[wiki]Category:How_To[/wiki]
Basics

Before diving into the specifics, a few essential checks which will often diagnose or even fix the problem.
Visual inspection

This one doesn't even require any specific equipment, other than maybe a magnifying glass. Give the item a thorough visual examination and note anything out of the ordinary. Things to look for include:
  • Scorched or burnt-out components
  • Residue from leaking components
  • Broken wires, connectors or [wiki]PCB[/wiki] tracks

It helps to clean dirty equipment up before inspection, although excessive scrubbing can of course itself lead to yet more problems!
Reseat cards, cables and ICs

Sometimes, simply [wiki]reseating[/wiki] components is enough to revive even a completely dead machine. It's easiest to start with larger components (expansion cards, cable connectors) first and, if this doesn't help, work down to smaller parts ([wiki="Integrated Circuit"]IC[/wiki]s, crystals, etc.).
Power supply

A faulty power supply can cause all manner of strange and often otherwise inexplicable symptoms. Use a [wiki]voltmeter[/wiki] to ensure all power rails are within the manufacturers tolerances (or if the tolerances are unknown, that the voltages are at least sane). If an [wiki]oscilloscope[/wiki] is available, this can be used to check for [wiki]ripple[/wiki] on DC power rails.
Common, model-specific problems

Similar machines often fail in similar ways. Use your favourite search engine to look for common problems with your specific hardware (and hopefully their solutions).
General Problems

  • [wiki]Dead computer[/wiki] (blank screen, no response)

Display

  • [wiki]Garbage screen[/wiki] (random characters/colours)
  • [wiki]Distorted image[/wiki]
  • [wiki]Snow (TV screen)[/wiki]

Keyboard

  • [wiki]Sticking keys[/wiki]
  • [wiki]Intermittent keys[/wiki]
  • [wiki]Nonfunctional keys[/wiki]

Disk Drive

  • [wiki]Cleaning heads[/wiki]