Tip & How-To about Computers & Internet
The most common cause of no display is a faulty back-light. Liquid Crystal Displays work by selectively blocking light. In most cases, the light source is a pair of fluorescent tubes that run quite hot and are prone to failure. Smart chips which sense any over or under-current then shut them down. Sadly, most tubes are difficult to get at, and require a clean-room to replace, unless you don't mind staring at a screen full of dust.
The transformers that generate high voltage to run them can also fail. Bad solder joints are becoming less common but can still happen. Typically, connections fail wherever there are high levels of heat, current, frequency, or voltage. The back-light circuit has all of the above.
No power at all is usually caused by one of two conditions: either the standby voltage that runs the microcomputer is too low, or there is no voltage at all. The former is almost always due to a bad filter capacitor, while the latter can be anything from a blown fuse to a weak start up capacitor.
Switch mode power supplies or inverters work by using transistors to produce their own alternating current (AC) at much higher than the line frequency. This allows them to convert a lot of power with a very small transformer. Filter capacitors act like a reservoir soaking up rapid pulses of current and turning them into a steady voltage. Unfortunately, the high frequency they run at (typically 30 to 50 kilohertz), generates a lot of heat which causes them to wear out more quickly. Sometimes they even blow their tops! A startup capacitor gets everything going by sending a burst of power to the switching transistor kind of the way the starter gets a car running. If it wears out you won't know it until after the next power failure.
Blown fuses fall into two categories: melted and vaporized. A vaporized fuse means you have a shorted component with access to a lot of current. Common causes are switching transistors, rectifier diodes (one way valves that turn AC into DC), filter capacitors, and anything across the AC line such as surge protectors and transient suppressors. A melted fuse can be more tricky to diagnose, since it can be anything from a shorted component to the gradual rise in current caused by aging.
Filter capacitors become less efficient over time, causing the whole set to draw more current. Fluorescent lamps also draw more current as they age. A good design will use micro-fuses or fusible resistors to protect the drive circuit.
If all the power supplies are there and it still won't turn on, you may have a bad switch. Membrane switches have been known to fail altogether.
Finally, as reliable as they are, microcomputers, signal processors, and multi-layered circuit boards all have one mortal enemy: heat. Even a soft metal such as copper can only take so many expansion and contraction cycles before metal fatigue causes it to break.
Tip: Turning the back-light intensity down just a little bit can make it last a lot longer.
Posted by tech_talk on
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