You will need Phillips screwdriver #P2, jewelers flat-blade screwdriver 2-mm width, Torx T15 (known also as Mac Cracker), blunt thin pry tool such as table knife, and muffin tin to put screws in per order of removal.
1. Wait about half hour from shutdown of computer to allow high voltage in CRT circuits to dissipate. Unplug computer and practice anti-static grounding precautions.
2. Remove system tray by removing four screws on back of unit. This website has good explanation:
http://www.scienceman.com/pgs/trev_g3aio.html
Take system tray out completely.
3. Remove lower faceplate, which surrounds speakers and CD and floppy drive slots. Be patient with all Mac plastic case parts - they can be removed but process is slow. Pry tabs on bottom of faceplate down to clear tangs, and slowly work faceplate away from unit. When it won't go any farther than a few millimeters, get table knife and pry into screwdriver slots on sides of faceplate. Secret is to push knife into slots and only pry sideways a little amount. There is a tang inside slot that will be disengaged with sufficient work. When tangs are disengaged, faceplate will pull away from unit. Rotate faceplate around upper edge to disengage tabs along top, and remove faceplate.
4. Use Torx driver to remove four screws above and below speakers.
5. Use jewelers screwdriver to remove two 12-mm round plugs along top front edge of case. They are on the translucent part of the cover, about 10 cm from center on each side. When plugs are removed, use Torx driver to remove two screws which plugs covered.
6. Plastic cover can now be slid back off of unit. If needed, gently pry with kitchen knife along front seam to get cover to move.
7. Use Phillips screwdriver to remove six screws which secure the perforated metal cover at the upper rear of unit. Two screws are on lower edges of cover on each side, one is on lower edge on back at center, and one is below power cord receptacle. With screws removed, cover will rotate up about front edge, disengaging tabs, and then cover can be removed.
8. The circuit boards containing the power supply and the CRT power and driver circuits can each be removed easily if the tang at lower aft corner of board is first disengaged. Board then slides up and out.
I discovered that my unit was sparking from CRT high-tension transformer, although I originally suspected the power supply.
Linear Mode
