Flat screen TVs and monitors

This page covers flat screen TVs and computer monitors, how they work, how to diagnose problems and how to fix them.

Summary

Flat screen TVs, especially the larger sizes, are expensive items, but they quite ofen fail and are very often just thrown away, even though some faults are quite easily fixed. In this page we review the types of device, their common modes of failure, and what can be done by way of repair.

Safety

As with all mains electrical devices, dangerous voltages may be exposed when opened. Be aware that capacitors can store a dangerous voltage long after power has been disconnected. Also, there may well be high voltages associated with the backlight on an LCD screen, whether it uses LEDs or cold cathode fluorescent tubes.

Large screen TVs can be very heavy. Lift with care and get help if in any doubt about your ability to manage.

Classes of device

For completeness we should mention the original CRT screens which were the norm from the first days of television to around the 1980's when plasma and newer technologies started to replace bulky CRTs. CRTs required a very high voltage of 10-20kV (10,000-20,000 Volts) which meant great care was required for any repair.

There are several different technologies used to create a flat display panel for TV's and monitors, well summarised in Wikipedia under Flat-panel displays. LCD panels are perhaps the commonest. They require a separate light source which, in older devices, is often one or more cold cathode fluorescent tubes (which require their own high voltage supply). However, in newer devices it's almost invariably a row of LEDs. Illumination is normally along one edge and spread evenly over the whole panel by a diffuser made of transparent plastic to spread the light, or from behind the panel in larger screens.

So what is the distinction between a TV and a monitor? The distinction comes from the source of the picture and sound signals. A TV has an RF tuner (receiver) that takes the signal from the aerial (or a satellite dish or a cable connection), selects the required programme and extracts the sound and vision.

A monitor doesn't have a tuner and often won't have speakers for sound, instead being fed from an external source. This could be a PC or a laptop. However, a monitor can be fed from a Freeview or satellite TV set-top box and a TV can generally be used as a computer monitor. Either can be connected to a DVD or video player or a games controller, provided it offers compatible inputs.

A smart TV perhaps forms a third class of device, containing both a tuner and its own computer with an Internet connection. Furthermore, you may be able to "cast" a video stream to it from your smartphone or tablet using WiFi or Bluetooth.

The first step is to clearly identify what type of TV or monitor you have because that will help you determine what the basic electronics blocks are. Depending on which functions (if any) still work, you may be able immediately to narrow your diagnostic search. Usually it's fairly obvious what class of device you have from the inputs on the back but if unsure, search the internet for information about the make and model type. The most useful model information is usually found on the back or underneath the item.

Diagnosis and Fixing

CRT TVs and monitors

Fixing these requires a very clear understanding of high voltages and any opening or repair should be done with full knowledge of how to identify the danger zones. The CRT tube itself requires careful handling as it contains a vacuum. Droppping it, dropping something on it, or excessively straining the neck could cause it to implode, showering a wide area with shards of glass and causing serious injury. Consider carefully whether you should even be attempting a fix at a repair event.

Flat Panel TVs and monitors

The first thing to do is to narrow down the fault as far as possible and eliminate anything simple.

• Is it showing any kind of diagnostics, e.g. the power LED flashing a diagnostic code?
• Could it just be a faulty connector or cable, which might be fairly easily repaired or replaced?
• Do the control buttons still do anything, both on the device and on the remote, if there is one?
• Does the remote need new batteries?
• Is something wrong in the setup menus, e.g. wrong aspect ratio?

Next, work out what still works, if anything. If you can display and navigate the setup menus then check which inputs work. Maybe just the tuner is faulty.

If TV sound still works then the tuner can probably be eliminated. If nothing works or if it spontaneously switches itself off then the problem is almost certainly in the power supply. However, the power supply probably produces several different voltages for the different modules, some of which may still be functional.

If the display is distorted, or has lines or bars across it, or uneven illumination, or is extremely faint, or has a colour cast, then the problem is probably with the display panel and its drivers or with its backlight.

Initial disassembly is generally fairly straightforward. Lay the device face down on the workbench and first of all, remove the stand or wall-mount fixing. The back cover should then come off after removing a good number of screws. If these are not all of the same size, store them separately (e.g. in the cells of an egg box) and make a note of which came from where. If the back doesn't lift off there may be one more screw that you've missed (very easily done), or sometimes there may still be clips that need to be released. Try to work out at which points the back still seems to be retained.

Power Supply Problems

Power supply problems are common, especially in older devices, and are often the commonest fixable problems. The device may be completely dead, or it may switch off spontaneously, or the screen backlight doesn't come on (you may still see a faint ghost of a picture) or maybe nothing but the standby light comes on.

The power supply module is easily located as the one that the mains power feeds directly into. Check for any signs of overheating or burning. If so, a rectifier diode may have failed. These are easy to recognise and easily tested with a multimeter equipped with a diode testing function, as most do. This is explained in How to use a Multimeter. Be aware, though, that signs of burning may well be the result of a much less obvious fault elsewhere, and that other nearby components may have suffered collateral damage. Test any you can.

If there are no signs of overheating then it's quite likely an electrolytic capacitor has failed or is failing. Again, these are easily recognised as a cylindrical aluminium can, often with a plastic sleeve. A domed top or any signs of leakage is a sure sign of a failing or failed component, but if it looks good that's no guarantee that it is good. How to test an electrolytic capacitor is described towards the end of the section on Capacitors in Basic Electronic Components.

