Hughes JVC Projector 250 Service Manual

Chapter 4---Optical
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Model 250 Service Manual
Figure 4-5  
Upper  level view of Optical Path (top view).
Optical Path - Inputs
Arc Lamp high intensity white light
Component RGB Image light from each of the ILA
®
Optical Path - Outputs
Component RGB polarized light to the ILA®
Output image light to the Projection Lens
Optical Path - Operation
The light travels from the Arc Lamp into the Optical Path (see Figure 4-5).  The
light reflects off a cold mirror and passes through a light pipe. From the...

Chapter 4---Optical
Model 250 Service Manual 4-9
The Dichroic Beamsplitter Assembly separates the white light into its red, green,
and blue components. Each component light beam goes into the Prism Assembly
where it is polarized.
Light can be viewed as having two electromagnetic components: a Horizontal-
electric field and a Vertical-electric field. These fields are perpendicular to each
other. When unpolarized light travels through a polarizing beamsplitter, one of
these fields is reflected and one is...

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Model 250 Service Manual
Figure 4-6  
Optical Path 

Chapter 4---Optical
Model 250 Service Manual 4-11
Figure 4-7  
Optical Path
Optical Path - Service Adjustments
Tools Needed
3-mm Hex wrench
Parts Needed
No serviceable parts
The one adjustment that can be performed on the optical path is with the XY
Positioner that moves the #1 Condensing Lens in the X- and Y-axis. The XY 

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Model 250 Service Manual
Positioner adjusts the light beam coming out of the Light Pipe so that it projects
squarely onto the face of the ILA®.
NOTE:
  Do not adjust the XY Positioner unless there is a dark edge visible on
screen.
If the XY Positioner is misaligned, there will be a dark edge on the left, right, top
or bottom edge caused by the edge of the Light Pipe. Perform the Arc Lamp
alignment before adjusting the XY Positioner unless the dark edge makes Arc
Lamp alignment...

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Model 250 Service Manual 4-13
4.3 ILA®
ILA® Main functions
!
 Modulates image light from the CRT onto the high intensity polarized
light from the Arc Lamp
!
 Reflects high intensity light received from the Prism Assembly back into
the Prism Assembly after modulating with image light
!
 Adjustable bias voltage and frequency (ILA® Sensitivity)
!
 Adjustable offstate with Super Contrast ILA®s
!
 Image light from CRT is blocked from output. Image is electrostatically
coupled to output...

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Model 250 Service Manual
ILA® Operation
The ILA® plays a critical part in bringing the image to the screen. The ILA®
receives the image light when the CRT projects the image through the Relay Lens
and focuses it onto the photoconductive layer on the input side of the ILA®. The
image does not pass directly through the ILA® but is transferred by a change of
impedance of the photoconductive layer to the Liquid Crystal Layer on the output
side of the ILA
®. The light coming from the...

Chapter 4---Optical
Model 250 Service Manual 4-15
NOTE:
  Reset the ILA® bias after setting the Compensator and check G2
Sensitivity Offset, and Threshold Offset level (see Model 250 User’s
Guide, section 5.8 Black Level G
2 and Sensitivity Offset).
ILA
® Overlap
This adjustment positions the ILA® assemblies in their sockets so that the image
from each ILA® will overlap (be placed on top of) the other two ILA®s. Make this
adjustment whenever replacing an ILA®.
Tools Needed
4-mm hex-head wrench
Flathead...

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Model 250 Service Manual
  CAUTION!  To avoid damaging the connector,
grasp the ILA® assembly itself, not 
the connector at the top.
 
Figure 4-10  
ILA® Assembly top view. The overlap screws shown are under
the ILA®. They are visible only after the ILA® assembly is removed.
13. If the overlap is at the top or bottom, be sure the projector is level. Slide
the spring clip (see Figure 4-10) at the top of the ILA
® assembly back.
14. Loosen the two hex nuts, using a 4-mm hex-head...

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Model 250 Service Manual 4-17
19. Retighten the two wing nuts and tighten the two hex nuts.
20. Reset the ILA
® biases to their previous levels from Step 2.
ILA
® Back Focus Adjustment
The ILA® Back Focus adjustment moves the ILA®/Relay Lens/CRT assemblies
forward or backward as one unit to focus the ILA® output on the screen. When
using a zoom lens this adjustment allows the zoom lens’ tracking to remain
focused on the screen throughout the entire zoom range. Use the Focus Pattern...