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Hughes JVC Projector 250 Sc Service Manual

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    							Chapter 5---Electronics
    5-46 Model 250 Service Manual
    Figure 5-27  
    Video Amplifier PCB I/O Diagram for the Video Processor PCB and
    CRTs.
    Video Amplifier PCB - Outputs
    RGB_BEAM - Cathode beam current sense lines (about 1 V per 100 µ A).
    RGB_CATHODE - image output signal to CRTs (about 40 Vpp with peak
    voltage of 70 V).
    RGB_G
    1 - G1 voltage to CRTs (Blanking pulse to CRT).
    RGB_G
    2 - G2 voltage for CRTs (about 600-800 V).
    RGB_HEAT (pos.) - positive side of the CRT heater voltage (about 6.2 V). 
    						
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    Model 250 Service Manual 5-47
    RGB_HEAT (neg.) - negative side of CRT heater voltage (ground level).
    ARC_GND - CRT ground back to High Voltage Power Supply (CRT Protection
    against arcing).
    /VA_OK - CRT Protection signal to the High Voltage Power Supply (low = good,
    high = bad). This signal is becomes /HV_ENA at the High Voltage Power Supply.
    Video Amplifier PCB - Operation
    Image Amplifier
    The main function of the Video Amplifier PCB is to amplify each of the image
    signals going to the cathode of the CRTs. The Video Amplifier PCB amplifies the
    image signal with a gain of approximately 80.The DC Restore from the Video
    Processor PCB is added to the amplifier circuit and the image signal is sent to the
    CRT. The Video Amplifier PCB uses the +80 V from the Low Voltage Power
    Supply to bias the image amplifier circuits.
    G1 Regulation and Blanking
    The Video Amplifier PCB provides G1 regulation for the blanking pulses. The
    Video Amplifier PCB receives the blanking pulse timed to both the horizontal and
    vertical syncs from the Raster Timing Generator PCB. It also receives the DC
    offset (G
    1_BIAS) control voltage (0-5 V) that comes from the brightness control
    on the Video Processor PCB. The G
    1_supply (-200 V) comes from the High
    Voltage Power Supply. The G
    1 regulator regulates the -150 V G1 supply up to
    about -80 V. The blanking pulse is added to the -80 V and the G
    1 bias adds an
    offset for brightness control. This voltage is distributed to each of the CRT’s G
    1
    control grids.
    G
    2 (Black Level) Voltage
    The Video Amplifier PCB receives the G2 supply voltage (1200) from the High
    Voltage Power Supply. It receives the G
    2 control lines from the Video Processor
    PCB. The Video Amplifier PCB regulates the G
    2 supply voltage down to about
    1100 V and distributes it to each G
    2 regulator (one for each CRT). The G2 control
    line from the Video Processor PCB controls the output of each G
    2 regulator
    according to the G
    2 (Black Level) command setting in the Convergence Menu.
    The output goes to the G
    2 grid on the CRT.
    CRT Filament (Heater) Voltage
    The Video Amplifier PCB receives the Filament (heater) voltage (+6.2 V) from
    the Low Voltage Power Supply and distributes it to the filament of each CRT.
    The filament or heater heats the cathode and causes it to emit electrons that are
    accelerated toward the anode of the CRT to form the image on the face of the
    CRT.
    CRT Protection Circuit
    The Video Amplifier PCB plays a major role in the CRT Protection circuit.
    The/SWEEP_OK from the Horizontal Vertical Deflection PCB becomes /VA_OK
    on the Video Amplifier PCB. If the /VA_OK goes high, the Video Amplifier PCB
    shuts down the G
    1 and G2 regulators effectively shutting down the all the CRTs. 
    						
