the video processor are not normally compatible levels. Also, the current sense output from the CRT drivers contains some DC idle current error that must be compensated for. The AKB level shift circuit establishes a DC reference during blanking. Since no beam current is drawn during blanking, any voltage generated from the CRT driver measurements, must be internal leakage and bias errors. By nulling this out, any current measured during the AKB period and passed back to the AKB sense line will be beam current. A voltage divider consisting of R15185, R15186 and R15187 set bias voltages on Q15107 and Q15108. Another voltage divider consisting of R15181, R15182, R15183 and R15184 bias Q15105-B at about +4.8V. +4.8V is required as a reference to offset any erroneous currents within the CRT driver IC that may occur when the cathode current measurement output is below +4.0V. With no input, if Q15104-B is above +4.8V, current flow is from +12Vr, R15194, Q15105-C/E, R15107 and C15109. C15109 begins to charge, increasing the voltage at Q15109-B.
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C15181 470 Q15180
R15197 1000 C15180 10
+12Vr
+4.8V
+12Vr
R15194 20K
+12Vr
R15100 51K R15180 100
C15108 10
R15181 2700 Q15105 Q15110
Q15104
+4.8V
R15182 660
Q15103 +12Vr R15104 1000 R15105 33K R15108 1000 R15103 10K Q15101 R15101 150K
+12Vr
+12Vr
+12Vr
R15183 1000
Q15106
R15102 1000
From Red CRT Driver, U15101-7 R15192 100
R15188 3600
R15185 430
R15184 1100 Q15108 Q15107 Q15109 R15186 1200 R15107 1000
C15107 + 100uF R15193 5100 +12Vr R15110 10K
Q15102 AKB Pulse To Video Processor U22300-56
From Green CRT Driver, U15102-7
R15109 1000
R15196 20K R15195 1000 Q15112
From Blue CRT Driver, U15103-7 R15191 2000 R15189 100 R15187 2700
+
C15109 100uF
R15190 270K
Vertical Sync from U13101-34
Figure 14-15, AKB Pulse Level Shift Circuit
As current flow increases in Q15109-C/E, the voltage on Q15109-C decreases. When Q15109-C decreases, Q15104 begins conducting more current, decreasing current in the mirror transistor, Q15105. C15109 voltage now decreases and the process begins again. The servo loop acts to maintain a constant +4.8V on Q15104-B during the AKB pulse period, cancelling error currents. Q15112 and Q15110 disable the loop during active scan allowing the AKB pulse to pass. Vertical sync at Q15112-B is zero volts during active video, turning on Q15112 and Q15110. With Q15110 on, supply is removed from Q15104/05 cutting the servo loop. During vertical sync, Q15112 and Q15110 are off returning the +12V supply to Q15104/05-E. The remainder of the circuit is for waveshaping and buffering. Q15180 and associated components reduce overshoot of the AKB pulses that might have upset AKB action. Q15101 and Q15103 remove the +4.8V bias from the cathode drivers, leaving about +0.8V. Q15102 is the final buffer, raising the nominal AKB pulse to around +1.6V. The entire AKB system is now acting as a servo loop to maintain beam current during cutoff at around 25uA. Once the AKB pulse is "read" by the video processor during the three lines of AKB, the bias is "locked" for the remainder of active video. At the start of the next field, the process begins again.
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AKB Start up When the MM101 is first turned on the CRT takes a short time to reach stable levels of operation. During this time, adjustment of any bias voltages would not be effective and would require greater adjustment to bring the biases back to normal after CRT warm-up. For that reason, early chassis versions will have AKB disabled for the first 12 seconds of the startup sequence. Later versions will reduce this time to around 6 seconds. At the end of the startup sequence, AKB reference voltages on the RGB drives are fixed at a predetermined level. That level is stored in the main EEPROM and downloaded to the appropriate video processor registers during startup. This assures bias values close to nominal levels during the first 10-20 seconds of operation. Once startup is complete, these AKB values are discarded in favor of real-time operation and AKB becomes independent of microprocessor control. When the set is shut down, there are no AKB values stored for the next startup. AKB begins self-alignment and operation automatically, following the above sequence. AKB Adjustment There are no direct AKB adjustments. The AKB system completely self-contained, real time and continuous after the CRT warm-up period. CRT screen control adjustment and color temperature alignment are required for proper AKB operation.
TECH TIP
Color Temperature Shift If a noticeable shift in color temperature occurs during startup, time when the shift happens. If it is noticed at about 12 seconds after startup, the shift is normal. This is when the factory settings for AKB are downloaded to the video processor. The factory settings are values aligned and stored in the EEPROM at the time of manufacture and cannot be changed without Chipper Check. However, as the CRT ages, they may no longer provide an acceptable picture. As soon as AKB begins working, it should adjust the CRT back to acceptable color temperature.
