IT Services



Hardware Fault Diagnosis


Contents



1. Troubleshooting monitors

To undertake component level repairs, test equipment, circuit diagrams and access to spare parts are all needed. However, the following points will allow a "rule-of-thumb" approach to be followed.

An oscilloscope is very useful for making many of the above checks, but great care is needed in positioning the earth return of the probe. It should not be clipped to the metalwork where the possible presence of a potential up to several hundred volts may well be sufficient to damage the oscilloscope. It should instead, be connected to the power supply's zero volt line.

As ever with component level repairs, a thorough visual inspection to uncover dry solder joints can pay dividends in monitor repairs.



2. CRT Monitor Warning

This guide is NOT intended for the novice or non-technical user. When troubleshooting monitors, it should be noted that Cathode Ray Tube (CRT) based monitors have high voltages inside the casing and in particular on the CRT connector, at certain parts of the Printed Circuit Board (PCB) and on the flare of the CRT. In the latter case, the flare is part of the construction for the anode, which typically will be at a potential of 25,000 volts relative to earth and therefore very dangerous to touch. Due to a built-in capacitor, this voltage cannot be assumed to disappear when the monitor is disconnected from the mains supply. The charge can remain at a high level for an hour or more following disconnection, so a method of carefully discharging this capacitor is necessary if working in this area of the monitor's circuitry.

It should also be borne in mind that there is an implosion hazard at the neck of the tube where the glass is thin and easily broken to allow a sudden inrush of air into the near vacuum inside. Generally, it is advised only to attempt simple procedures when tackling monitor repairs due to the necessity to fault find at component level.



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.



4. Video Adapter Problems

Before assuming the monitor to be faulty, it is worthwhile substituting it with another in order to establish that the video card of the PC is working. Normally such faults as intermittent assortment of odd characters, perhaps some flashing in vivid colours on the screen are indicative of a video card fault.

If the problem appears to be related to mode changing, e.g. form VGA to Super VGA, this may well be due to malfunctioning logic circuitry, relays or solid-state switches, which are best left alone unless the repairer is prepared to spend some considerable time to effect the repair. Many problems arise from inappropriate configuration of the video card. It is often forgotten that many monitors are not designed to operate at high resolutions and high vertical and horizontal frequencies. Using monitors of modest specifications in excess of their design limits will display out-of-sync images possible together with audible buzzing. Persistently overdriving monitors in this way can lead to physical damage of the monitor's circuitry. This phenomenon can be checked by temporarily resetting the PC's display to the standard VGA setting of 640 by 480 pixels.



5. Troubleshooting Video Interfaces

In the event of an absent screen image, it is more likely that the monitor is at fault than the video interface. If after substitution, the monitor proves to be operational, suspicion falls on the interface and its associated cables.

Checking that the video card is fully seated in its connector and that the motherboard is designed to accommodate the card, is a useful first step in diagnosing a video communication problem. Reseating the card often cures the fault.

An intermittent assortment of unusual characters in vivid colours is a classic sign of electronic problems with the video card. Replacement of the card is the only solution. With the increasing complexity of these cards and the use of ever larger amounts of high speed video RAM, their ability to dissipate large quantities of heat is crucial to long term reliability. To facilitate this, fans and ductings are increasingly used. If these become encrusted with dust or the fan stops working, the electronics will rapidly fail.

Due to the several different interface ports in current use such as the analogue VGA and the digital DVI and HDMI, the interface may be incompatible with the monitor. In the case of dual head interfaces, where there are two or more output ports, the signal could be directed to the wrong connector. The interface driver software can correct this. Where the monitor has several ports, it may be expecting signals from an unused connector. The monitor’s setup will redirect the signal appropriately. If the driver software has not been installed, the interface is likely to operate only in VGA mode. Installing the software, together with any updates, will make available the full range of modes of the interface.



6. Does the monitor match the PC?

Early methods of communication between PC and monitor relied on monochrome digital signals but were soon superseded by low resolution colour versions such as CGA (colour graphics adaptor) and EGA (enhanced graphics adaptor). In 1987, the VGA (Video Graphics Array) was introduced with a resolution of 640*480 pixels, which was followed in 1990 by the SVGA standard with 800*600 pixels. Both used analogue data signals but maintained the digital control lines of their predecessors. With increased screen sizes, further increases of resolution were needed, starting with the IBM developed XGA as shown in the table below.

Name X Pixels Y Pixels Typical Screen Sizes
(measured diagonally)
Video Graphics Array VGA 640 480
Super Video Graphics Array SVGA 800 600
Extended Graphics Array XGA 1024 768 15 inch
Super Extended Graphics Array SXGA 1280 1024 17/19 inch
Ultra Extended Graphics Array UXGA 1600 1200 20 inch
Quad Extended Graphics Array QXGA 2048 1536 30 inch

The output ports provided by the computer to supply data to the monitors have progressively increased their performance to accommodate the higher speed data flows required by the interfaces. Below is a list of the most common of these.

Computer Data Bus Year of Introduction Throughput
Industry Standard Architecture ISA 1985 16 MBps
Peripheral Component Interconnect PCI 1993 133 MBps
Accelerated Graphics Port AGP*1 1997 266 MBps
AGP*2 1998 533 MBps
AGP*4 1999 1.1 GBps
AGP*8 2002 2.1 GBps
PCI Express 2004 5.0 GBps

Other analogue video standards have originated in the television and video equipment fields. Composite video, is in essence, a television RF (Radio Frequency) signal before being combined with sound and the RF carrier. Just two wires are needed, signal and ground, using a phono connector. Of the three analogue standards discussed, composite video produces the lowest quality of image. No audio is provided.

