LAMP AND DRIVER
- A high-pressure, gas filled, arc lamp is used to generate sufficient light to pass through the LCD panels and optical lens, and is reflected from a mirror to the rear of the screen. A power supply devoted to generating the required high voltage to “arc” the lamp is located at the rear of the chassis just above and to the left of the air intake for the lamp.
- This power supply receives 260VDC from the G1 board and has a control line (on-off) from the C2 board via the A board. There is also a current and a high-voltage-detect line that are monitored by the self-diagnostics circuits. The current-detect line is responsible for signaling a weak or failed lamp. The high voltage detect line monitors the slight radiance from the lamp driver to determine if it has turned on. A small length of wire is used as antenna to accomplish this. Be certain this wire is routed in the general configuration.
- This high voltage power supply is designated as “Power Block” in the electrical parts list of the manual (PN 146879811). It should not be confused with the main power supply. Referring back to Figure 4-4, take note of the small, white, cylindrical device located on the right side of the board next to the label. This is a spark-gap device designed to protect the lamp. It will always fire whenever the unit is turned on from a “cold start” (this will be defined shortly). It will sometimes make a high-pitched buzzing noise for about a second. If a customer hears this and is concerned, it is quite normal. This spark gap will prove to be a useful tool in isolating a defective lamp or lamp driver.
- One last important piece of information to know is the lamp startup and shutdown sequence. This will help prevent any potential misdiagnosis of what may be an otherwise normal function.
- When the unit is first turned ON, the standby/power LED will turn from red to green. The lamp-cooling fan will immediately start and in less than one second, the lamp driver spark gap will fire. The lamp will slowly increase in brightness with full illumination occurring in about 45 seconds. If the unit is turned off by the remote or power switch, the lamp will remain lit for seven seconds before extinguishing. If the unit is powered up before the lamp goes out, the picture will appear within four seconds with full brightness. Turning on the unit after the lamp has gone out will require a wait of 27 seconds before the lamp will restart and will begin its gradual climb to full brightness. Once the power has been turned off and nothing else is done, the cooling fan will run for two minutes and seven seconds. This is a predetermined time regardless of what the temperature inside the lamp assembly was at power-down.
TROUBLESHOOTING
- The use of many digital processing circuits, along with a lamp and LCD panels, requires new techniques and approaches when diagnosing failures in the video circuits. Symptoms normally encountered with CRT-based display devices rarely occur on LCD displays. The digital circuits can produce some very strange phenomenon on the screen, but fortunately, they appear to be digital in nature by manifesting themselves as random or stationary pixel, blocks or line distortions. The distortions will have defined edges, unlike analog signals, which are susceptible to smearing and brightness level symptoms.
- A no-picture condition also presents new challenges. Besides lamp failures (which are easily detected), a loss of video can be caused by failure of the signal path or the inability to change the liquid crystal alignment within the LCD panels. The remainder of this chapter will focus on some suggested procedures and tips to help gain a better understanding of troubleshooting this type of display format. By following the procedures and using flow charts, you should be able to accurately determine whether the failure is in the optical block or on one of the circuit boards and if so, what board is most likely the cause.
LAMP DOES NOT TURN ON
- Since the LA-2 chassis uses a lamp to pass light through LCD panels, verifying the correct operation of the lamp must be checked first whenever video cannot be displayed on the screen. In most cases, lamp turn-on can be seen by the backlighting of the screen. If ambient room lighting is high, going to the back of the unit and looking inside the cooling fan intake vent is the best way.
- If the lamp is not lighting, the cause of the video failure has been isolated. The lamp should be checked for obvious physical signs of failure. Access is achieved by loosening the two thumbscrews located at the lower rear of the cabinet back. The front panel can then be pulled off. The remainder of the panel is attached with plastic snap locks, so some effort is required to pull it loose.
- Once the panel is removed, the access door to the lamp can be seen as seen. Loosen the thumbscrew and remove the panel. The lamp assembly is now visible and can be taken out by removing the two screws. A 3mm hex wrench is required.
- If the lamp appears OK, it could still be faulty and must be eliminated as the cause. The lamp driver power supply could have failed and there is a rather simple way to determine which is the likely cause:
- If the front panel has not been removed (as described above), do it now. Access and remove the lamp housing as described above.
- Remove the two small screws securing the high voltage lead connector to the lamp and pull the connector loose. Position the connector away from any ground potentials.
- Remove the rear cover if this has not been done already.
- Apply AC power and turn the unit on with the remote. As mentioned earlier in this manual, the remote receiver is located at the top right corner so you will have to reach around with the remote to turn it on.
- The spark gap on the lamp driver will now arc within one second. You may miss the spark but you should hear a high-pitched buzz lasting about one second.
- Continue observing the spark gap. In 21 seconds, it should fire again. In another 21 seconds, it should fire one more time. The lamp driver will cease any more attempts and the lamp LED on the front panel should now be flashing.
NOTE: The initial startup voltage required to “arc” the lamp and place the
mercury into a gaseous state can be up to 21.5KV. Caution should be exercised
to keep the lamp socket connector away from ground return paths and the
technician should stay clear of the connector when performing this test.
This is a simple test to determine if
the lamp driver is functional. There is a high probability of a lamp failure if the above sequence of events
occurs.
LAMP LIGHTS BUT NO VIDEO IS DISPLAYED
One of the key symptoms to observe
whenever a LCD-type display does not produce any trace of a video image is
whether a bright or dark raster is displayed. This is extremely important.
Liquid crystals are, by nature, orientated in an identical pattern to pass
light on a single plane. By polarizing the light source from the lamp on the
same plane of the crystalline structure, light will pass through. When a
voltage is applied, the crystals “twist”. The more voltage applied, the more
the crystals will twist. This is how the brightness of each pixel is determined
at a given time. A bright or dark “raster” will determine the next step in
locating the potential cause of the failure.