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IRE
Blanking
7.5 15 (Setup)
22.5
30
37.5
45
52.5
60
67.5
75
Figure 14-16, Typical Staircase Pattern Color Temperature Alignment Patterns Color temperature alignment for the MM101 will vary from previous chassis. Some form of staircase pattern similar to Figure 14-16 and 14-17 will be required. Proper identification of the "0" (if available) and "7.5" or "setup" bars on screen and the waveform produced at the cathodes of the CRT will be needed. Consult the specification manual for the pattern generator to confirm the location of these bars. The oscilloscope waveform shows the relationship between the bars and the video signal at the cathodes of the CRT. This waveform is present on all three cathodes. With the oscilloscope adjusted to provide a full peak to peak readout of the waveform at the horizontal rate, the 7.5 IRE setup bar will be the critical area. Be certain this bar can be identified using the equipment available. If a 7.5 IRE bar is not available, 10 IRE may be used. It should also be noted that bar patterns differ. Some vary from 10 to 100 IRE in various steps and in different directions, but most should have an identifiable 7.5 or 10 IRE bar.
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CRT Management Screen and Color Temperature Setup As with previous AKB systems, there is no direct AKB setup. AKB is automatic and requires no technician adjustments. However, the screen control must be setup and color temperature must be adjusted to proper levels for AKB operation. AKB will then maintain the cutoff bias of the CRT based on these initial adjustments. There are additional color temperature related setups that must be performed prior to the final color temperature alignments.
To Adjust the Screen Control:
NOTE: AKB will remain "ON" for screen control adjustment. Bias and drive settings should not be changed unless the eeprom has been changed and no longer has the typical values for these settings. The screen control alignment with AKB on is very accurate and not significantly dependent on these bias/drive settings. 1. Apply a vertical gray bar staircase pattern (at least 8 bars from "7.5" to ">75" IRE similar to Figure 14-17) to the video input and adjust the set for operation from that input. Identify the "7.5" IRE bar location both on-screen and using an oscilloscope. The 7.5 IRE bar is "black" or "cutoff" and will be used for reference. On most patterns, the remainder of the bars will progressively become brighter. 2. Use the "Reset" function on the consumer picture quality controls to place black level, contrast, color, tint and sharpness to their nominal factory default settings. Use the "Picture Presets" to place the set in "Normal Lighting" mode. 3. Allow the instrument to warm up with the staircase pattern or active video on screen for at least 15 minutes. Make certain the staircase pattern is again on screen before proceeding. 4. Using an oscilloscope, measure all three CRT cathode voltages and identify the cathode whose "7.5" IRE pattern is the greatest. 5. Connect the oscilloscope probe to the cathode identified in step 4. Slowly adjust the Screen Control until the 7.5 IRE bar is at 170V. 6. Do not change the screen control after this setup is complete. Any further adjustments will be done with the color temperature controls. Measuring CRT Cathodes It is difficult to monitor cathode voltage with the shield on the kine driver board. Oscilloscope probes may also cause some loading error. To assist the technician in performing these measurements, a test lead may be made using a 2200-3300 ohm 1/2 Watt resistor with an alligator or hook type clip soldered to one lead. The clip will easily connect to the end of R15116 (Red), R15126 (Green) or R15136 (Blue) where they connect to the kine driver IC U15101/02/03 at pin 13. These resistors are easy to locate and connect to from the open side of the shield. They are mounted on metal standoffs next to the IC's. This test point will give an accurate measurement of cathode voltages. Once the highest cathode is determined, the connection to this cathode is used to set the 0 IRE or 7.5 IRE cathode voltage by adjusting the screen control. NOTE: The connection between the clip and the resistor should be fairly short. The connection between the resistor and the probe can be longer. It may be easier to have three lead assemblies, connecting all three prior to screen adjustment. The probe should be at least 10x or 100x. Connecting the probe without the resistor can result in oscillation of the kine driver.
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IRE
Blanking
7.5 15 (Setup)
22.5
30
37.5
45
52.5
60
67.5
75
170V
Figure 14-17, Typical Staircase Pattern (Repeated) Color Temperature Background The purpose of color temperature setup is to assure uniform gray level from black to the brightest scenes. If a uniform gray screen is displayed, no matter the brightness level, no tinting in either the red, green or blue direction should be apparent. This is known as "color tracking". Once the proper color temperature is set, AKB will maintain the cutoff of the CRT to assure proper low light performance. Assuming color tracking of the CRT does not change, this will also maintain the high level picture color temperature. 6500K is a much "warmer" picture than 9300K conforming more closely to the original NTSC standards. 9300K is much "brighter" resulting in an apparent brighter display at the expense of shifting the color temperature towards blue. There are two alignable user selectable color temperature settings, 6500K and 9300K. Each requires color temperature and light output alignment. The 6500K alignment should be performed first, followed by 9300K. YUV Cutoff or RGB Cutoff will be adjusted only during the 6500K setup. A single adjustment is good for both temperature settings. If a CRT or video processor IC has been replaced, the technician should perform screen control alignment first. Then a critical evaluation of the color temperature setup of the CRT should be done. In most cases, color temperature will be acceptable and not require alignment!
