3. Troubleshooting Monitors

It is extremely dangerous to remove the back cover of a monitor, particularly a CRT (Cathode Ray Tube) type, as voltages in excess of 25,000 volts can be present, which could remain for some considerable time after the power has been removed. Although the front face of a CRT has a thick glass wall, the rear neck of the tube is thin and easily broken. If it is broken, there is an implosion hazard due to the inrush of air into the vacuum of the glass envelope.

If no image is seen, the signals being sent by the computer may be incompatible. CRT Monitors will only accept a certain range of vertical and horizontal scan ranges. Unsuitable scan rates will produce unstable or incoherent images. Flat Panel Monitors only display sharp images at a specific resolution, called the native resolution. This varies between monitors, but is usually consistent for a given size of screen. For a 15 inch screen, this typically would be 1028 * 768 pixels. If a monitor is presented with any resolution other than the native resolution, the image will appear less sharp or distorted or both.

Both the computer and monitor may have more than one interface port. The absence of an image may be due to the computer sending an output to the wrong port. This can be altered at the video interface by means of the driver software and at the monitor by its setup controls. Incompatibility of ports and their connectors are a frequent source of problems.

CRT Monitors are susceptible to various geometric distortions such as pincushion and barrel. Access to adjusting controls may be either external, on the rear panel, or internal. Please note the dangers of delving inside a CRT monitor. Convergence errors cause primary colour separation in white areas. The solution is to adjust the electron beam current of one or two of the three beams. If there are localised patches of background colour on parts of the screen, usually blue or purple, colour purity errors may be the problem. Correction is usually achieved by attaching small permanent disc magnets to the flare of the tube near the affected areas.

Manufacture of Flat Panel screens has, until recently, proved difficult and expensive due to low yields. If just a few of the many thousands of pixels are defective, this can lead to annoying and misleading effects on the images. These dead pixels may emit a single primary colour, or be always “on” (a white spot) or be always “off” (a black spot). For economic manufacture, customers previously have had to accept a small number of failures, but processes have now improved such that screen should have none or very few of these aberrations.

Unlike CRT monitors which have built in power supplies, Flat Panel units often have external supplies in the form of an adaptor, as used in laptop computers. In cases where the monitor shows no signs of life, a useful test is to test the adaptor by measuring the voltage at the output plug.

External influences on monitors can produce some odd effects. If a CRT monitor is subject to excessive jitter, first try looking for any nearby peripheral devices or appliances that could be responsible rather than blaming the monitor itself. Magnets such as those contained in loudspeakers or desktop fans could cause jitter due to magnetic interference effects and might also explain the appearance of patches of background colour. Moving the offending item a short distance away should cure the problem.

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