For many CCTV engineers the monitor is seen as a complete component which, when it fails, is replaced as a whole item.
Repair, if the cost of the monitor warrants it, is performed by someone with suitable qualifications and experience, very often an engineer from the TV/VCR service industry.
Although servicing is complex and often does require the attention of an engineer with specialist skills, many of the common faults are not beyond the ability of a competent CCTV engineer. The aim of this article is to introduce you to some of the more common faults which occur in CCTV monitors, and offer some guidance on how to tackle these safely and effectively.
Making adjustments
A quality monitor will produce images of acceptable quality for at least four years, remembering that it is the CRT (cathode ray tube) which ultimately decides when the image resolution deteriorates. You may recall colour monitors back in the 1980s which began to take on a pink or green cast after a few years. This was because the three separate electron guns in the CRT do not deteriorate at the same rate, so subsequently one of them (the one with the least deterioration) became dominant.
An engineer who had been trained in what is known as grey scale adjustment could re-set the R, G, B (Red, Green and Blue) drive levels and perhaps give the CRT a few more months of effective use, depending on how quickly the guns continued to deteriorate (see Fig 1).
Modern colour monitors employ a system known as auto grey scale tracking, where the drive chip continually checks the beam currents and adjusts the drive voltages accordingly. This not only compensates for CRT ageing, but also for changes during the initial warm-up period of around ten minutes. However it does mean that the engineer is given no warning of tube deterioration because the auto circuit will continue to correct until the guns fall below a certain threshold.
So … as happened to me on one occasion … you may view a perfect colour picture on the day of your service visit, and be called out just a week later because the picture is completely unwatchable.
Auto grey scale has greatly improved the effective CRT life, however some monitors still incorporate three R, G, B drive adjustments for what are termed the low light adjustments. These are usually mounted on the CRT base. Two other bias voltages on the CRT which are adjustable are the focus and screen (also known as A1 because it is the first anode in the gun assembly) potentials. These are normally built into the line output transformer, and many monitors have apertures in the rear cover to allow adjustment.
Focus adjustment is quite straightforward; you simply set it for the sharpest picture. However, it is advisable to know that the lens focus at the camera you are using during adjustment is correct! A better alternative it is to use a portable test pattern generator.
The screen adjustment is more involved. Altering the screen grid bias voltage at the CRT alters the beam current, and thus the screen control appears to function like a brightness control. However its setting is critical and incorrect adjustment can reduce the CRT life to as little as six months. This happens when the engineer sets the screen potential too high, resulting in a bright picture. He then corrects this by reducing the main brightness control. This has two effects; The operator may later complain of seeing a series of faint, horizontal white lines across the screen on dark scenes. These are the vertical flyback lines which are normally suppressed showing through (Fig 2). More important, the CRT bias conditions are completely incorrect, resulting in the Barium oxide coating on the cathode being stripped off over a period of a few months. When this happens, the electron emission falls, and the CRT becomes unserviceable.
The screen may be set by measuring the potential at the CRT electrode with a multimeter and adjusting for a specified voltage. However this is not usually necessary. A reliable method is to set the main brightness control to minimum, and the contrast to its mid point. Now adjust the screen control until the white areas of the picture just become visible. Finally adjust the brightness for correct viewing. Some fine adjustment may be necessary, especially if flyback lines appear when the contrast level is reduced.
Where R, G, B adjustments are incorporated, these are intended for factory alignment, or in cases where a new CRT has been fitted. The auto grey scale circuit means that it is rarely necessary for the CCTV engineer to re-set these, however where a coloured cast has come over the entire image, adjustment is worth attempting. In this case, remembering that these are low light controls, perform the adjustment with the brightness and contrast set to give a dark picture, and the colour level set to give a monochrome picture. It is preferable to display a white screen produced by a test signal generator whilst adjusting the CRT drives, and first make sure that the screen control has been correctly adjusted.
Degaussing … an easy cure
Another thing to watch out for in colour CRTs is a magnetised shadow mask. This metal mesh is located behind the glass screen, and should it become magnetised it upsets the landing points of the three electron beams, resulting in coloured patches on the screen. The CRT can become magnetised very easily. Any external magnet placed near the monitor can instantly cause the problem, the most common culprits being loudspeakers and isolation transformers.
This problem is usually easy to cure. The monitor has an in-built demagnetising circuit known as the degaussing circuit, comprising of a set of coils wrapped around the rear of the CRT bulb (not to be confused with the scanning coils) and a timer circuit based around a thermistor (see Fig 3). Each time the monitor is switched on from cold, the degauss circuit operates for about one second. Where the level of magnetisation is only slight, the short burst of alternating 50 Hz magnetic energy is sufficient to clear the problem. But remember, the circuit only operates from cold.
In some cases the monitor has a timer over-ride switch on the front. Pressing this causes the degauss circuit to operate for a few seconds. Whilst working, an impressive rainbow effect appears on the screen. Some computer monitors also have this facility, and I am often asked what it is for.
But what do you do when the internal degauss circuit is not sufficient to deal with the problem? In this case, for about £20 you can purchase a degaussing wand (Fig 4). Connected to the mains, this produces a very strong 50 Hz field. With the monitor switched on, the wand is moved in a circular pattern directly in front of the screen three times. Then, whilst continuing to move the wand in a circular fashion, you move progressively backwards so that the field strength at the screen steadily reduces.
A word of warning! Use of a degaussing wand at the customer's premises must be considered carefully. For example, in a CCTV control room you could very easily destroy the information on floppy disks, video tapes and possibly even take out the odd hard disk – although these are normally shielded.
