Troubleshooting and Repairing Monitor ProblemsMany common monitor problems can be easily repaired if you know what you're looking for. In this article from PC Support Advisor, David Stott provides a practical how-to guide to monitor troubleshooting. This is a plain html version of the original article. For the original version in PDF format complete with pictures, click here. Despite the fact that monitors are fairly robust when compared with the internal logic circuitry of personal computers, display monitors can have a considerably higher failure rate. In fact, repairing monitors is one of the most common PC support tasks. There are two major reasons for this: firstly, the circuitry in monitors operates at a much higher voltage than that in the PC, and secondly, while PCs are digital circuitry, which either works or doesn't, monitors are mostly analogue circuitry which can gradually drift off and give inferior performance. Most of the logic circuitry inside a PC operates at around 5 volts, and such a low voltage induces virtually no stress on the circuitry. However, monitor circuitry can operate at voltages up to 10, 000 volts for monochrome displays, and anything up to around 25, 000 volts for colour displays. Such high voltages impose considerable stress on the components and may cause them to drift in value, and this will cause corresponding drifting and non-linearity in the displayed image. Furthermore, digital logic gates are either on or off, with essentially no in-between states, whereas the analogue circuitry of a CRT monitor can take any voltage in a continuously varying range, and this gives a lot more scope for things to go wrong. In this article, I will discuss the basic principles of operation of display monitors, discuss the faults and mal-adjustments which can arise, and describe how to fix them. It is also worth considering using a monitor test utility - see below. Important: Lethal voltages exist in a monitor long after it has been turned off and disconnected from the electricity supply. Do not touch anything inside the case unless you know exactly what you are doing. CRT Principles The primary component in a display monitor is the display tube or cathode ray tube (CRT). This fan-shaped glass tube contains a vacuum through which electrons may travel. At the rear end of the CRT is a (usually nickel) cylindrical cathode, or electron gun, which is heated by a small 6.3 V heater in order to "boil off" electrons - this is the principle of thermionic emission. Electrons, being negatively charged, are attracted to the positively charged anode at the other end of the tube, and accelerate, in a stream, to strike the phosphor coating on the in-side of the glass face of the tube. When the electron beam hits the phosphor, the phosphor emits light; this light can be green, orange, pale blue or white, depending on the type of phosphor, and the glow might persist for some time or fade quickly. The anode has a very high positive voltage applied - typically tens of thousands of volts - and it can be easily identified by the large rubber cap over its connector behind the tube face, and the thick rubber insulation on its wiring. Left to its own devices, the electron stream will simply strike a single point in the centre of the tube face. However, because a magnetic field exerts a force on charged particles we can apply a magnetic field around the electron beam, and "bend" the stream to strike any spot on the face of the tube. The magnetic field is supplied by two sets of electromagnets mounted on a yoke and wrapped around the neck of the tube; one set deflects the beam horizontally while the other bends the beam in the vertical axis.
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