- Bright Raster:
Each pixel of the LCD panel(s) receives a voltage known as VCOM. As the acronym implies, it is a common voltage. Another voltage (usually called Xn) is applied and its potential is controlled by a voltage usually known as Yn. This is the voltage that determines the amount of “un-twisting” (in relation to VCOM) to perform to each LCD pixel and will determine the brightness of that particular pixel. Under normal conditions, VCOM will completely twist the crystals in the absence of a control signal and cut off light attempting to pass through. If VCOM is lost, the pixels will have no reference (similar to an open ground path in a traditional circuit) and will be unable to respond to level signals. The crystals will all remain in phase with the incoming polarized light and pass most of it. This is a good indication of a failure of this voltage and will usually be caused by circuits outside of the LCD panels and driver circuits. In this chassis, suspect a loss of +17V to the C1 board. If 17V is present, the C1 board is likely at fault and the optical block must be replaced
- Dark Raster:
The liquid crystals are being twisted
to block out incoming light. VCOM is present and it now becomes necessary to
determine if the brightness control voltage is being applied to control the
pixels. In this chassis, the source becomes rather complicated to isolate. The
failure could lie on the C2 or DIC2 board and, even though VCOM +17V is
present, could be the C1 board. The
only reliable clue is the presence of OSD. If the customer menu can be called up,
valuable information is presented. As mentioned earlier, the customer and page
one of the diagnostics screen are generated by the
main micro located on the C2 board. The presence of OSD indicates a high
probability of a functional C2 board and a definite clue that the C1 and
optical block are functioning properly. The failure now lies at the DIC2 board
and back into the input switching circuits. The twin-view feature helps to
determine if an input problem exists. The main and sub video paths are
processed independently on the DIC2 board.
If a picture appears in one of the
boxes but not the other, make sure that both boxes are set for tuner or video 1~4
inputs. The left box will display the main picture source. If it is set for
video 5~7, you will need a component or DVI source to display a picture. If a
picture does appear in the left box, the main video path is OK. Further
elimination of the video switching circuits can be achieved by selecting the
main box (left and right on the remote joystick) and changing to another input.
The ability to display any 480i source in the left box whenever there is no
video on the main screen indicates a failure or the DRC circuit and the DIC2
board must be replaced.
Video Distortion: Since all video
sources are converted to digital and processed in that domain, some unusual symptoms
can appear on the screen. Missing colors, stripes, random pixels, double
images, and missing sections of video are some of the conditions one can
encounter. Keep updated on the service website. The service bulletins and field problem reports are updated on a
regular basis to assist with some of the unique symptoms that can occur with
this television design. Using the flowchart at the end of this chapter should
also help to isolate the cause. Below are some symptoms you might encounter followed by
suggestions on where the cause may be located.
Missing Color: Once the digital video
signals enter the C2 board, they are converted to RGB where they remain in that
state onto the C1 board. Since it is highly unlikely that an LCD panel will
fail where the crystals block the light, the problem is located on the C1 or C2
board. Of the two LVDS transmitters between the C2 and C1 board, one of
them transmits the B and G component and the other, the R. The Red LVDS cable
(marked with a red band near the connector) transmits the LCD driver pulses. If
this cable comes loose or damaged, the result will be no picture. The B/G LVDS
cable will generate a cyan picture if it fails to send data. There is also a
possibility of physical blockage of light to the particular panel due to damage
within the optical block. This is easily tested by removing the flexible cable
to the suspected LCD panel. If the picture is now dominant with the perspective
color, the optical light path is OK.
Dominant Color: If the color is very
bright with no video signal present, the LCD panel may have failed or the
connector to it may be loose. If video is present, picture settings may have
become corrupted.
Vertical Stripes: It is important to
note the width of the stripes. A twelve-phase sampling method is utilized to
increase the charge time per pixel. This signal is output by each LCD driver to
the panels. If uniform lines, 12 pixels in width are observed, the driver, flex
cable or LCD panel is at fault. Single or random lines are usually caused by a
defective LCD panel.
Color Shifting at Specific Video
Levels or Loss of Detail: This can be checked by turning on the internal ramp
test signal (mentioned below) to check for uniformity from black to white. If
the distortion is present, the problem lies on the C1 or C2 board. If not, the
DIC2 board is the cause.
INTERNAL PATTERN GENTERATOR
Several internally generated test
patterns can be accessed in the service mode to assist in diagnosing a video
problem. The greatest advantage of using these patterns is to verify proper
processing of the digital video signals from the C2 board all the way to the
LCD panels. These patterns are generated on the C2 board and mixed with the
digital RGB lines. To access these test patterns:
- Enter the service mode.
- Use the “2” or “5” key on the remote to scroll through the groups of adjustment pages (it is faster to use the “5” key). Note: The “1” and “4” keys scroll to individual adjustments within each of these groups.
- Locate adjustment group D9671TPN.
- Use the “1” key to scroll down to TPNRGB. Set data to 7 with “3” key if not 7 (max value).
- Scroll to next adjustment TPNMODE. Set data to 0 if not 0.
- Use “4” key to scroll back to TPNSW. Set data to 1 with “3: Key. The unit should now display an all white pattern.
- Press “1” key to scroll back to TPNMODE.
- Use “1” key to select the various test patterns based on the data value selected:
- Flat Field
- One Dot Stripe
- Two Dot Stripe
- Toggle Dot
- Crosshatch
- Cross Point
- Ramp
- Stair-step
- Checkered
- Diagonal Line
- Window
