Twentieth Anniversary Macintosh General Maintenance

Twentieth Anniversary Macintosh with its bass unit (photo: Wolfgang Stief, CC0)

The Twentieth Anniversary Macintosh (TAM) was serviced by Apple only at central repair centres, and its Service Source volume contains no take-apart procedures. Routine care therefore rests on the owner's manual that Apple reproduced in that volume and on two community disassembly guides, the iFixit teardown and the longer guide on mac27.net. The machines are now close to thirty years old.

Safety

The power supply is in the bass unit. Its primary reservoir is a pair of 470 µF 200 V capacitors, and the silkscreen on the supply board warns that heatsinks HS1 and HS2 are live. Apple notes that the only way to disconnect power completely is to unplug the power cord. Before working on the supply board, unplug it, discharge both reservoir capacitors through a resistor and confirm with a meter that they are flat.

• Turn the computer off before connecting or disconnecting any cable. Apple warns that connecting or disconnecting the keyboard or trackpad with the power on can damage the computer.
• Touch the metal fan frame inside the back cover before touching any internal part, to discharge static.

Setting up and positioning

Apple gives these limits in the setup chapter of the manual:

• Keep the screen vertical, or tilted back by no more than 15° or forward by no more than 5°, so that the vertically mounted CD-ROM drive works correctly.
• Put the bass unit within six feet (1.8 m) of the computer, near an earthed outlet, with free air flow round it for cooling. It works best acoustically on the floor.
• The bass unit contains a very strong magnet. Do not stand it next to the computer, and do not put floppy disks on it.
• Check the voltage switch in the well under the bass unit before plugging in. Set it to "115" for 100–130 V supplies and to "230" for 200–270 V supplies; it slides with a small screwdriver. Apple warns that a wrong setting can damage the computer.

Voltage switch in the well under the bass unit (Apple user manual, p. 13)

Umbilical coupling: line up the bumps, push together, then turn the metal ring (Apple Service Source, Basics p. 6)

Connecting the umbilical

1. Line up the bumps on the two halves of the connector, so that the internal ribs and grooves match.
2. Press the connectors together without forcing them.
3. Turn the metal band firmly until it clicks.
4. Press the cable back into the groove under the bass unit and route it out through the cut-outs in the base.

The plug has 23 pins in a circular shell. Look for bent or recessed pins before connecting a machine of unknown history; the Twentieth Anniversary Macintosh Troubleshooting Guide has the pin assignments.

Cleaning

Apple's cleaning instructions, from the manual's maintenance appendix:

• Turn the computer off and unplug it before cleaning the case. Wipe it with a soft, lint-free cloth dampened with water, and keep moisture out of the openings. Do not use aerosol sprays, solvents or abrasives.
• Wipe a dusty bass unit the same way.
• Clean the screen with household glass cleaner put on a soft cloth, not sprayed on the screen.
• Wipe condensation off the trackpad with a clean cloth before use.
• Keep every ventilation opening clear.
• If thin, clear liquid is spilt on the keyboard, unplug it, turn it upside down to drain and let it dry for 24 hours at room temperature. Apple sends sticky or greasy spills to a service provider.

Leather palm rests

Apple recommends this routine several times a year:

1. Clean the leather with a soft cloth moistened (not wet) with a mild soap solution, using light circular movements, then repeat with clean water.
2. Let the leather dry completely without a hair dryer or other heat.
3. Apply a leather conditioner with a soft cloth and light circular movements, let it dry, and buff with a dry cloth.

Avoid hot water, abrasive polishes, solvents, sprays, stiff brushes and long exposure to sun. Blot drinks immediately; Apple warns that stain treatment may change the look of the leather.

CD-ROM drive

Keep the drive door closed, because dust on the lens causes read problems. Do not force the door open by hand, do not wipe the lens with a paper towel or anything abrasive, and never move the computer with a disc inside. The door hinges and the door arm break easily; one owner snapped the arm the day after buying the machine.

Under the back cover

DAV connector, DIMM slots, cache slot, PCI slot and communication slot under the back cover (Apple user manual, p. 161)

Apple describes the area under the back cover as the only user-serviceable part of the machine, and recommends an Apple-certified technician even for that.

1. Choose Shut Down from the Special menu.
2. Unplug the power cord from the wall.
3. Remove any security cable or padlock from the cover latch.
4. Lay the computer face down on a clear, flat, cushioned surface, with padding under the speaker fabric.
5. Push the two latches in and lift the back cover off.
6. Store the cover in a plastic bag to protect its metal shielding.
7. Touch the metal fan frame before touching anything inside.

Source: user manual, Chapter 10, pp. 163–167.

Memory

Each of the two DIMM slots takes an 8, 16, 32 or 64 MB module: 64-bit, 168-pin, 5 V, EDO or fast-page mode, 60 ns or faster, with a 2K refresh count. 3.3 V DIMMs, DIMMs that need a 4K refresh and SIMMs from older Macintosh models will not work. Mixed speeds all run at the speed of the slowest module. To reach 128 MB, remove the factory modules and fit two 64 MB DIMMs.

