The video and audio processing circuits are virtually identical to those used in other display devices (e.g. CRT and Plasma). Particular attention will be given to the circuits that control and drive the panel. The Key components common to all LCD displays include the LCD panel assembly (there will be 3 of them in a projection unit), a method of passing back lighting through the LCD crystals, and circuits to scale the incoming video data to the resolution of the panel.
Regardless
of the signal source, the video signal is in a digital format. The initial video
process circuits digitize all analog signal sources. LCD panels can only accept digital signals. Most designs accept
the individual RGB components in an 8 or 10-bit word size. The RGB signal will
have been scaled to meet the fixed resolution of the panel. This digital RGB
data is allocated to the correct pixel position on the panel by the panel logic
circuit.
Failures
occurring with the LCD panel itself or the above mentioned circuits usually
manifest themselves in an easily recognizable symptom.
The
most common are:
- No picture
- Missing or fully lit horizontal lines (individually or in groups)
- Missing or fully lit vertical lines (individually or in groups)
- Distortion in the displayed video.
Other
failures can occur such as a unit that will not turn on or turns its self off
for protection purposes. The latter will usually generate a diagnostic indication
that may help in determining what the cause is. Failure of the audio processing
and amplifying circuits must also be considered. All of the previously
mentioned items are diagnosed in different ways that are unique to the
chassis/model design. This chapter is geared towards generic approaches to
servicing LCD televisions. Additional chapters in this manual will contain
troubleshooting flowcharts specifically tailored for a model or group of models.
Diagnostics procedures that are unique to
specific models are best followed with troubleshooting flowcharts specific to
them. This manual will contain such charts located in individual chapters
labeled with the model or chassis they are designed for.
NO VIDEO
A trues
“no video” condition assumes that none of the various inputs or tuner sources
are displaying a picture. As mentioned earlier, it is important to immediately
isolate the cause and determine if it lies within the video process or the panel
control circuits. All Sony televisions generate what is known as “On Screen
Display” graphics (OSD). Any display of on-screen graphics (such as channel,
video input numbers, or customer setup menus) immediately disqualifies the
panel and the drive circuits as the cause. On-screen graphics can be a powerful
troubleshooting tool but its use is unique to the design of the unit. A flowchart
specific to that chassis or model should be used.
BACK-LIGHTING
All
LCD televisions, whether a direct-view or projection type, require a light
source be present to pass through the LCD crystals and out to the viewer. In
direct-view units the current choice is multiple fluorescent tube lamps whose
light output is spread by a diffuser panel. Projection units utilize a
high-intensity lamp. In Sony LCD projection units, the lamplight is split into
red, green, and blue light components. These light components are sent to
individual LCD panels for pixel control and recombined for projection to a
screen.
Since
current LCD technology is unable to completely block back-lighting a small
amount of light passes through the LCD crystals and can be seen as a dark gray
raster. Ambient room lighting will determine how easy this is to see. In most
cases it is easier to watch the screen as the unit is being turned off to see
if the raster becomes slightly darker.
All Sony LCD televisions contain protection
circuits to monitor the circuits driving the back-light lamps. If the ballast
control circuits or the lamp(s) fail, the unit will usually shut down and
display a diagnostics indication. How this is done is unique to each model.
Utilizing a troubleshooting flowchart for that specific model is the best way
to isolate the cause. Verification of back-lighting should always be the first
step in isolating a “no video” condition regardless of the presence of
protection circuits.
NO-VIDEO WITH BACK-LIGHTING
If back-lighting is confirmed, the next step is to observe for the presence of any
on-screen display (OSD) graphics. The main microprocessor or video process
circuits generate these. The presence of OSD is a clear indicator that the LCD
panel and its associated drive circuits are functioning. The focus of attention
is now in the video input and process circuits. Use a troubleshooting flowchart
for the specific model to assist in isolating the problem.
DISTORTED VIDEO
Distortions in the video can be a difficult challenge since
it can manifest itself in many ways. Fortunately, many of the distortion issues
that are caused by the panel control and driver circuits are unique and usually
easy to identify. Distortions can be classified into the following groups:
- Unlit or fully lit rows or columns of pixels
- Digital distortion across the screen
- Improper video level
- Dark or colored spots on the screen.
ROWS OR COLUMNS OF PIXELS STAYS LIT OR UNLIT
Fully lit rows or columns of pixels are more
common in LCD panels than ones that are not lit at all. Drive circuits that are
linked to the panel via flexible PC cables control the horizontal and vertical
address lines. These cables are bonded to the outer edge of the panel. If the bond
is lost at one or more of the lines, the control voltage will be lost. This
allows the liquid crystals to twist to their normal position and allow full light
to pass through. Since an entire line is affected all of the red, green, or
blue pixels will light. This is a clear indication of a panel failure since very
expensive and specialized equipment is needed to repair the bond. Any unit
under warranty will need to have the panel replaced. Figure below illustrates
and example of this failure.
DIGITAL DISTORTION ON ENTIRE SCREEN
Unless the LCD panel has been damaged in any
way, the process circuits for the video signal usually cause this type of
distortion. The proper step is to isolate the cause to a particular board. The
two key circuits for processing video are the initial circuits to perform
analog to digital conversion, and another to scale the incoming video data to
the panel resolution and allocate that information to the proper pixels. Distortions caused by digital process
circuits are unique and, in most cases, easy to identify as to the source.
Random points of pixel lighting and loss of detail in the displayed image are
examples of distortion caused by the initial video process stages. If this
happens, using the OSD functions of the unit can help to verify this. OSD is
usually inserted near the end of the digital processing, before it enters the
panel scaling circuits. Figure 2-2 illustrates an example of a digital process
failure in the front end of the process circuits. Note how the OSD is unaffected.
The OSD has made it clear that there is nothing wrong with the panel or the
scaling and drive circuits. Distortions
caused by the scaling and panel drive circuits usually generate symmetrical
patterns. Erroneous highlight and black level can also occur here since gamma
correction is performed at this stage.
