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Monitors are Strong enough compared with the internal logic circuitry of personal computers. The failure rate is higher in display monitor. The two major reasons for this:
• The voltage rate is much higher in the circuitry in monitors than that of in the PC.
• Monitors are mostly analogue circuitry while PCs are digital circuitry.
PC operates at around 5 volts with its most of the logic circuitry . It has low-voltage . This low voltage include virtually no stress on the circuitry.
But in the case of monitor required 10,000 volts for monochrome and for trychrome required 25,000 volts.The ana-logue circuitry of a CRT monitor can take any voltage in a continuously varying range. This gives a lot
more scope for things to go wrong.
CRT Principles:
The display tube or cathode ray tube (CRT)is the primary component in a dis-play monitor.This is a fan-shaped glass tube. Electron may travel through this.The principle of thermionic emis-sion is , the CRT is heated by a 6.3 V heater in order to “boil off”.
Negatively charged, Electrons attracted to the positively charged anode which situated at the other end.When the electron beam hits the phosphor
the phosphor through up light. This light can be green, orange, pale blue or white depending on the type of phosphor. The anode which has a very high positive voltage applied , it can be easily identified through large rubber cap , which is connected behind the rubber face. The electron stream will strike a single point in the centre of the tube face.The magnetic field is supplied by two sets of electromagnets which is mounted on a yoke and wrapped around the neck of the tube. One set averts the beam horizontally while the other bends the beamin the vertical axis. For the horizontal deflection coils the electron beam repeat-edly move slowly from left to right
across the screen and then quickly “fly-back” to the left hand side again. The vertical deflection coils, we can cause the beam to slowly move down the screen and quickly re-turn to the top.
Flyback transformer required for the CRT anode. Before insulate the fly back transformer we shold keep our fingure clean. If will not do so, then the
electron beam will turned off and will turned on when illuminate particular spot on the screen. This is known as modulation.By doing this under the control of a character generator as part of an adapter card we can display text on the screen or, by writing directly into videomemory,we can create graphics.
The electron beam is modulated during the horizontal forward scans. Turned off during both the horizontal and vertical retrace inter-
vals. This cause a complication it will synchronised to the of the electron beam or vice versa.
Some video cards provide separate horizontal and vertical sync signals. Others combine them into a composite sync signal. CGA have a composite video output in which the syncs and video are all combined into a single signal. It is usually use with domestic televi-sion sets or television monitors rather than computer monitors.
Colour:
Earlier days colour monitors used a single electron gun and special phos phors which changed colour depend-ing upon the energy of the electron beam, in turn controlled by the anode voltage or a grid. These were very limitted functionality. But in modern displays use a more com-plicated arrangement of three electron guns and three different kinds of phos-phor spots on the screen, corresponding to the colours red, green and blue.
A special power system called a “ shadow mask”,ascertains that the red gun can hit only red dots, the green gun can hit only green and so on.
all three electron beams are averted by the same coils. If all three beams strike an adjacent group of red, green and blue dots, the screen will looks white.If green and blue guns are on at full strength, the result will be a cyan pixel.This is the difference between TTL/RGB monitors and analogue monitors.An analogue monitor can cope with a continuously varying voltage on each colour input. a digital computer can never generate a perfectly
variable analogue voltage . but, in practice, the 256,256 colours available
froma VGAprovide close to pure analogue video quality.
Monitor Faults:
monitor faults are hardware technicians with special tools and test equipment particularly in view of the risks of working with high-voltage circuitry.it quite easy and less expense.
Test Patterns
The faults are exaggerated to identify them more easily. To find out the fault
Start Windows Paint program. Use the Fill Tool to flood the background with black and then use the Line Tool to draw a grid of white lines; hold down the shift key to snap the lines to the standard vertical and horizontal grid. Once created, BMP file full-screen using the View Bitmap option .
Safety:
• When working on exposed highvoltage circuitry well insulated ground should be there.
• Keep one hand in pocket all times, to minimise the risk of current flow
through the chest cavity,
and make alladjustments with an insulated trimming tool.
Simple Adjustments:
mostmonitors are easily accessible.
in order to get access to the potenti-ometers. use a long plastic trimming tool and keep fingers well away from high-voltage circuitry.Controls accessible throughthe cover are generally labelled on the outside.Internal potentiometers are usually labelled as part of the silk-screening onthe printed circuit board.This is often unsuccessful.