Electrolytic capacitors are easy to replace except that the high value ones used tend to be quite fat and so difficult to fit comfortably behind a flat screen. You may find the one or ones you need to replace are very long and thin, and harder to obtain. A simple work-around is to put several in parallel, since their capacitances will then add up. For example, for a 1,000µF you could substitute two 470µF capacitors, or three 330µF. (The value doesn't have to be exact.) Always choose replacements with the same, or better still, higher voltage and temperature ratings. And make quite sure you connect them the right way round otherwise they will fail very rapidly, quite possibly with a bang (could be entertaining) and a lot of mess (much less fun)!

If it's not clear what the problem is, or if you don't feel able to repair it, it's well worth searching online for a replacement power supply board. Look for a part number on the board, and search for that on eBay or any a search engine. If you find one, check it's visually identical. These boards are often commodity items used in a range of different devices by the same or different manufacturers. Sometimes the same board comes in several variants with different options according to the outputs needed for a particular TV or monitor. In this case you may find that yours has markings on it for components which are not fitted, and another variant has a different set of components missing, and so is unlikely to be compatible.

Display Problems

In newer devices, display problems may well outnumber power supply faults.

Problems with the actual display are unfortunately much harder and often not easily fixable. There are firms which specialise in this and it may well be an economic option and certainly better for the planet, especially for larger sizes.

If you discover that backlight problems are prevalent for your make and model of TV then you can probably extend its life by turning the brightness down, or at least turning it down in the evenings when you don't need it so bright. You might even get a better night's sleep!

In the case of laptops, a replacement panel is quite often available as they are commodity items. By extension, there may be some chance for TVs and monitors. If you can remove the panel as a unit, look for a model number and search for it, not omiting any suffixes as these may designate one of several incompatible variants. If you have no luck on eBay, try Aliexpress.com or Banggood.com. For larger screen sizes, it's less likely that you could get a replacement, and in any case, on account of its size, weight and fragility, shipping costs may be high and fitting may be difficult.

If the glass is cracked then replcement is the only option.

Even if it isn't fixable, you can often deduce what the problem is from an understanding of how a display panel works. This in itself will be instructive.

How the picture is formed:

Whether it's an LCD, OLED or plasma display, the pixels will be arranged in rows and columns, with connections along the bottom (or the top) to each column and down one side to each row (though probably also brought out at the bottom). With separate red, green and blue elements for each pixel, the number of these connections will be easily into the thousands. A controller chip receives brightness and colour information at very high speed, column by column and row by row. A series of multiplexer chips, each serving a subset of the rows or the columns sends this information to individual pixels addressed in sequence by row and column numbers. These multiplexors are usually mounted on a narrow printed circuit board running the width of the screen along the bottom, and connected to the glass panel by a flexible printed circuit ribbon. This is bonded to the glass with conducting adhesive, in order to conduct the electrical signals to conductive traces on the glass.

In smaller displays (and laptop screens) the multiplexor board also contains timing and control circuitry and is connected to the main video controller chip on the mainboard (or laptop motherboard) via a ribbon cable or maybe 8 twisted pairs. Larger screens may have a separate TCON board for the timing and control.

The backlight:

The edge backlight from an LCD TV.

Only LCDs require a backlight as they generate no light of their own.

In small to medium displays the backlight usually consists of a row of LED chips (or in older displays, a CCFL or cold cathode fluorescent tube) along the bottom. This lights the edge of a plexigass sheet which is covered with a pattern of tiny dimples causing the light to exit from the sheet evenly over its surface. In front of this, a cunning arrangement of several plastic sheets further diffuse and polarise the light before it enters the LCD panel itself. (If you disassemble a scrap display panel you'll find these sheets have curious optical properties.)

In larger displays, a number of CCFL tubes or LED strips may be located behind the panel, with a frosted diffuser sheet to spread their light evenly. This increases the thickness of the entire device but allows more LEDs (or CCFL tubes) to be used in order to adequately illuminate the greater area.

A faulty backlight will manifest itself as a black screen or uneven light distribution. In the case of a black screen, you should still be able to make out a ghostly image with a torch held at certain angles. If an irregular shaped area of the screen is darker, this indicates the light distributing plastic sheets are delaminating or liquid has got into and between the light-diffusing sheets.

Backlight repair:

It's possible you may able to find a replacement for a failed CCFL tube if you search online. The high voltage it needs will be generated by the main power supply board or by a separate board adjacent to the tube.

In the case of a LED backlight, there is some possibility of obtaining a drop-in or cut-to-length universal replacement. Replacing a failed individual LED chip would be hard, but if they are standard surface mount LEDs this might be possible. Since a considerable number of LEDs will be connected in series they will require a high voltage, very possibly well over 100V and so will be driven by their own output from the power supply board. Needless to say, such a voltage will present a serious electric shock hazard.

Display panel faults:

If the display is marred by individual horizontal or vertical lines or groups of lines, then it's likely to be the connections between the flexible printed circuit and the glass of the panel. Often in such a case, some of the lines will appear or disappear if you press on the edge of the screen. Inserting packing to apply pressure to the affected area will sometimes cure the fault, temporarily at least.

A wide black or coloured band with well defined edges indicates that one of the multiplexor chips or its connection to the TCON board is faulty.