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This page covers data projectors.

Summary

Data projectors are often quite expensive items and so well worth fixing when they go wrong. With care, disassembly and fixing are not generally too difficult, but an understanding of their principles will help.

Safety

Projector lamps get extremely hot. Always leave a projector to cool down for 30 - 45 minutes before working on it, so as to avoid the risk of burns.

Take great care not to touch any of the glass of a projector lamp as finger grease can shorten its life, and in the worst case, cause it to explode.

How projectors work

There are two basic types of projector: DLP and LCD. Except for pocket-size projectors, they almost invariably use a discharge tube as the light source.

DLP projectors

DLP (Digital Light Processing) projectors are probably the commonest. The image is created by a digital micromirror device (DMD) fabricated on a silicon chip. Ech pixel is controlled by a tiny mirror which can be electrically deflected to reflect light either through the lens and on to the screen, or into a "light sink" where it is absorbed. By oscillating between the two states at high speed and spending more or less time in one than the other, the apparent brightness of the pixel can be controlled.

The DMD is essentially a monochrome device. A colour image is produced by a colour wheel in front of the lamp which passes each of the three primary colours in turn. The DMD then creates each of the red, green and blue fractions of the image, one after the other. If you sweep your gaze rapidly across the screen you will see the red, green and blue images separating. Some people find this distracting, and it is sometimes cited as a disadvantage of DLP projectors.

In passing only one colour at a time the colour wheel rejects two thirds of the light. High end home theatre and event projectors sometimes have 3 DMD chips. The light from the lamp is split into red, green and blue beams which are focussed onto the 3 DMD chips before being recombined into a colour image for projection onto the screen.

LCD projectors

These typically have 3 LCD modules, and as in high end DLP projectors, the light from the lamp is split into red, green and blue beams, which are focussed on the 3 LCDs before being recombined. When you take the lid off you may be able to see 3 identical ribbon cables at 90 degrees descending from the main logic board to the 3 LCD modules.

Lamps

In the last few years, a range of pocket projectors has appeared which use LEDs as the light source, but these have a limited brilliance making them unsuitable for projecting more than quite a small image for a very few people. LEDs intrinsically have a very long lifetime, though they are likely to be operating to the limits of their power, which may reduce their life or result in elevated failure rates.

Virtually all other projectors use a xenon or metal halide discharge tube as the light source. These lamps run very hot and have a life of only a few thousand hours. However, the rated life tends to assume that it's used in long stretches at a time, and the actual life may be considerably shorter if the projector is only used for an hour or so at a time.

These lamps take a minute or so to reach operating temperature and full light output. They require a high voltage to initially strike the discharge but thereafter they run at a much lower voltage. However, the striking voltage is significantly higher still if you try and switch them on while still hot from previous use, and in this situation a projector may have to automatically retry several times in order to strike the discharge.

Diagnosis and fixing

Many of the same problems as are found in any electronic equipment can affect a projector. Unless you have specific symptoms pointing to a particular fault, signs of burnt or overheated components, or bulging electrolytic capacitors, may quickly lead you to the problem. The power supply in a projector will have to supply more power than in many other types of electronic equipment, subjecting them to greater stress.

In a DLP projector, the colour wheel may fail due to a worn or dry bearing. Indicator lights may specifically indicate a colour wheel problem, or the image may flicker different colours, or the projector may refuse to start up. The bearing may be virtually impossible to get at but if it is, you could try lubricating it with a tiny drop of sewing machine oil. Failing that, it should be possible to source a replacement wheel online, but having found what seems to be a suitable one, it may be worth taking a photo of the original and sending it to the seller to ensure the replacement is identical.

Fans, also, can fail, and can be replaced and air filters (if fitted) can get clogged and need cleaning. Such problems may have resulted in the projector overheating.

A lamp coming to the end of its life can run excessively hot and can also cause overheating. There may be a thermal cut-out switch which may have tripped as a result of either fault.

Dust or other forms of contamination can be sucked into the projector by the fan. If this lands on any of the optical surfaces it may cause blurry shadows on the projected image. Clean off with great care as it is easy to make things worse, and any finger marks may not be easy to remove. Optical surfaces will have anti-reflective coating which could be damaged by unsuitable solvents o cleaning agents, and the colour coatings on a colour wheel of a DLP projector or the colour separation filters of an LCD projector may similarly be damaged.

Replacement projector lamps are often quite expensive, often several hundred pounds. Generally, there is a choice of 3 options. The branded replacement offered by the manufacturer is likely to be the most reliable or trouble free and would be the professional choice, but at a premium price. An unbranded alternative may be available, identical in every way except for the branding and coming from the same factory. This will offer a considerable saving. Still cheaper third party alternatives may be available, built to the same specification or remanufactured with a new discharge tube in a recycled housing. they may even offer an increased life as compared to the branded alternative, by using a higher specification of discharge tube. Finally, you may be able to obtain a replacement bare discharge tube and fit it to the old housing, offering the cheapest alternative. However, you need to take extreme care to avoid touching the glass.

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