Power Macintosh 7500 General Maintenance

Rear of a Power Macintosh 7500/100: power sockets and voltage switch, AV panel, PCI covers and I/O ports. Photo: Marco Stadelmaier, Wikimedia Commons, CC BY-SA 3.0

These procedures cover the Power Macintosh 7500, Power Macintosh 7600 and Power Macintosh 7300, which share a case, a logic board design and a Service Source, Power Macintosh 7300/7500/7600 & WS 7350. The Power Macintosh 7200 uses the same case and power supply; its own manual and memory rules are on Power Macintosh 7200 General Maintenance. Where the models differ, the step names the model.

Mains voltage is present inside the power supply whenever the computer is plugged in. Unplug it before opening the case.

Before you start

Check the voltage switch on the back of the power supply, reached through the rear panel, before plugging in a machine of unknown history; Apple says it must be set to 115 V in the US (SS pp. 112, 119). The owner's manuals give the same warning. The Electrical page of the same Service Source describes the supply as automatically configured (p. 52); go by the switch.

Opening the case

1. On the 7300, the 7600/200 and the WS 7350, first remove the security bar: take out its mounting screw and rotate the bar up to free its tab from the front of the case (SS pp. 164–165). The bar covers the top housing's latches.
2. Press the two tabs at the front corners of the top housing, pull the housing forward 1 to 2 inches and lift it straight up (SS pp. 85–87).
3. Release the chassis latches, flip open the chassis support foot and swing up the internal chassis that holds the drives and power supply (p. 91).
4. Flip down the support arm and seat its tab in the hole in the bottom chassis. Apple warns never to work with the chassis up unless the arm is down and fastened (p. 115).

Low End Mac warns that the internal SCSI cable in these machines comes loose from the logic board easily as the case is opened; it must go back on SCSI bus 0, the internal fast bus, not bus 1, which serves the external port.

Memory

The logic board has no RAM of its own, and at least one DIMM must be fitted. The eight slots take 5 V, 168-pin, 64-bit DIMMs; earlier SIMMs do not fit (SS pp. 19–21). The slots form two banks, A1–A4 and B1–B4. DIMMs work singly, but for interleaving fit matching pairs of the same type and size in paired slots (pp. 21–23).

DRAM DIMMs by model (SS pp. 20, 43–44)

Model	DIMM type	Fitted	Maximum
7500/100	70 ns fast-page	8 or 16 MB	512 MB
7600/120, 7600/132	70 ns fast-page	16 or 32 MB	512 MB
7600/200 (Japan)	60 ns EDO	16 or 32 MB	512 MB
7300 series	60 ns EDO	16 or 32 MB	512 MB

The memory controller treats EDO DIMMs as fast-page DIMMs, so either type works in the 7300, the 7600/200 and the WS 7350 (SS p. 20).

The board has no soldered VRAM. Two 1 MB VRAM DIMMs (112-pin, 70 ns or faster) come fitted and must be present for any picture; two more give 4 MB, in matching pairs in paired slots. Push down the release latch beside a VRAM DIMM and lift it straight out; press a new one in evenly until the latch clicks (pp. 49, 147–152).

The cache slot takes a 160-pin, 11 ns cache DIMM of 256 KB, 512 KB or 1 MB (p. 44). On a 7500/100, a cache DIMM with 820-0719-B printed on its board should be replaced with part 661-1310 (pp. 60, 65).

Processor card

The card stands upright in its slot beneath the plastic PCI guide. Swing open the expansion card cover, grip the card by its corners and pull it straight up; the heat sink may be hot. To refit it, position it evenly over the slot and press down gently without forcing it. When fitting a 132 MHz card, slide its EMI grounding clip over one wall of the top chassis so that the clip also touches the card's heat sink (SS pp. 134–137).