    							Chapter 5---Electronics
    5-48 Model 250 Service Manual
    The Video Amplifier PCB detects all the necessary supply voltages (+80 V, +15
    V, and the +6.2 V) for the CRT and includes that as part of the /VA_OK signal. If
    any of the supply voltages is not present, /VA_OK goes high, again shutting down
    the G1 and G2 regulators. The /VA_OK goes out to the High Voltage Power
    Supply as the /HV_ENA. If the /VA_OK goes high, the /HV_ENA goes high and
    shuts down the high voltage amplifier section of the High Voltage Power Supply
    (see CRT Protection section of the Ch 7-- Troubleshooting).
    The CRT Protection circuit on the Video Amplifier PCB also has a beam current
    sense line for each CRT that comes from the output of the image amplifier circuit.
    It samples each CRT beam current and sends that back to the Video Processor
    PCB. The Video Processor PCB compares the beam current to a reference value
    (250 µ A). If the beam current of any CRT meets or exceeds that reference value,
    the Video Amplifier PCB reduces the Contrast of the CRTs (a global adjustment)
    until the beam current is below the reference.
    The CRT Protection circuit also provides isolation of the anode voltage return
    through the Arc Ground circuit. Occasionally, particles flake off metal
    components inside CRTs that cause shorting inside the CRT. On these occasions,
    the Arc Ground circuit is an isolated ground path back to the High Voltage Power
    Supply. This isolation protects the other supply voltage returns.
    The Video Amplifier PCB has three CRT Socket PCBs that are attached to the
    back of each CRT. These PCBs provide an interface between the Video Amplifier
    PCB and the CRT for the image signal, G
    1, G2, and filament voltages (see Figure
    5-28). The Arc Ground cable is also routed through the CRT Socket PCB. The
    Anode cables are routed directly from the High Voltage Power Supply to each
    CRT. The red Focus Voltage cables are routed directly to each CRT Socket PCB.
    Video Amplifier PCB - Remove and Replace
    The Video Amplifier PCB is located under the CRT assemblies at the rear of the
    projector.
    Tools Needed
    #1 Pozi-drive Phillips-head screwdriver
    2.5-mm Hex wrench
    Parts Needed
    Video Amplifier PCB  p/n 105232
    To remove a Video Amplifier PCB:
    1. Power off the projector by IR Remote or PC, and allow the cooling fans to
    run until they shut off automatically.
    2. Turn the AC Circuit Breaker to the OFF position and unplug the AC
    Power Cord.
    3. Remove the rear cover.
    4. Tilt the Electronic Module up. 
    						
    							Chapter 5---Electronics
    Model 250 Service Manual 5-49
    5. Remove the green CRT by loosening the green CRT Holder Ring (see
    Figure 5-29). Perform this step with the CRT Socket Connector still
    connected to the Green CRT and the ground wire still connected from the
    CRT/Yoke Assembly to the CRT Socket Connector. The Green CRT
    Socket Connector is close to the rear of the projector. It is easier to
    disconnect it from the CRT after the CRT/Yoke Assembly is loose and
    moved away from the rear of the projector (see CAUTION
     below).
    NOTE:
     The holder ring should only be hand-tight. If it cannot be removed
    by hand, place a Flathead screwdriver in the slots on the CRT Holder Ring
    (see Figure 5-29) and tap lightly to help loosen it. Be careful not to let the
    Flathead screwdriver slip out of the slot and possibly damage components.
    Do not use a screwdriver or any other tool to tighten this ring. It should be
    hand-tight only.
      CAUTION! The connections at the rear of the CRT
    are very fragile and can be damaged easily. Use extreme care
    when removing the CRT/Yoke Assembly to prevent damage to any
    CRT connections.
    6. Carefully remove the Green CRT/Yoke Assembly by sliding it backward
    and upward as far as possible before disconnecting the Green CRT Socket
    Connector from the CRT neck. Use care to avoid bumping the CRT while
    removing it from the projector, to avoid damaging it. Also, be careful not
    to pull any of the wires that connect from the CRT Socket Connector to
    the Video Amplifier to avoid damaging them.
    7. Move the Green CRT/Yoke Assembly away from the rear of the projector,
    then disconnect the Green CRT Socket Connector and the ground wire
    between the CRT/Yoke Assembly and the CRT Socket Connector.
    NOTE:
     It may be helpful to perform this step with a partner-one person to
    hold the CRT/Yoke Assembly firmly and the other to remove the CRT
    Socket Connector and the ground wire.
    8. Carefully lay the Green CRT Socket Connector on the top of the Video
    Amplifier PCB.
    9. Place the Green CRT Assembly in a safe location on or against one of the
    other CRT Assemblies.
    10. Disconnect the Red and Blue CRT Socket Connectors and carefully lay
    them on the top of the Video Amplifier PCB.
    11. Disconnect the three connectors (push in slightly, squeeze the tabs, and
    pull out), J67, J68, and J69 from the Video Amplifier PCB (see Figure
    5-28). Move the cables out of the way. 
    						
    							Chapter 5---Electronics
    5-50 Model 250 Service Manual
    12. Disconnect the three Focus cables that run from the CRT Socket
    Connectors to the Focus connectors on the HVPS.
    NOTE:
     Observe that each of the three Focus cables (large, red wires that
    run from each CRT Socket Connector to the Focus connectors on the
    HVPS) is joined near the CRT by a coupler plug and jack. Disconnect
    (and label) the focus cables at these couplers.
    13. Loosen the six Pozi-drive Phillips-head screws that secure the Video
    Amplifier PCB to the projector frame.
    14. Make sure everything is out of the way, then slide the Video Amplifier
    PCB toward the left so the mounting screws will clear the access holes and
    remove the board from the projector.
    15. Reinstall in the reverse order from above.
    Figure 5-28  
    Video Amplifier PCB showing CRT Socket Connectors. 
    						