CRT-basedmonitors only reach their best performance after a
warm-up period. Some of the energy from the beam that produces the image on the screen is converted to heat and the heat causes the CRT to change
shape slightly. This is enough to change the screen performance, in-
cluding focus, colour purity and geometry. After 20 to 30 minutes the
temperature will reach working levelsand.,
Common Faults:
• If a Screencompletely black , then we can to do this.
Keep monitor is plugged into power (at both ends) switched on (at both ends).
Then the signal cable is connected to the graphics adapter (and especially not a Token Ring card).That the user hasn’t simply turned down the brightness overnight, orthat a screensaver utility has not cut the video.
Turn the brightness right up andlook for the screen to go grey and the appearance of the retrace lines If they are there, then the problem lies either in the input circuitry on the monitor, the video cable or the circuitry of the graphics adapter.If turning up the brightness has no effect, yet the power indicator on the front of the monitor is glowing then it’smost likely that a fault in themonitor circuitry has causedthe power supply circuitry to “crow bar” or shut itself off internally Send out for repair.
• Screen grey (green, amber) when brightness increased (not shown).
In this case the monitor is displaying the raster pattern correctly, so
the power supply, high voltage and both horizontal and vertical circuitry are working okay. Then check the cabling to the graphics adapter. If
that is okay, then the fault must be in the video input or sync circuitry.
• If the image is squashed vertically.Identify the “vertical size” adjustment potentiometer and use it to attempt to restore the image to full size. After adjustingvertical size, check vertical linearity.
• If the image is reduced to a single horizontal line.In this case, the vertical output has gone entirely, and it is unlikely thatadjusting the “vertical size” will have any effect.
• Image is squashed horizontally.Identify the “horizontal size” adjustment potentiometer and use it to attempt to restore the image to full size. After adjusting horizontal size, check horizontal linearity.Image is reduced to a single vertical line .
• As for case Dabove, this means that horizontal output has gone entirely.
• Image is “tearing” horizontally.This is caused by degradation of the filter capacitors in the monitorpower supply.As theybreak down, they generate noise and cause this problem. Adjustment of the“horizontal sync” controlmight provide some relief.,
• Image is twisted to one side.This is caused by the yoke carrying the deflection coils having come free from the tube and shifting around.On colour monitors, it may be accompanied by colour shifts.The fix is to restore the yoke to the correct position and (in the case ofa colour monitor) reconverge the guns, but I wouldn’t recommend this job for anyone who is not confident around highvoltage circuitry, for safety reasons.
• Pincushion distortion.this is caused either by power brown-outs, and is usually quite short-lived, or by problems in the power supply.
• Retrace lines visible.Adjustment of the brightness andcontrast controls will usually fix this problem.However, if it persists and no combination of these controls will give satisfactory results, then locate the “black level” potentiometer, which is probably set toohigh, and adjust it accordingly.
• Picture rolls vertically.The monitor is failing to “lock on”to the vertical sync signal from thegraphics adapter. The fact that horizontal sync is okay suggests the graphics card is probably okay, so adjust the “vertical hold” control.If substitute monitor exhibits thesame fault, then check the video cable for continuity on the sync lines and, if that’s okay, suspect a lines and, if that’s okay, suspect a fault in the graphics card.
• Picture “tears” and rolls horizontally.Try adjusting the “horizontal hold” and, if necessary, “vertical hold”. If these have no effect, proceed as for K above, suspecting the horizontal sync signal.
• Test pattern square size varies top to bottom.Otherwise known as vertical non-linearity. Locate and adjust the“vertical linearity” potentiometeruntil all squares are the same size.
• Horizontal non-linearity (notshown).This looks just the same as verticalnon-linearity, only sideways. Lo-cate and adjust the “horizontallinearity” control.
• Colour changes across or down screen.If a screen changes colour particularly from top to bottom one prime suspect is a stray magnetic fieldfrom the PC underneath the monitor, particularly the power supply area. Try moving the monitor or inserting a sheet of steel under neath it. If this does not fix the problem, then it could be a convergence problem: send out for repair.
Monitor Test Program
basic bitmap test pattern is the most common and simple monitor faults.
The more sophisticated correct things like colour balance or image focus.
CheckScreen is a freeware utility that can be usedto test monitors or LCD flat panel displays. The program can be downloaded from www.csf.org.uk,a n druns under Windows 95, 98 and NT.CheckScreen provides a series of test
screens to help you adjust your monitor to get the most from it.
Conclusion
The monitor is the major PC component least likely to become over-the-hill
and require advancing. System components, such as processors,
hard disks.etc, have been replaced by more cutting-edge versions.
Therefore it is well worth performing some diagnostic tests and adjustments
periodically makethe monitors stay in A-one shape.