S-video (for super or separated video) carries the video data as two separate signals, for colour and luminance. A resolution of 700*486 pixels is available, again with no audio. Cables with DIN connectors with at least 4 pins are commonly used. As S-video has effectively the two component parts of composite video, the colour and luminance can be combined is a simple adaptor to form composite video. The reverse conversion is only possible with electronic manipulation. Due to lack of bandwidth, S-video is not suitable for high definition pictures.

Component Video is the analogue standard that has found favour for use with DVD players, high definition displays, video projectors as well as with computers. RGB colour signals are carried by three separate cables, each having their own phono connector. Video projectors and other professional video applications in this country utilise BNC connectors, whereas Japanese equipment tend to use “D” Type connectors. The line and frame synchronising signals are sent during the blanking periods of one or more of the colour signals and typically use Green for this purpose. Some variants of the system have separate synchronising lines, requiring a total of five cables. Audio is not inherently included but can be accommodated with two additional cables for stereo. Computer video cards typically label component video as “TV Out” or “VIVO” (Video in Video Out). These variations in the encoding of the RGB signals generate a significant lack of standardisation increasing the risks of improper picture generation on the monitor. Transmission of the component video signal is more robust than for the DVI and HDMI digital systems, enabling cables of up to 60 metres to be used without the use of boosters. There is a gradual deterioration in picture quality with length of cable, which is contrasted against the behaviour of digital signals, which experience a sudden occurrence of picture breakup at a specific cable length. Although the quality of the digital systems is better in theory, it is often a matter of conjecture as to whether the viewer would, in practice, see much difference.

Of the two common digital formats, the DVI (Digital Visual Interface) was the first to gain popular acceptance in computer displays. Introduced in 1999, it is the only widespread video standard that includes analogue and digital options within the same connector. Audio is not included. The connector is specific to the standard and has between 19 and 29 pins. Three main designs of connector are available. DVD-D (digital only), with 19 pins provides digital signals, DVD-A (analogue only) is analogue having 23 pins and DVD-I (Integrated, digital an analogue) has 29 pins. As the analogue signals of a VGA connector and the analogue section of a DVI connector use the same signals, it is possible to use a straightforward adaptor to convert between the two without having any intervening electronics. Because there is uncertainty as to which version of the connector is to found on the video interface and the monitor, the following summary may be useful in establishing which cable is needed to connect the two.

For displays of 1920*1200 pixels, cable lengths of up to 4.5 metres are possible. For 1280*1024 pixels, up to 15 metre cables are generally attainable. For longer distances, boosters are recommended. These are either self-powered or will need an external power supply. As already mentioned, digital signals are subject to sudden degradation if excessive cable lengths are used. This is known as the “digital cliff” and will occur at a specific distance for a given system and cable quality. There is no gradual deterioration as in analogue systems.

In 2003, a video/audio interface with a compact connector was introduced to provide high definition digital video with up to 8 audio channels and other features such as remote control. This HDMI (High Definition Multimedia Interface) system can support 2560*1600 pixels. The 8 channel audio is transmitted at a 192 KHz sample rate with 24 bits per sample. Digital Dolby is supported. Cable lengths of up to 5 metres are achievable for cheaper cables and better cables can be a maximum length of 15 metres. Degradation, when it occurs, is often in the form of instability or blinking. The connectors have 19 pins and are hot-pluggable. As the DVI digital signals are compatible with HDMI, a DVI-D or DVI-I cable may be used to drive a HDMI monitor, or vice versa, using an adaptor. In this case, but audio and remote control features would be lost.

A major feature of HDMI is the incorporation of High-Bandwidth Digital Content Protection (HDCP), which prevents the unauthorised user from viewing HDCP protected content. This has made it particularly attractive to software suppliers and some 800 companies in the video and PC fields have now adopted the format. Sales of the interface are set to exceed those of DVI in 2008.

Increasingly, monitor manufacturers are offering both types of digital interface and so one is faced with the decision of which to use. For normal screen sizes, the picture quality is the same. Both use the same encoding scheme and simple adaptors are available to convert between the two. The principle advantage of HDMI is that it also carries high quality audio. Also, the connectors are smaller and the cables are of smaller cross-section. The analogue Component Video is also capable of good quality reproduction though it achieves it with different colour information. Cable lengths can be as long as 60 metres, producing a gradual reduction of quality with increasing length.

The Apple range of computers deserves an individual mention as they have used a variety of video interface standards, often combining them to form proprietary connectors. Apple’s standard DVI connector, which can provide VGA via a short connector cable, also accommodates composite and S-Video on the Power Mac G5 and the Mac mini. A mini-VGA connector may be converted to normal VGA via a short cable and is available on the white iBook, eMac, iMac G4 and G5 as well as the first generation 12-inch Power Book G4. Later models also support composite and S-Video. A micro-DVI connector is used in the MacBook Air to cater for the restricted space available. This is also used on later 12 inch PowerBook G4, Intel based iMacs and MacBooks. A full DVI connector is supplied on the MacBook Pros.

Looking to the future, the current display standard most able to be upgraded to higher resolutions, is HDMI. This is the only one that has inbuilt audio capability. The Video Electronics Standards Association (VESA) has proposed a licence free new standard called DisplayPort which has been designed to succeed DVI which includes DRM (Digital Rights Management).