    							Chapter 5---Electronics
    Model 250 Service Manual 5-51
    5.10  CRT/Yoke Assemblies
    CRT/Yoke Assemblies - Main Function
    !
     CRTs converts image signal to image light
    !
     Yokes provide horizontal and vertical deflection for CRTs
    !
     Yokes provide x-axis and y-axis convergence correction for CRTs
    CRT/Yoke Assemblies - Inputs
    To Yokes
    H_RGB_YOKE (pos.) - top of red, green, and blue horizontal yokes.
    V_RGB_YOKE (pos.)  - top of red, green, and blue vertical yokes.
    RGB_LOCK (pos.) - interlock for red, green, and blue yokes (5 V when closed or
    15 V when open).
    X_RGB_YOKE (pos.) - top of red, green, and blue X-axis Convergence yokes.
    Y_RGB_YOKE (pos.) - top of red, green, and blue Y-axis Convergence yokes.
    To CRTs
    Anode Voltage from High Voltage Power Supply (25 kV).
    Focus Voltage from High Voltage Power Supply (7 kV).
    RGB G
    1 - G1 grid signal from Video Amplifier PCB.
    RGB G2 - G2 supply voltage from Video Amplifier PCB (about 600-800 V).
    RGB_CATHODE - video output signal from the Video Amplifier PCB (about 40
    Vpp with a peak voltage of about 70 V).
    RGB_HEAT (pos.) - positive side of CRTs heater voltage (about 6.2 V).
    CRT/Yoke Assemblies - Output
    From Yokes
    H_RGB_YOKE (neg.) - bottom of red, green, and blue horizontal yokes.
    V_RGB_YOKE (neg.)- bottom of red, green, and blue vertical yokes.
    RGB_LOCK (neg.) - interlock for red, green, and blue yokes (5 V when closed or
    0 V when open).
    X_RGB_YOKE (neg.) - bottom of red, green, and blue X-axis Convergence
    yokes.
    Y_RGB_YOKE (neg.) - bottom of red, green, and blue Y-axis Convergence
    yokes.
    From CRTs
    RGB_HEAT (neg.) - negative side of CRTs heater voltage (0 V).
    ARC_GND - CRTs ground (Anode supply return line). 
    						
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    5-52 Model 250 Service Manual
    Image - image light that passes through Relay Lens and focuses on photosensitive
    layer on input side of ILA®.
    CRT/Yoke Assemblies - Operation
    The CRTs receive image signals that come from the VICs to the Video Processor
    PCB, to the Video Amplifier PCB. The image information drives an electron gun
    inside the CRT. The Cathode of the CRT is a small, metal oxide disk that covers
    the filament heater. The filament heater heats the Cathode causing it to emit
    electrons. The electrons travel forward to the positively charged CRT Anode. As
    the electrons travel forward in the CRT, they pass through control grids, each one
    more positively charged than the next.
    The first grid G
    1 is a small aperture that is negatively charged with respect to the
    Cathode. The negative bias voltage allows the beam current to be controlled by
    the video signal applied between G
    1 and the Cathode to modulate brightness.
    The G
    2 control grid restricts the beam of electrons to a narrow path and
    accelerates them because of its positive voltage. The G
    3 grid works with G2 to
    form an electrostatic focusing lens that focuses the electron beam on the CRT
    phosphor.
    The Deflection Yokes use magnetism to deflect the electron beam horizontally or
    vertically inside the CRT. The Deflection Yokes deflect the electron beam at right
    angles, so the Horizontal Deflection Yokes are on the top and bottom of the CRT.
    The Vertical Deflection Yokes are on the left and right sides. Both the horizontal
    and vertical deflection waveforms come from the Horizontal Vertical Deflection
    PCB. Geometric, Convergence, and Shading correction information is add to the
    deflection waveforms to adjust them so they create a centered, linear uniform
    image on the screen.
    Each CRT is encased in a metal shroud made of Mu metal. This shroud protects
    the CRT from stray magnetism from PCBs, cooling fan motors and the
    magnetism from other CRTs. The stray magnetism can influence the CRT Yoke
    magnetism causing noise and distortion on the screen.
    CRT/Yoke Assemblies - Service Adjustments
    NOTE:
     It is recommended that the CRT Mechanical Focus procedure and the
    CRT Rotation procedure be done sequentially. This means that the CRT
    Mechanical Focus procedure should be performed immediately followed by the
    CRT Rotation procedure or vice versa. The reason for this is there are two
    setscrews, one on either side of the Guide Plate, that hold the CRT in position.
    Loosening the setscrews to perform one adjustment may cause the other
    adjustment to change; therefore, both adjustments should be performed at the
    same time. 
    						
