Macintosh LC III General Maintenance

Macintosh LC III logic board 820-0650-B with the electrolytic capacitors marked. The circled part at lower right, beside the −5 V pin of the power connector, is C22, which Apple fitted and silk-screened backwards. Photo: Recap-a-Mac.

The Macintosh LC III (1993) uses logic board 820-0650-A or 820-0650-B, which is also used in the Performa 450. The case, power supply, battery and take-apart procedure are shared with the other LC "pizza box" machines. The machine is now more than thirty years old, and the surface-mount electrolytic capacitors and the PRAM battery are the parts that damage the board.

The machine

Apple's specification for the Macintosh LC III

Item	Figure
Processor	Motorola 68030 at 25 MHz, 32-bit data path, burst-mode RAM access, socket for an optional 68882 FPU
DRAM	4 MB soldered to the board; one 72-pin SIMM socket; 36 MB maximum with 80 ns or faster SIMMs
ROM	1 MB on the board (the Service Source prints "512K expandable to 1 MB")
VRAM	512 KB soldered; one VRAM SIMM socket takes 256 KB for a 768 KB total
Expansion	114-pin processor-direct slot, which also accepts 96-pin LC and LC II cards
Sound	8-bit mono input at 11 or 22 kHz; 8-bit mono output feeding both channels of the jack
Maximum power	30 W

Sources: Service Source pp. 6, 8–9, 12, 14, 16 and Developer Note pp. 2–5.

Mono sound is not a fault: Apple specifies one 8-bit generator sending the same signal to both channels on the LC, LC II and LC III.

FPU socket

The socket for the optional 68882 is a PLCC (plastic leaded chip carrier) socket, and some configurations shipped with the FPU fitted. Use a PLCC extraction tool to remove the chip.

Power supply

Every machine covered by Apple's LC Series/Quadra 605 manual takes the same power supply, part 661-1733. It runs from 100–240 VAC, 47–63 Hz, and configures itself, so there is no voltage selector. The 30 W figure above is the LC III's maximum consumption, not the supply rating: Apple gives 50 W for the LC and LC II and 26 W for the Quadra 605 on the same supply.

Apple publishes no rail voltages, tolerances or adjustment for this supply, and every power symptom in the Service Source ends in replacing the supply or the logic board. The "+5 V 4.85–5.15 V / +12 V 11.9–12.7 V" band printed for some Macintosh models is the Macintosh Plus procedure and does not apply here. Measure +5 V, +12 V and −5 V at the logic board power connector, with the machine running, against their nominal values; a rail that sags when the hard drive spins up points at the supply.

Recapping the supply

The supply's electrolytic capacitors leak and dry out, and a dead supply is a common reason an LC III will not start. The primary side runs at mains potential. The bulk capacitor charges to the peak of the mains, about 170 V on 120 V mains and about 340 V on 230 V mains, and can hold a charge after the plug is pulled. Unplug the supply, wait, and measure across the bulk capacitor before touching the primary side.

Apple bought the 661-1733 supply from several makers. Console5 lists six versions (ASTEC AA16250, AA16251 and AA19140, Dyna Comp DCF 353 and DCF 704, and TDK 614-0003), each with a different capacitor set, so read the maker's label before ordering parts. The lists are on Macintosh LC Capacitor Replacement Guide.

To remove the supply, disconnect its cable from the logic board, release the two latches that hold its front end to the bottom case, and lift it out.

PRAM battery

The battery is a 3.6 V lithium half-AA cell, Apple part 742-0011, in a holder on the logic board with a snap-on cover.

Apple's check: set the meter to the 10 V DC range, put the probes on the ends of the cell, and replace it if it reads below 3.0 V. Apple gives no replacement interval. To remove the cover, push a small flat screwdriver down into the top of the holder until the side tab releases, then do the same at the other end.

A cell left in a stored machine is the usual source of board corrosion; remove it before the machine goes into storage. See Battery Explosion, Capacitor or Corrosion Damage and Battery Refurbishment.

If it has leaked

Lithium cell leakage is acidic. Neutralise it with a paste of sodium bicarbonate and distilled water, not vinegar, which is for alkaline cells. Rinse with distilled water, then isopropyl alcohol, and dry the board.

Then check for damage:

• Inspect the traces around the holder under magnification for dull, green or thinned copper.
• Check continuity on every track leading away from the holder. Corrosion travels under the solder mask.
• Look at the underside of the board, where electrolyte comes through the vias.

Opening and cleaning

Opening the case

1. Disconnect all cables.
2. Remove the case screw, if one is fitted.
3. Lift the tabs at the back of the lid and lift the top cover straight up.

Apple's procedure, Service Source p. 38.

Inside

• Blow dust out of the logic board, supply and fan with compressed air, holding the fan blades still so the fan is not spun up.
• Clean grime with 99 % isopropyl alcohol and a soft brush.
• Check the underside of the board around the capacitors and the battery holder.

Fan and speaker

The fan has its own cable to the logic board. Disconnect it, release the plastic latches and lift the fan out. Refit it logo side down: Apple warns that otherwise "the fan may damage the computer".

When refitting the speaker, thread its cable through the top two brackets that hold the hard drive before connecting it.

Outside

Clean the case with a damp cloth and mild detergent. The platinum plastic yellows through its thickness, so cleaning will not remove the colour; see Retrobrite.

Connectors and the expansion slot

Reseat the DRAM SIMM, the VRAM SIMM, any processor-direct card and the logic board cables if a fault comes and goes. Clean oxidised contacts with isopropyl alcohol; do not rub gold SIMM contacts with an abrasive.

The 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 W in total across the three rails. Apple warns that cards dissipating more than 4 W "may overheat and damage the computer's circuitry", and recommends that every card ground the /FANSPEED signal to speed up the fan (Developer Note p. 37, Table 1-16).

Capacitors and C22

The logic board has eleven surface-mount electrolytics: C2, C4, C5, C7 and C18 at 10 µF 16 V; C8, C19, C20, C21 and C22 at 47 µF 16 V; and C13 at 100 µF 6.3 V. The full guide is Macintosh LC III Capacitor Replacement Guide.

C22 is the bulk capacitor on the −5 V rail, and Apple's silkscreen and factory fitting are both reversed: the pad marked positive connects to −5 V and the pad marked negative to ground. Fit the replacement the opposite way round to the silkscreen. Check it with the meter in continuity mode: the negative terminal of a correctly fitted C22 connects to the −5 V pin of the power connector.

The original aluminium capacitor tolerated the reversal. A tantalum fitted the silk-screened way round does not: one owner measured the −5 V rail at −2.3 V with the capacitor drawing 1.3 A, and another reported one catching fire on an aftermarket supply. Apple's supply is rated at only 75 mA on −5 V, and the −5 V rail is used mainly by the RS-422 serial ports. Serial port faults after a recap point at C22.

Floppy drive

Apple's notes for the 1.4 MB SuperDrive: remove the dust shield, if present, before fitting a replacement auto-inject drive, and replace drives like for like, auto-inject with auto-inject and manual-insert with manual-insert. Head cleaning and mechanism service are on Macintosh Floppy Drive Maintenance.

Hard disk

Apple's SCSI checks: give an external device a different ID from the internal drive, terminate both ends of the external chain, and make sure the internal drive carries its terminator. Solid-state SCSI emulators such as BlueSCSI and ZuluSCSI are the usual replacement for a failed original drive. See Macintosh HDD Maintenance.

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