iMac G3 General Maintenance

A slot-loading iMac G3 (katz2110, Wikimedia Commons, CC BY 2.0)

Routine work on the iMac G3 means getting at the memory, the clock battery and the boards. The procedures differ between the tray-loading models of 1998-1999 and the slot-loading models of 1999-2001, and both are given below. Fault-finding is on iMac G3 Troubleshooting.

Warning: the iMac G3 contains a CRT and mains circuits on the boards above the logic board. Memory and the battery can be reached from below without touching them. For anything behind the top rear housing, Apple's Service Sources call for the CRT to be discharged first; see CRT Discharge Procedure.

Tools

Apple lists for the tray-loading iMac a spring hook tool (922-1547) for the processor card, a plastic screwdriver (922-3504) for the housing tabs, a size 00 jeweller's Phillips screwdriver for the CD-ROM bezel, a CRT discharge tool, a nut driver or pliers for the A/V board thumbscrews, needle-nose pliers, a Phillips screwdriver and an ESD mat. See Recommended Tools.

Opening the case

Tray-loading models

1. Lay the computer face down with the screen on an ESD mat or other soft surface.
2. Loosen the captive screw on the lower access cover, then pull the cover up and towards you by its handle with a sharp tug; Apple notes it is difficult to remove.
3. The logic board, hard disk and CD-ROM drive come out together on the logic board/mass storage chassis.

Slot-loading models

1. Lay the computer face down on an ESD mat or other soft surface.
2. For memory only, turn the coloured latch on the access door anticlockwise with a coin or flat screwdriver and pull the door down.
3. For the logic board, remove the VGA vent cover with a plastic flat-blade screwdriver and the two screws near the flip foot, take off the bottom housing and the EMI cover. The logic board and the down converter board come out together.

Owners report that the slot-loading iMac's inner bezel is extremely brittle and can break into many pieces when the case is taken apart; MacEffects makes a replacement.

Memory

Memory rules

Item	Tray-loading models	Slot-loading models
Module	144-pin SDRAM SO-DIMM; EDO is not accepted	168-pin PC100 SDRAM DIMM, 3.3 V, unbuffered, no more than 16 chips
Slots	Two, on the processor card: the factory module in the lower slot, the upper slot free	Two, behind the access door in the bottom of the case
Soldered RAM	None	None; the machine will not start with both slots empty
Maximum	128 MB on the 233 MHz /A model; 256 MB on the /B, 266 MHz and 333 MHz models	512 MB (slot loading, DV) or 1 GB (Summer 2000 onwards)
Notes	The lower slot takes only modules 1.0 to 1.5 in tall. Modules can go in singly, in either slot	Registered or buffered DIMMs are not supported

Sources: Apple's tray-loading Service Source and Memory Guide, and Apple's specification pages.

On the tray-loading models the memory is on the processor card. Remove the lower access cover, the logic board chassis, the processor top shield and the processor card, then pull the metal snaps at each side of the module outwards so that it tilts up. Apple notes that customers may already have filled the upper slot, so do not assume it is free. The tray-loading logic board has 2 MB of SGRAM video memory soldered on and one slot for an SGRAM SO-DIMM to reach 6 MB. Use only SGRAM; the 256K and 512K VRAM modules of older Macs must not be fitted.

Clock battery

Apple's specification pages give a 3.6 V lithium cell. On the slot-loading models, measure it with the machine unplugged across the BT1 terminals: 3.3 to 3.7 V is good, and below 3.2 V it should be replaced, followed by a PMU reset. Apple recommends removing the battery whenever the logic board is handled, because touching the circuitry can crash the PMU, and a crashed PMU drains the battery in about two days. Dispose of the old lithium cell as hazardous waste.

Display adjustments

The CRT's geometry, cutoff, white balance and focus are set in software with Apple's Display Adjustment Utility (version 4.0 for the tray-loading iMac), with a light meter for the cutoff and white balance steps. Apple's light meter set-up covers the R77 (076-0310), L-248 and 246 meters. On the slot-loading models the focus and screen voltage potentiometers are on the flyback transformer, reached by removing the bottom and top rear housings, and the adjustments are made with the machine running. Set the screen voltage in a dimly lit room before the cutoff and white balance.

Service parts

• Tray-loading iMacs have original and "Version 2" analogue/video boards, power supply boards, CRTs, CRT frames, fans and cables. Version 2 parts only work with other Version 2 parts; identify the board before ordering.
• Slot-loading iMacs have two power/analog/video boards, 661-2465 (with a switch at SW901) and 661-2466. Exchange like for like.

Related pages

• iMac G3
• iMac G3 Troubleshooting
• iMac G3 Capacitor Replacement Guide
• CRT Discharge Procedure
• Battery Explosion, Capacitor or Corrosion Damage
• Hard Drive Maintenance and Repair