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    Model 250 Service Manual 5-53
    CRT Mechanical Focus
    The CRT Mechanical focus is factory-set and normally will not require additional
    adjustment unless a CRT has been replaced. After replacing a CRT, reset the
    mechanical focus. After completing the CRT installation and mechanical focus,
    check the other CRTs for convergence and shading.
    Tools Needed
    1.5-mm Hex-head wrench
    #0 Pozi-drive Phillips-head screwdriver
    To adjust the CRT Mechanical Focus:
    1. Remove the rear cover.
    2. Pull the Interlock Switch for the rear cover up to the Service Mode
    position.
    3. Power the projector ON and allow it to stabilize for at least 15 minutes.
    4. Tilt the Electronic Module to the full up position.
    5. Select the Focus pattern (test pattern #6).
    6. Zoom the Projection Lens to the widest angle (largest image) and adjust
    the Projection Lens focus for the sharpest image of the Focus test pattern.
    7. Loosen the two setscrews (see Figure 5-29) near the top of the CRT sleeve
    using the 1.5-mm Hex-head wrench.
    8. Loosen the CRT focus lock screw located on the Guide Plate at the top of
    the CRT shield (see Figure 5-29). The CRT is now free to slide.
    9. Grasp the CRT thumbscrew (see Figure 5-29) and slide the CRT forward
    and backward until the image is focused as sharply as possible (be careful
    not to loosen the CRT thumbscrew, which could allow the CRT to rotate).
    10. Tighten the setscrews first, then the CRT focus lock screw.
    NOTE:
     Tighten the setscrews before tightening the focus lock screw. If
    the setscrews are loose the focus lock screw can slightly defocus or rotate
    the CRT as it is being tightened.
    11. Repeat the above steps for the Red and Blue CRTs. 
    						
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    5-54 Model 250 Service Manual
    Figure 5-29  
    CRT Mechanical Focus adjustments.
    CRT Rotation
    The CRT tilt adjustment aligns the CRT level with the screen. This is a factory-set
    adjustment and should not need adjusting unless the CRT or yoke is moved.
    Tools Needed
    1.5-mm Hex-head wrench
    #0 Pozi-drive Phillips-head screwdriver
    3-mm Hex-head wrench
    To perform the CRT Rotation adjustment:
    1. Remove the rear cover.
    2. Power the projector ON and allow it to stabilize for at least 15 minutes.
    3. Tilt the electronic module to the full up position.
    4. Select Center/Lin pattern (test pattern #8). 
    						
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    Model 250 Service Manual 5-55
    5. View the color for the CRT that requires rotation. Hide the other two
    colors.
    6. In the Geometry menu, set “Reg Enable” to off by unchecking the box.
    Unchecking the Reg Enable box temporarily disables any convergence
    corrections that have been performed with a different CRT/Yoke assembly
    and sets these values to the default levels so that CRT Rotation can be
    accomplished correctly.
    7. In the Geometry menu, access Bow, Skew, Pincushion, and Keystone
    adjustments. Record the values for each setting.  After recording the
    values, set each adjustment to the default level of 128 so the CRT Rotation
    can be performed without the influence of any previous corrections. For
    the Skew adjustment, select the color of the CRT that is to be rotated (the
    other functions are not color-selectable).
    8. Loosen the two setscrews (see Figure 5-29) near the top of the CRT sleeve
    using the 1.5-mm Hex-head wrench.
    9. Loosen the CRT Thumbscrew (see Figure 5-29) on the top of the CRT
    Assembly using the 3-mm Hex-head wrench if necessary.
    NOTE:
     The CRT Thumbscrew is used more as a handle to move the CRT
    position as in rotating the CRT or moving the CRT back and forth for
    Mechanical Focus. When the positioning is complete, only hand tighten
    the CRT Thumbscrew.
    10. Rotate the yoke by grasping the Thumbscrew and moving it in whichever
    direction is necessary to make the CRT raster square with the screen.
    11. When the CRT raster is square on the screen, tighten the two setscrews
    first, then the CRT thumbscrew.
    12. In the Geometry menu, put a check the Reg Enable box.
    13. Reset Bow, Pincushion, Keystone, and Skew (select proper color for
    Skew) to the values recorded in Step 4.
    14. Recheck Bow, Pincushion, Keystone, and Skew and readjust if necessary.
    Also recheck Convergence (see the Setup Adjustments section, Chapter 5
    in the Model 250 User’s
     Guide).
    NOTE:
      If replacing a CRT/Yoke Assembly, the horizontal size coils on
    the Scan Reversal PCB may need adjustment to compensate for the new
    yokes.
    15. Repeat the above procedure for any other CRT that is not level. 
    						
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