TECH TIP
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Color Temperature Setup (YUV):
NOTE: Perform YUV Cutoff and 6500K setup first. 1. Apply a vertical gray bar staircase pattern (at least 8 bars from "7.5" to ">75" IRE) to the video input and adjust the set for operation from that input. Identify the 7.5 IRE bar location. It is the "black" or "cutoff" bar. For these adjustments, this bar and the next brighter bar will be used. On most patterns, the remainder of the bars will progressively become brighter. If a CRT or main EEPROM has been replaced, use Chipper Check or the front panel menu to set the Red, Green and Blue "Cutoff" values to midrange. 2. If all three colors (red, green and blue) appear too dim or too bright, adjust "YUV Cutoff" until any single color disappears from the black bar, but is still visible in the adjacent bar. Note that color. For instance, the black bar is plainly visible and close to gray. By decreasing the YUV Cutoff control all three colors will begin to dim. Blue may the first color to disappear from the black bar, leaving a red/green (yellow) tint to the black bar. YUV Cutoff adjustment should stop at this point. 3. Adjust the appropriate remaining red, green or blue "Cutoff (6500)" controls until any red, green or blue tint disappears from the black bar, but is still visible in the adjacent bar. When properly adjusted, the adjacent bar should be a very low level gray with no color tinting. Low level color temperature setup is now complete. 4. Now observe the brighter portions of the bars. Adjust the red or blue drive controls to remove any signs of red or blue tint in the higher brightness bars. Observe the bars for signs of CRT overdrive. Overall brightness may be adjusted using the "YUV Light Output-6500)" control. Some compromise may be required, but the higher IRE sections should be as free from color tinting as possible. Color temperature setup is now complete.
>75 IRE "White"
Shades of Gray
7.5 IRE "Black"
>75 IRE "White"
Shades of Gray
7.5 IRE "Black"
Shades of Gray
>75 IRE "White"
Figure 14-18, Typical Grayscale Setup Patterns
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Color Temperature Setup (YUV) continued:
NOTE: 9300K Setup. 1. Apply a vertical gray bar staircase pattern (at least 8 bars from "7.5" to ">75" IRE) to the video input and adjust the set for operation from that input. Identify the 7.5 IRE bar location. It is the "black" or "cutoff" bar. For these adjustments, this bar and the next brighter bar will be used. On most patterns, the remainder of the bars will progressively become brighter. If a CRT or main EEPROM has been replaced, use Chipper Check or the front panel menu to set the Red, Green and Blue "Cutoff" values to midrange. 2. Adjust the appropriate red, green or blue "Cutoff (9300)" controls until any red, green or blue tint disappears from the black bar, but is still visible in the adjacent bar. When properly adjusted, the adjacent bar should be a very low level gray with no color tinting. Low level color temperature setup is now complete. 3. Now observe the brighter portions of the bars. Adjust the red or blue drive controls to remove any signs of red or blue tint in the higher brightness bars. Observe the bars for signs of CRT overdrive. Overall brightness may be adjusted using the "YUV Light Output-9300" control. Some compromise may be required, but the higher IRE sections should be as free from color tinting as possible. Color temperature setup is now complete. Overall Brightness Levels (Drive) There is no control for the green drive (high brightness). Even though there is a front panel control to toggle the red drive to green, there is no EEPROM location to store the resulting adjustment. Any value for green would be stored to the red location. If overall brightness is not adequate, increase it by using the YUV Light Output control. This control should not affect color temperature at this level, however large changes may disturb the low brightness level color temperature (cutoff). After high brightness alignment is complete always recheck low level brightness areas. If color temperature is not acceptable, begin color temperature alignment again. Generally, only small changes should be required at this time.
Color Temperature Setup (RGB):
There is no color temperature setup required for RGB mode. However, note the brightness levels of RGB are generally lower than YUV for the same or similar inputs. There are two reasons. First, consumers using the MM101 as a computer monitor will be sitting much closer than when viewing commercial program material on the same set. Second, text characters tend to blur at high brightness levels. To be certain this does not occur, the MM101 reduces high brightness output levels in computer text mode. RGB cutoff should be adjusted in the "Computer/Text" mode until the black bar is unlit, but the adjacent bar remains slightly visible. RGB Light Output is more subjective and critical for the reasons given above. Light output should be set so the brightness of the brightest bar is 1/2 the level of the same bar in YUV mode. If a light output meter is available, measure the output level of the brightest bar in YUV mode. Then adjust "RGB Light Output" to provide a brightness level approximately 1/2 the YUV level. If no light meter is available, the adjustment is left to the technicians judgement. The "6500K" mode should be adjusted first, followed by "9300K".
