Flat screen TVs and monitors

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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.