Hughes JVC Projector 250 Service Manual

							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). 
						

							Chapter 5---Electronics
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). 
						

							Chapter 5---Electronics
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. 
						

							Chapter 5---Electronics
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. 
						

							Chapter 5---Electronics
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). 
						

							Chapter 5---Electronics
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.