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CRT Management AKB Troubleshooting Diagnosing AKB problems can be troublesome. The AKB pulses occur on lines 20, 21 and 22 of video. Unless test equipment can be procured allowing a technician to view each of the lines individually, the AKB pulses cannot be viewed directly. Failure of AKB to maintain cathode bias levels will result in incorrect color complaints. More than likely it will be in the area of low illumination scenes tinted towards or away from a specific color. As with any "wrong color" complaints, traditional troubleshooting techniques may be applied. Check the RGB output waveforms and voltages, then continue to the CRT cathode and grid voltages. Finally check the CRT cathode drive waveforms. If all these are acceptable, suspect CRT problems. If not, troubleshoot the appropriate circuits. Placing a static pattern on the screen will allow measurement of DC voltages in the AKB circuits. The level shift components expect a 4-6V pulse from the CRT drivers and deliver a 1-2V pulse to the video processor IC. If the incoming pulse is too high or low, suspect CRT driver problems. If the incoming pulse is OK, but the pulse to the video processor is too high or too low, suspect component failure in the level shift circuitry. If all waveforms and voltages agree with service data, suspect the video processor IC. If AKB fails, as the CRT is warming up, a picture (or static depending upon input selection) may appear for a short time. At about 12 seconds, (coinciding with the start of AKB operation), the screen will either go black, or abnormally bright. The first check should be at the video processor IC, U22300-56. This is the AKB feedback pulse from the CRT drivers. Although the technician will not be able to look at the AKB pulses directly, some sort of pulses about 2Vp-p would indicate the AKB level shift circuits were working properly. If they are present, suspect the video processor IC. An oscilloscope probe connected directly to the CRT cathode may cause a measurement error because of the interaction of AKB with the probe, not just loading. The probe connection changes the current that AKB measures and can cause as much as a 10V error. Always follow the recommendation on page 210 of this manual for measuring cathode voltages. AKB Mode TECH Do not change front panel adjustment 37, AKB mode. These are used for factory TIP alignments only. 02 is the default setting. 01 disables AKB, but 00 removes all video, including OSD! If 00 is selected, Chipper Check must be used to reset the value back to 02. All other adjustment values may set at nominal or half of their full range values when there is a question of what value is needed to begin alignments.
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Pa ra me te r #
Pa ra me te r N a me
Va lue R a ng e
N o te s /Co mme nts /N omina l Value s
01 02 03
Erro r Detectio n (1st) Erro r Dete ctio n (2 nd) Erro r De tec tio n (last)
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37
Re d C uto ff (N o rmal 9 30 0 °) Gre en C uto ff (N o rmal 9 30 0 °) Blue C uto ff (N o rmal 9 30 0 °) S ub C o ntra st Re d Drive S e le ct (N o rmal 9 30 0 °) Re d Drive (N o rmal 9 30 0 °) Blue Drive (N o rmal 9 30 0 °) YUV Light O utp ut (N o rmal 9 30 0 °) RGB Light O utp ut (N o rmal 9 30 0 °) Re d C uto ff (Warm 65 0 0 °) Gre en C uto ff ( Warm 6 5 0 0 °) Blue C uto ff (Warm 65 0 0 °) Re d Drive S e le ct (Warm 65 0 0 °) Re d Drive (Warm 65 0 0 °) Blue Drive (Warm 65 0 0 °) YUV Light O utp ut (Warm 65 0 0 °) RGB Light O utp ut (Warm 65 0 0 °) YUV C uto ff RGB C uto ff AK B Mo d e
00 . . 2 5 5 00 . . 2 5 5 00 . . 2 5 5 00 .. 31 00 .. 01 00 . . 1 2 7 00 . . 1 2 7 00 . . 1 2 7 00 . . 1 2 7 00 . . 2 5 5 00 . . 2 5 5 00 . . 2 5 5 00 .. 01 00 . . 1 2 7 00 . . 1 2 7 0 0 . .1 2 7 0 0 . .1 2 7 00 . . 2 5 5 00 . . 1 2 7 00 .. 03 Ad justs all thre e d rives Ad justs all thre e d rives Ad justs all three c uto ffs Ad justs all three c uto ffs DO N O T C HAN GE!!! Defa ult is 02 Alwa ys 0 0 Ad justs all thre e d rives Ad justs all thre e d rives DO N O T ADJUS T!!! Alwa ys 0 0
Figure 14-20, Service Menu Color Temperature Adjustment Locations