TV screen defects

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In the days of CRTs, tube faults were well defined and usually easy to diagnose. Sometimes they could be spectacular. Their glass, very hard, was virtually immune from faceplate damage except the abrasion and scratches caused by laying the set on its front. The same cannot be said of its solid-state successors. In this era of LCD and plasma panels fault-finding may not be easy, and failure of the panel itself leads to the whole set being written off. It’s important, then, to get the diagnosis right, in the workshop or on site.

Liquid crystals

The surface of an LCD panel is relatively soft, making it vulnerable to scuffs and scratches. It’s not really possible to remove them but the application of Vaseline or rubber pencil eraser can reduce their visibility. Impact and stress results in mura defects, where the liquid crystal is compressed or unevenly distributed. It shows on the picture as dark marks or patches, a non-uniformity of brightness. Time and usage often reduces the effect; gentle and even rubbing with a micro-fibre cloth can help. A heavy sharp blow on an LCD screen will break its surface, dispersing the liquid and severing internal drive connections. The effects can be bizarre as the picture above shows.

Although it’s not as noticeable or serious as with plasma (see below) there can be an ‘image-sticking’ effect on an LCD screen: it arises from a bright and prolonged image and usually fades away with time and use. Internal failure of an LCD image panel has various symptoms: very fine horizontal or vertical lines, white or coloured, superimposed on the picture; columns of rainbow colours, typically in two bands above and below the horizontal centre-line; screen suffused with a single colour; and others.

The most common fault associated with LCD panels, however, is some kind of failure of the backlight. Except in new and expensive sets this will be of the fluorescent type, whose tubes can fail but whose energy source, the invertor assembly, is a more common offender. An internet search may well find both of these available as spare parts.

Gas panels

Though they are harder, surface-wise, than LCD types, plasma screens are more vulnerable in other ways. Most dealers are now aware of the possibility of burning of the screen whenever any bright picture feature is allowed to linger there: there is no miracle cure for this, but some relief may come from prolonged use with full-screen pictures having no stationary highlights. A physical risk with plasma is breakage of the bond between the faceplate and its underlying cell-wall structure. This causes bad colour staining, and comes from rough handling – for instance bumpy transportation with the panel lying flat. Keep it vertical at all times.

While plasma TVs do not have any backlights to give trouble, they have many peripheral circuits associated with the panel’s operation, quite apart from the video processing section. These are complex and run under some stress, which combined with the high temperatures and greater power dissipation of plasma (compared with LCD) can lead to poor reliability. The problem here is to make a correct diagnosis amongst the Y-sustain, X-drive, scan and other panels, then to source a replacement, hopefully at a reasonable price. Single-line defects and pixel errors, as with LCD, are always due to the panel itself.

An occasional minor irritation with a plasma panel is an audible buzz, sometimes a roar, which may vary with image brightness or operating temperature. Little can be done about this, but a layer of felt or similar material under the stand can prevent the latter from acting as a sounding-board.

Take care

Whenever you handle, move or store LCD or plasma panels, be sure to provide a substantial screen protector of one sort or another. When transporting a thinscreen larger than 26in, get someone’s help so that all four corners can be held, preventing any risk of flexing: this can stress and detach internal connections. If you rent out the TV have the customer bring in their home insurance documents along with their identification document etc. at the outset so you know you’re covered. Finally, if you have the room, store written-off and scrapped TVs for cannibalisation of their screens, panels and other bits – you probably won’t be able to get them anywhere else…

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