Expansion cards

The PCI slot takes a PCI 2.0 card with Macintosh drivers, up to 6.88 inches long and 15 W. The card fits into a PCI card adapter and an expansion panel, and the machine is then closed with the deeper expansion back cover in place of the standard one. Press the card straight into the slot without rocking it, and if it has a DAV connector, connect it to the DAV socket on the logic board with the longer ribbon cable. A communication card may draw no more than 2.5 W.

Clock battery

The clock battery is a pack on flying leads, held to the chassis with Velcro and plugged into the logic board; the iFixit teardown removes it in step 3 along with the hard disk and fan bracket. The teardown photograph shows a Rayovac "Computer Clock Battery". Apple's technical specifications page lists the battery as 4.5 V alkaline, but the Service Source specifications call it a long-life lithium battery. Owners report finding dead 4.5 V Rayovac packs and replacing them with AA battery packs.

Read the label on the pack before cleaning up any leakage, because the neutraliser depends on the chemistry; see Battery Explosion, Capacitor or Corrosion Damage and Battery Refurbishment.

Fans

The main unit has a fan on the hard disk bracket over the processor and a case fan behind the backlight inverter board. The bass unit has a fan in the power supply.

TAM fans

Fan	Access	Notes
Hard disk bracket fan	Back cover off; unplug the fan from the logic board and remove the four #0 Phillips screws holding the hard disk and fan assembly	One owner's original was an Elina 0PF6015L-12MB-2. A Noctua NF-A6x15 FLX fits under the standard (slim) back cover; the deeper NF-A6x25 FLX does not. The Noctua has a 3-pin plug and the logic board header is 2-pin, so an adapter is needed. The owner tied the fan to the bracket with thread, because the head of a zip tie fouled the slim cover.
Case fan	Full head unit disassembly; remove the inverter board (two Torx screws and one connector), then the fan bracket (two captive Torx screws)	Replaced in the mac27 restoration because it had become loud.
Power supply fan	Full bass unit and power supply disassembly	Leave the new fan's leads short. A loose green cylindrical part (possibly a ferrite bead) on the transformer wires can slide into the blades and click; push it behind the fan's support arm.

Hard disk

The original drive is a 2 GB 2.5-inch IDE notebook drive, mounted on the bracket with the processor fan. Image the original drive before replacing it. Replacements used by owners:

• a CompactFlash card on a passive IDE adapter, which one owner chose because CF cards are natively IDE (the mac27 restoration used a 32 GB SanDisk card);
• an mSATA or SD card on an IDE adapter, or dosdude1's 2.5-inch IDE SSD.

The mounting holes on the TAM drive bracket do not match newer drives, so owners fix the replacement with double-sided tape. One owner had lock-ups while reading the CD-ROM drive with an SD card adapter fitted. Low End Mac quotes Steve Wozniak's warning that a volume over 4 GB does not work in a TAM, and notes that other owners have had mixed results.

Disassembly cautions

Follow the mac27.net guide, which is adapted from the iFixit teardown and adds the steps it misses. Points that cause damage:

• The plastics are brittle, and many parts are taped, zip-tied or glued at the factory.
• The four main case clips between the front and rear of the head unit are fragile. Release each by pulling its tab towards the centre with a spudger.
• When the front fascia comes away, the large black latch bar for the CD-ROM door is exposed. Knocking it pops the door open, and sideways force on the open door breaks its hinges.
• The speaker covers are press-fit; lever them off with a spudger.
• The front case is still connected by the inverter, speaker, CD-ROM, display and front panel cables when it comes loose. Do not pull it away until they are disconnected.
• On reassembly, refit the ground cable loop under the lower right tuner card screw, and route the two coaxial cables through their slots before refitting the power distribution board.
• In the bass unit, the decorative cap is held by three plastic posts that may snap (superglue holds it again), and the rubber dome is stuck down round its rim. Route the subwoofer cable back through its groove with equal lengths each side before refitting the subwoofer, because it cannot be moved afterwards.

First checks on a newly acquired TAM

1. Check the voltage switch under the bass unit against the local supply before plugging in.
2. Inspect the umbilical pins, then connect and lock the coupling.
3. Confirm that the green status light in the well under the bass unit comes on.
4. Take off the back cover and look at the clock battery and the board round it for leakage.
5. Listen for fan noise and for the "Bose buzz" from the speakers.
6. Image the hard disk before it is used further.

Faults found here are covered on the Twentieth Anniversary Macintosh Troubleshooting Guide. Capacitor values are on the Twentieth Anniversary Macintosh Capacitor Replacement Guide.

Related pages

• Twentieth Anniversary Macintosh
• Twentieth Anniversary Macintosh Troubleshooting Guide
• Twentieth Anniversary Macintosh Capacitor Replacement Guide
• Hard Drive Maintenance and Repair
• Recommended Tools