Macintosh LC III General Maintenance

This page covers routine care for the Macintosh LC III (logic board 820-0650-A/-B, 1993), which is also the board in the Performa 450. The chassis, power supply, battery and take-apart procedure are shared with the rest of the LC "pizza box" family; the logic board is not.

Figures below come from Apple's Service Source for the LC series and Quadra 605 and from Apple's Macintosh LC III Developer Note, both hosted on this wiki.

What this machine is

Apple's specification for the Macintosh LC III

Item	Figure
Processor	Motorola 68030 at 25 MHz, 32-bit internal data bus, burst-mode RAM access, coprocessor socket
DRAM	4 MB standard, expandable to 36 MB (80 ns or faster SIMMs)
ROM	512 K, expandable to 1 MB
VRAM	512 K on board, upgradable to 768 K
Expansion	114-pin PDS taking an 020/030 direct-slot card
Sound	Monaural 8-bit input at 11 or 22 kHz; monophonic 8-bit generator feeding both channels
Video	Up to 16-bit on 12-, 13- and 16-inch monitors, 8-bit on 15-inch portrait and 21-inch; NTSC and PAL supported

Apple's figures. The LC III is the first of the family with a full 32-bit data path and the first with a coprocessor socket — an LC or LC II has neither. The 16-inch Colour Display is supported on the LC III but not on the LC 475 or Quadra 605.

Mono sound is not a fault. Apple specifies a monophonic generator feeding both channels on the LC, LC II and LC III.

The FPU socket

The LC III has a PGA socket for an optional Motorola 68882. Fitting one is a five-minute job — align pin 1 with the chamfered corner of the socket, press straight down, and the machine detects it at the next boot — but it is also five minutes in which a bent pin is easy. Support the board underneath while you press, and take the chip out with a proper PLCC/PGA extractor rather than a screwdriver.

Power supply

Every "pizza box" machine in this family — LC, LC II, LC III, LC III+, LC 475 and Quadra 605 — takes the same power supply, which Apple lists as a single service part, 661-1733. It is a universal auto-ranging switch-mode unit covering 100–240 VAC, so there is no voltage selector to set wrongly and no separate 120 V and 230 V board to confuse.

Apple's electrical specification

Item	Figure
Line voltage	100–240 VAC, automatically configured
Frequency	47–63 Hz
Maximum power, LC III	30 W, monitor excluded

Note what that figure is: the machine's maximum consumption, not the supply's rating. The same 661-1733 supply is fitted to a 50 W LC and to a 26 W Quadra 605.

What Apple does and does not publish

Apple publishes no rail tolerance and no power-supply adjustment procedure for this machine. The Service Source carries no acceptable-range table for the +5 V, +12 V or −5 V rails and no trimmer adjustment; every power fault in the symptom charts ends in "Replace power supply". The "+5 V 4.85–5.15 V / +12 V 11.9–12.7 V" table that circulates on pages of this era is Apple's Macintosh Plus procedure — genuine there, where it uses voltage test cable 077-0135 on the external floppy port — copied onto machines it was never written for. It has been removed from this page rather than reproduced.

What you can do instead is compare against a known load. With the machine running, each rail at the logic-board power connector should sit close to its nominal value and should not move when a drive spins up. A rail that is visibly low, or that sags under load, points at the supply. A rail that measures as a short to ground with the supply disconnected points at the logic board.

Recapping it

You are allowed to open this supply — tired electrolytics in it are one of the commonest reasons one of these machines will not start. Understand what is live inside first. It is fed straight from the mains, so with the cover off and the lead in, the fuse, input filter, bridge rectifier and bulk capacitor all sit at mains potential. The bulk capacitor charges to roughly the peak of the incoming waveform — about 170 V on 120 V mains, about 340 V on 230 V mains — and holds a useful fraction of that after the plug comes out. Unplug, wait, then measure across the bulk capacitor and confirm it reads a few volts before a tool goes near the primary side.

