Macintosh LC 550

Macintosh LC 550

Macintosh LC 550
Specifications
Manufacturer	Apple Computer, Inc.
Type	All-in-one personal computer
Released	1994
CPU	Motorola 68030 at 33 MHz with built-in MMU
Memory	4 MB on board, expandable to 36 MB with one 72-pin SIMM (100 ns or faster)
Storage	3.5-inch internal 80, 160, 250 or 320 MB SCSI hard drive
Display	Built-in 14-inch Trinitron, 13-inch viewable, 0.26 mm aperture grille pitch, 640 × 480 at 70 dpi
Sound	Records at 11 or 22 kHz, plays back at 22 kHz; two speakers, built-in microphone, front headphone jack
Graphics	512 KB VRAM on board, expandable via a 68-pin VRAM SIMM
ROM	1 MB
FPU / coprocessor	Socket for an optional math coprocessor
Media	Internal 1.4 MB Apple SuperDrive (manual-inject); internal Apple CD 300+ CD-ROM drive
Ports / interface	Two ADB, two RS-232/RS-422 mini DIN-8 serial, DB-25 SCSI, sound in and out
Expansion	96- or 114-pin processor-direct slot
Power	100–240 VAC universal, 47–63 Hz, 120 W maximum
Dimensions	17.9 × 13.5 × 16.5 in (45.5 × 34.4 × 42.0 cm)
Weight	40.5 lb (18.4 kg)
OS / Firmware	—
Predecessor	Macintosh LC 520
Successor	Macintosh LC 575
Model no.	661-1830 (logic board)

The Macintosh LC 550 is the LC 520 chassis with a 33 MHz 68030 in place of the 25 MHz part, a manual-inject floppy drive and the caddyless CD 300+ in place of the caddy-loading AppleCD 300i.

Everything on this page that is a number comes from Apple's own Service Source for the LC 520, 550 and 575, hosted on this wiki. Apple published no separate developer note for the LC 550, so where a figure exists only for the LC 520 or the LC 575 this page says which machine it belongs to rather than assuming it carries across.

Specification

Apple's specification

Item	Figure
Processor	Motorola 68030 at 33 MHz with built-in MMU; socket for an optional math coprocessor
RAM	4 MB on board, expandable to 36 MB, 72-pin SIMM connector, 100 ns or faster
ROM	1 MB
PRAM	256 bytes, long-life lithium battery
VRAM	512 KB on board; 68-pin VRAM SIMM connector — see below
Floppy	Internal 1.4 MB Apple SuperDrive, manual-inject, requiring the keyhole-shaped bezel
Hard drive	3.5-inch internal 80, 160, 250 or 320 MB SCSI
CD-ROM	Internal Apple CD 300+, caddyless
Expansion	Processor-direct slot for 96- or 114-pin cards
ADB	Two ports; three devices recommended; 500 mA maximum
Display	14-inch diagonal, 13-inch viewable, Trinitron with high-contrast glass, 0.26 mm aperture grille pitch, 640 × 480 at 70 dpi
Sound	Records at 11 or 22 kHz, plays back at 22 kHz; two speakers with enhanced stereo
Line voltage	100–240 VAC, universal power supply
Frequency	47–63 Hz, single phase
Maximum power	120 W
Dimensions	17.9 × 13.5 × 16.5 in (45.5 × 34.4 × 42.0 cm)
Weight	40.5 lb (18.4 kg)

All of that is Apple's. Corrections to figures that circulate:

• 120 W, not "150 W typical, 200 W maximum". Apple publishes one maximum-power figure for this chassis and it is 120 W.
• 47–63 Hz, not 50–60 Hz.
• The 68030's address bus is 32 bits wide, not 24. A page claiming "24-bit address, 16 MB addressable space" while also claiming 36 MB of RAM is contradicting itself; the 36 MB figure is Apple's.

What Apple changed between the LC 520 and the LC 550

Apple's Basics chapter identifies the differences directly. The LC 550 and LC 575 ship with:

• the newer manual-inject 1.4 MB Apple SuperDrive, which requires the keyhole-shaped bezel;
• a revised chassis harness assembly that does not use a floppy drive adapter;
• the newer, caddyless CD 300+ CD-ROM drive.

Apple also states that the chassis harness assemblies are interchangeable in one direction with conditions: you may fit the newer LC 575 harness to an LC 520 or LC 550, but the newer harness requires the manual-inject floppy drive and the caddyless CD-ROM drive. Fitting the harness without the drives that go with it is a self-inflicted fault.