Clock battery

The battery is near the front left corner of the logic board. Swing up the internal chassis, prise up one side of the battery cover with a screwdriver and lift the battery out (SS pp. 90–93). Apple's specification pages give it as a 3.6 V lithium cell, and Low End Mac as a half-AA. Fit the new cell the right way round (p. 30). This Service Source gives no voltage below which to replace it; "Check battery" is a step in the chart for intermittent shutdowns (p. 64). A leaking lithium cell leaves acidic residue: neutralise it with bicarbonate of soda solution, rinse with isopropyl alcohol and check the nearby traces. See Battery Explosion, Capacitor or Corrosion Damage and Battery Refurbishment.

Resets

Press the Cuda reset button on the logic board to reset the Cuda chip, which controls power, resets, PRAM, ADB and the clock (SS p. 28). For a computer that will not power up, Apple's logic board reset comes before any part is replaced (pp. 29–30):

1. Unplug the computer and remove the battery.
2. Disconnect the power supply cable from the logic board and press the Power On button.
3. Wait at least 10 minutes, then refit the battery the right way round.
4. Reassemble and test. The reset clears PRAM, so set the date, time and other settings again.

Power supply

To remove the supply (SS pp. 112–120):

1. Disconnect the power cables from the CD-ROM drive and the hard drives.
2. Swing up the internal chassis and lock it with the support arm.
3. Disconnect both supply cables from the logic board: the 10-pin cable, which carries 3.3 V for the processor card, and the 22-pin cable, which carries 5 V and ±12 V.
4. Remove the two screws at the back that hold the supply.
5. Release the support arm, swing the chassis down, slide the supply forward and lift it out, feeding its cables up through the hole in the chassis.

When refitting, slide the supply back until the two metal tabs on the chassis lock into it, and check the voltage switch (pp. 119–120). The exploded view gives one service part, 661-0920, marked "refer to parts list" (p. 170).

The 7300 logic board's power connector differs. Owners have photographed a 24-pin upright connector on the 7300 board (820-0858-B) and a 22-pin right-angle connector on the 7600 board, and report that the 7200, 7500, 8500 and 9500 supplies share the 22-pin connector. Low End Mac also says a 7300 logic board needs the 7300 power supply. Check the connector before swapping a board or supply between these models. Connector pinouts and owners' supply repairs are on Power Macintosh 7500 Troubleshooting.

Logic board

Remove the top housing, processor card, PCI cards, speaker and power actuator first (SS p. 153).

1. Swing open the expansion card cover, then swing up the internal chassis.
2. Disconnect both power supply cables, the floppy, SCSI and CD audio cables, the LED and speaker cables, and on the 7500 and 7600 the AV module cable.
3. Remove the screw inside the top column support.
4. Press down the two tabs at the front of the board, slide it forward away from the rear panel and lift it out.
5. Lift the EMI clip off the board. Apple warns that leaving it on can damage the board.

Remove the cache, DRAM and VRAM DIMMs before sending a board away, but leave the ROM SIMM where one is fitted (pp. 153–161). To refit the rear panel, slide the board in only an inch short of its final position, fit the panel, then slide the board back until it locks with its connectors lined up in the panel openings (p. 133).

The AV module of the 7500 and 7600 is held to the rear panel by two Torx screws and connects to the logic board by a cable (pp. 141–146).

Cleaning

Residue on a PCI card's gold edge connector can cause a range of faults; inspect the pins with a magnifier and clean them gently with a pencil eraser (SS p. 57). Keep the vents clear: the thermal protection circuit shuts the computer down if it overheats (p. 64).

Drives

Apple's 2/3/4/6 GB Firmware Utility 1.0 (20 November 1997) updates some 2, 3, 4 and 6 GB drives with firmware other than 0F0J that fail to wake from sleep. Check the firmware revision with Apple System Profiler, and use Disk Copy 6.1 or later to write the utility's disk image to a floppy; Apple warns that a floppy made by dragging the file across will not update the drive (SS pp. 58–59, 65). Only the last device on each SCSI chain is terminated (p. 76). See Hard Drive Maintenance and Repair and Floppy Drive Repair.

Related pages

• Power Macintosh 7500
• Power Macintosh 7600
• Power Macintosh 7300
• Power Macintosh 7500 Troubleshooting
• Power Macintosh 7500 Capacitor Replacement Guide
• Power Macintosh 7200 General Maintenance
• Recommended Tools