Apple bought this supply from several vendors and their capacitor complements differ, so read the maker's label before ordering parts. The per-variant lists are on Macintosh LC Capacitor Replacement Guide.

Removing it: disconnect the power supply cable from the logic board, release the two latches that hold the front end of the supply to the bottom case, and lift it out.

PRAM battery

Apple's part is a 3.6 V lithium half-AA cell, part number 742-0011, in a clip-in holder on the logic board. It appears under that number in the exploded view Apple prints for the whole LC series and the Quadra 605.

Apple's check is a measurement, not a calendar. Set a meter to the 10 V DC range, put the positive probe on the positive end of the cell and the negative probe on the negative end, and replace the cell if it reads below 3.0 V. Apple publishes no replacement interval for this battery and none is given here: a cell above 3.0 V is doing its job, and one below it should come out today whatever its age.

If it has leaked

This is a lithium cell, so neutralise with bicarbonate of soda, not vinegar. Vinegar is the right treatment for an alkaline cell, whose leakage is basic. It is the wrong treatment here, and it adds water and chloride to an already corroded board. Use a paste of sodium bicarbonate and distilled water, rinse with distilled water, follow with isopropyl alcohol and dry thoroughly.

Then check the damage instead of assuming there is none:

• Inspect under magnification for traces that have gone dull, green or thin.
• Buzz out every track running away from the holder. Corrosion travels under the solder mask and does not show from above.
• Look at the underside of the board. Electrolyte wicks down through vias and comes out the other side.

See Battery Explosion, Capacitor or Corrosion Damage and Battery Refurbishment.

Take the cell out before the machine goes into storage. That is not a service interval; it is the one measure that reliably prevents this damage.

Cleaning

Opening the case

1. Disconnect everything.
2. Remove the case screw if one is fitted. Not every machine has one.
3. Lift the tabs at the back of the lid and lift the top cover straight up.

Inside

• Compressed air and an anti-static brush on the logic board, the supply and the fan. Hold the fan blade still while you blow it: spinning a fan with compressed air drives it as a generator and puts a reverse voltage back down its cable.
• Isopropyl alcohol (99 %) and a soft brush for grime, working outwards from each capacitor and from the battery holder.
• Look at the underside of the board too. Both battery and capacitor electrolyte travel through vias.

Outside

• A damp microfibre cloth and mild detergent. No abrasives, no solvents — the texture on these shells does not survive either.
• ADB keyboards come apart for washing: keycaps off, warm soapy water, dry them completely, then clean the contacts with isopropyl alcohol.
• Mouse ball out, rollers scraped and wiped with isopropyl alcohol.

The platinum-grey plastics yellow with age. The discolouration is in the body of the plastic rather than on the surface, so cleaning will not shift it; see Retrobrite for what can be done about it and what it costs in brittleness.

Fan and speaker

The fan is a separate part with its own cable to the logic board: disconnect the cable, release the plastic latches, lift the fan out.

Fit it back logo side down. Apple prints this as a caution because a fan installed the other way up blows the wrong way and, in Apple's words, may damage the computer. It is the classic self-inflicted fault on this chassis: the machine goes back together, runs hot, and nothing looks wrong.

When the speaker comes out, Apple's replacement note matters — thread the speaker cable through the top two brackets that hold the hard drive before reconnecting it, or the cable fouls the drive.

Connectors and sockets

Oxidised contacts are the commonest cause of intermittent faults on a machine this age, and the cheapest to put right.

• SIMM sockets. Clean with isopropyl alcohol on a swab and reseat. Do not take an abrasive to a SIMM's gold fingers — once you are through the plating, the base metal oxidises and then transfers to every socket the module touches.
• VRAM sockets. Same treatment. Video faults that come and go when the case is knocked usually start here.
• Expansion slot. Check for bent contacts and debris before a card goes home.
• Logic-board power connector. Look for browning or a distorted housing. A discoloured power connector is a warning, not a cosmetic problem.
• SCSI and floppy headers. Reseat, and check the ribbon for cracked conductors where it folds.

