Flat screen TVs and monitors

This is currently a skeleton page. Please help by adding your contributions either here or in the associated Discussion page.

This page will cover flat screen TVs and computer monitors.

Summary

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.

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 also 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 display. Of these, LCD panels are perhaps the commonest but these don't in themselves, create any light but require a separate light source. This is normally along one edge and spread evenly over the whole panel by a difuser made of transparent plastic. In older devices the light source is often a cold cathode flourescent tube which requres its own high voltage supply but in newer devices it's almost invariably a row of LEDs.

So what is the distinction between a TV and a monitor? Both display pictures and may provide sound, the distinction comes from the source of the picture and sound signals. A TV has an RF tuner (receiver) that the users controls to select the programme they wish to view. It's signals arrive by radio from a remote transmitter and it will have an antenna connected to receive those signals.

A monitor doesn't have the tuner and often won't have speakers for sound, instead it is fed from an external source, typically a PC or laptop via cables. That said, a monitor can be fed from a freeview or satellite TV set-top box and a tuner can 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, via 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 disgnostic 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 excessibly 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. Could it just be a faulty connector or cable, which might be fairly easily repaired or replaced? Do the control buttons work, 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 if anything, still works. If you can display and navigate the setup menus then check which inputs work. Maybe 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 extremely faint, or has a colour cast, then the problem is probably with the display panel and its drivers or with its backlight.

Power Supply Problems

Power supply problems are probably the commonest and certainly 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 indicate 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 values needed 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 (a lot 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 vartiant has a different set of components missing, and so is unlikely to be compatible.

Display Problems

Problems with the actual display are unfortunately much harder and often effectively not fixable. In the case of laptops, a replacement panel is quite often available and so there may be some chance for smaller TVs and monitors. If you can remove the panel as a unit and it has a single multiway connector on the back, look for a model number and search for it, not omiting any suffixes, which may indicate incompatible variants. If you have no luck on eBay, try Aliexpress.com or Banggood.com. For larger screen sizes, it's very unlikely that you could get a replacement, and in any case, its size and weight would make it difficult to fit.

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

Whether it's an LCD, OLED or plasma display, the pixels will be arranged in rows and columns, with connetions down one side to each row and along the bottom (or top) to each column. With separate red, green and blue elements to each pixel, the number of these connections will be well into the thousands. A controller chip receives brightness and colour information at very high speed, column by column and row by row. This is distributed to individual pixels addressed in sequence by row and column numbers by a series of multiplexer chips, each serving a subset of the rows or the columns. These multiplexors are typically connected to the glass panel by a flexible printed circuit ribbon which is bonded to the glass with some kind of adhesive, in order to conduct the electrical signals to conductive traes on the glass.

The backlight usually consists of a row of LED chips (or in older displays, a CCFL or cold cathode flourescent tube) along one edge. A cunning arrangement of several plastic sheets distributes this evenly over the back of the panel and reflects it forward. (If you disassemble a scrap display panel you'll find these sheets have curious optical properties.)

A faulty backlight will manifest itself as a black screen (you may still be able to make out a ghostly image) or uneven light distribution. If an irregular shaped area of the screen is darker, this indicates the light distributing pastic sheets are delaminating. Even if you can work out how this has happened, fixing it will be challenging.

There is some chance you might be able to replace a failed CCFL tube. There will be a driver board which generates the high voltage it needs, and this, equally, could have failed and might be replaceable.

In the case of a LED backlight, replacing a failed individual LED chip will be hard, but if they are standard surface mount LEDs this might be possible. Since they will collectively draw a substantial current, they are likely to be fed from a dedicated output from the power supply, or there may be two or more outputs, each supplying a subset of them. Check these out.

If there are well-defined horizontal or vertical black or coloured bands on the picture this indicates that one (or more) of the multiplexor chips is faulty. If there are individual horizontal or vertical lines or groups of lines, then it's the connections between the flexible printed circuit and the glass of the panel. Often in this case, more or fewer lines will appear if you apply pressure to the edge of the screen. Occasionally, inserting packing will cure the fault.