The Dead Monitor
Many years ago as a TV service engineer we had to deal with a particular model of Bush colour TV which became known as the biblical TV – i.e., the burning Bush! A design problem made this receiver particularly susceptible to catching fire under a certain fault condition.
You may recall that fires caused by TV receivers were not uncommon in the 1970s. This was partly due to the lack of circuit protection and isolation under fault conditions, bearing in mind that designers were moving from traditional tried and tested valve technology to the new transistor technology, and they still had lessons to learn.
By the 1980s the problem was all but licked. The inclusion of current monitoring circuits in every part of the receiver means that the power supply can be made to shut down quickly or go into what is known as its trip condition as soon as a fault occurs. The power supply employed is a switched mode type (SMPS), making it easy to control both in terms of power regulation, stand-by operation and trip control. This same technology is just as essential in a CCTV monitor, especially when we consider that these units may be required to operate 24 hours per day.
However this technology is not such good news for the service engineer as there is very little that can be done at the customer's premises when the power supply is in trip condition. This is because SMPS circuit repair requires specific training, a suitably equipped workshop, circuit diagrams, and a lot of patience. Trip condition can be identified by a repetitive squeaking or ticking which comes from the transformer in the power supply. Upon hearing this, the best course of action is to return the monitor to the workshop for service.
Where the monitor is completely dead with no signs of life whatsoever, it may be worth your while checking for the usual faults in the 13A plug top, as well as the plug fuse, and any fuses inside the monitor. Be aware that the latest regulations regarding monitor safety make the use of wired fuses something of a rarity because they simply do not rupture fast enough, and manufacturers have had to turn to solid state board mounted fuses which look nothing like a fuse to the inexperienced eye. However, a fuse of around 3.15A may still be used at the mains input, and these quite often fail after a few years … not through any fault in the equipment but simply because of deterioration in the fuse wire.
Where an internal fuse has failed, check to see if it has failed violently; that is, is the glass envelope blackened? If it is, then replacement would be unwise. Even if the monitor were to function after the fuse is replaced, the argument stands that something must have caused the problem, and it is not uncommon for rectifier diodes to fail intermittently. On the other hand, if the fuse wire appears to have simply broken, then a new fuse will almost certainly restore normal operation.
A word of warning
When you do replace a fuse, you must ensure that the correct replacement type is used. Most monitors employ a 3A fuse in the plug top, although some large screen units may specify a 5A fuse. 13A is definitely out! Internal fuses are more tricky, because you not only have to fit the correct rating, but also the correct type. The majority of fuses used in monitors are either time lag (denoted by a "T" next to the current rating on the metal cap) or anti-surge (denoted AS). Just remember that if there is a fire after you have fitted the incorrect type, you could be in serious trouble; perhaps even facing manslaughter charges. Thus, if in doubt, leave it alone! Intermittent faults such as loss of one colour or going dead are frequently caused by solder joints on the circuit board which have failed. Although these are not faults which you may wish to tackle in the premises, back at the workshop they are worth looking into.
Dry joints occur most commonly in places where the solder is put under stress. These are typically components which get hot, or are large and relatively heavy, or are subjected to mechanical pressure. Hot components are such things as power resistors, power transistors, or power ICs. "Heavy" components are transformers – most notably the line output transformer – and power transistors. Customer buttons, controls and the on-off switch are constantly subjected to mechanical pressure.
A visual inspection of these areas often reveals tiny cracks in the solder, but be aware that you may require a magnifying glass and a bright lamp to detect them. Note that when re-soldering components that run hot, a high melting point solder should be used, otherwise the joint may fail within a short space of time. For safety, the inspection should be performed with the monitor disconnected from the mains power supply.
Where the dry joint has caused arcing and subsequent burning of the surrounding PCB, the PCB should, according to the latest codes of practice, be condemned. This can be frustrating sometimes when the damage is slight and you feel that you could make a repair by using solid wire links, but once again, should the repair fail and result in a fire, you would be liable to prosecution.
You must at all times remember that monitors incorporate high voltage DC supplies, and these can be lethal should you manage to connect yourself across one of them. Thus, when operating monitors with the covers removed you must follow these rules.
The first rule to remember is that the monitor must be connected to the 230V mains supply via a 1:1 mains isolation transformer. This lifts the earth path from the secondary side of the monitor making it safe to connect earthed test equipment such as an oscilloscope or pattern generator. It also greatly reduces the chances of electrical shock should you come into contact with the mains input terminals.
Waiting to kill you!
Please note that the isolation transformer does nothing to reduce the chance of a fatal electric shock should you come into contact with a high voltage DC supply within the monitor. Thus, the second rule of servicing is to only ever put one hand into the monitor whilst it is switched on (see Fig 5). Keep your other hand away, perhaps in your pocket or by your side. Should you now accidentally touch a high voltage there is no current path back to the chassis of the equipment, which forms the negative side of the supply rail, and so – in theory – you will survive.
When servicing monitors, always consider them as something sitting there waiting for an opportunity to kill you.
This is not to say that you should not service them, rather view them as you should your ladder – this too is waiting to kill you, but you simply work in such a way that it is never given the opportunity.
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Fig 1: Engineering adjustments within a colour monitor
Other, Size 0 kbFig 3: Auto degauss coils inside a colour monitor
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Security Installer
Postscript
Joe Cieszynski is a CCTV engineer, author and contributor to SITO distance learning materials on CCTV. His latest book, Close Circuit Television, has recently been published in paperback by Newnes (Butterworth Heinemann) at £19.99.
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