A contact cleaner such as DeoxIT D5 earns its place on genuinely corroded contacts. It is not a preventive treatment and it does not belong on clean gold.

Expansion slot

The LC III's 114-pin processor-direct slot supplies +5 V at 800 mA, −5 V at 20 mA and +12 V at 165 mA, and Apple limits a card to 4 watts total however that is split across the three rails. Apple's warning is explicit: cards dissipating more than 4 watts may overheat and damage the computer's circuitry or make it inoperable, and Apple recommends that all cards ground the /FANSPEED signal to increase cooling.

The −5 V budget of 20 mA is a clue as well as a limit. That rail is weak by design, exists almost solely to feed the RS-422 serial drivers, and is the rail with the reversed capacitor described below.

Capacitors, and the C22 trap

Eleven surface-mount electrolytics on logic board 820-0650-A or -B: five 10 µF 16 V, five 47 µF 16 V and one 100 µF 6.3 V.

Before you recap an LC III, read Macintosh LC III Capacitor Replacement Guide. C22 is the bulk capacitor on the −5 V rail, and both the factory placement and the PCB silkscreen are wrong: the pad marked positive connects to −5 V and the pad marked negative connects to ground, so the part sits with 5 V of reverse bias across it. The replacement goes in the opposite way round to the silkscreen. Three independent sources agree, and you can confirm it yourself in continuity mode in about a minute.

The original aluminium part survived the mistake. A tantalum fitted in the silk-screened orientation does not: owners report the −5 V rail dragged down to about −2.3 V with the capacitor drawing on the order of 1.3 A. Apple's own supply is rated at only 75 mA on that rail, which is why the usual symptom is a wrong voltage and serial-port trouble rather than a fire.

If an LC III has serial-port faults after a recap, suspect C22's orientation before anything else.

Floppy drive

The 1.4 MB Apple SuperDrive is direct drive. It has no belt. Advice to fit a new belt to a sluggish or non-ejecting SuperDrive is wrong: there is nothing there to replace. What these drives actually suffer is hardened grease on the head carriage and eject rails, dust in the mechanism, and dirty or worn heads.

• Clean the heads with isopropyl alcohol on a lint-free swab, supporting the upper head arm so it is not sprung.
• If the rails are stiff, re-lubricate with a silicone grease — not white lithium grease, which gums up as it ages.
• Apple's own replacement notes: remove the dust shield, if present, before fitting a replacement SuperDrive, and replace drives like for like, auto-inject for auto-inject and manual-eject for manual-eject.

See Macintosh Floppy Drive Maintenance.

Hard disk

The original SCSI mechanisms are thirty years old and the usual failures are a stuck spindle, dried head-park grease and dead controller electronics. Solid-state replacements — BlueSCSI, ZuluSCSI, SCSI2SD — are quieter, cooler and considerably more reliable, and they let you keep the original drive on a shelf as a spare.

Whatever you fit, get the termination right: the internal drive on these machines is the end of the bus and must be terminated, and the computer itself is always SCSI ID 7. See Macintosh HDD Maintenance.

Storage

• Take the battery out.
• Store dry and at room temperature. Not a loft, not a garage.
• If the logic board comes out, bag it anti-statically and lay it flat.
• Nothing needs periodic running. "Power it up every few months to keep the capacitors formed" is repeated widely and supported nowhere for machines of this kind. The real hazard to a stored Macintosh is the cell in the battery holder, and removing it costs nothing.

Related pages

• Macintosh LC III
• Macintosh LC III Capacitor Replacement Guide
• Macintosh LC III Troubleshooting
• Macintosh LC Series/ Quadra 605 Service Source
• Macintosh LC III Developer Note
• Recommended Tools