VRAM: say which machine the figure came from

Apple's Service Source prints one VRAM row covering all three machines — 512 KB on board, expandable to 1 MB, with "68-pin VRAM SIMM connector (LC 520)" and "two VRAM SIMM slots (LC 575)" as the qualifiers. The two machine-specific developer notes disagree with the shared row in opposite directions:

• The LC 520 has 512 KB soldered and one socket taking a 256 KB module — 768 KB maximum.
• The LC 575 has no soldered VRAM and two sockets, which must both be filled with matching modules to give either 512 KB or 1 MB; Apple notes the machine will not recognise more than 1 MB.

Apple published no developer note for the LC 550, so its arrangement is not separately documented. Count the sockets on your own board before buying a module: one socket alongside soldered VRAM means the LC 520 arrangement and a 256 KB part; two empty sockets mean the LC 575 arrangement and a matched pair. Nothing has been assumed here to fill that gap.

Safety: this machine has a CRT

The analog board carries mains-derived voltages and the tube carries tens of kilovolts on the final anode, and both hold their charge after the machine is unplugged. Read CRT Discharge Procedure and follow it before anything goes inside the case.

Apple's own caution is worth repeating because it is counter-intuitive: never wear a grounding wrist strap until after the CRT has been discharged. A wrist strap earths you through a resistor; it is protection for the components, not for you.

Cleaning and take-apart

Opening the case

Five Torx screws hold the rear housing on, and the cover then lifts off. Apple does not state the driver size; a T-15 fits, and it needs to be a long one to reach. Behind the cover, the fan (Apple part 922-1803) comes out on two latches.

Work with the machine on its front on a soft surface, not on its back with the tube pointing up, and keep the neck board supported.

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.
• Check the underside of the board as well as the top. 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 and what it costs in brittleness.

• Clean the Trinitron screen with a cloth dampened with water or a dedicated screen cleaner, applied to the cloth and never sprayed at the tube. Nothing abrasive on the anti-glare coating.

Adjustments

Apple's Adjustments chapter for this chassis covers geometry, focus, cutoff and white balance. It contains no power-supply adjustment and no rail tolerance. Instructions to "adjust potentiometers on the analog board if the rails are out of range" are not Apple's, have no source behind them, and on a board that also drives the tube they are a good way to turn a working display into a broken one. They do not appear on this page.

What Apple does give are real, checkable figures:

Apple's geometry targets

Adjustment	Control	Target
Vertical size	VH	Raster height 7 in ± 1/8 in (176 mm ± 2 mm)
Horizontal size	HW	Raster width 9 1/4 in ± 1/8 in (235 mm ± 2 mm)
Vertical centre	VS	Raster centred top to bottom on an all-white pattern
Horizontal centre	HS	Raster centred side to side on an all-white pattern

Apple's procedure uses the Display Service Utility's test patterns, and its notes matter:

• Use a short hex-head plastic tool. A long one flexes and makes fine adjustment impossible. Never use a metal alignment tool — it is a shock hazard.
• Let the monitor warm up for at least 10 minutes before cutoff or white balance.
• Do cutoff before white balance, and do cutoff again whenever you replace the CRT or the analog board.
• Raster width varies by up to 3/16 in at each side depending on which Macintosh timing is driving the screen. That is documented behaviour, not a fault.

PRAM battery

A 3.6 V lithium half-AA cell on the logic board keeps the clock and parameter RAM alive with the machine unplugged.

• Set a meter to the 10 V DC range.
• Positive probe on the positive end of the cell, negative probe on the other.
• Replace the cell if it reads below 3.0 V.

3.0 V is this chassis's figure, from Apple's own manual for it. It is not the 2.8 V of the IIcx, IIci and IIvx family, nor the 3.2 V of the Centris machines and the larger Quadras. Apple publishes no replacement interval, and none is invented here.

If it has leaked

This is a lithium cell, so neutralise with bicarbonate of soda, not vinegar. Vinegar is the right treatment for alkaline leakage; it is the wrong treatment here and it adds water and chloride to an already corroded board. Bicarbonate paste in distilled water, rinse with distilled water, then isopropyl alcohol, then dry thoroughly — and afterwards buzz out every track running away from the holder and check the underside of the board.

See Battery Explosion, Capacitor or Corrosion Damage and Battery Refurbishment. Take the cell out before the machine goes into storage.

Power supply

The supply is part of the analog board (Apple part 661-0192, shared across the LC 520, 550 and 575), which also drives the tube. Apple publishes no rail tolerance and no power-supply adjustment procedure for this machine. 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 copied onto machines it was never written for; it is not reproduced here.

Apple's service parts: CRT 661-1666 (661-1667 Australia, 661-1668 Europe), analog board 661-0192, logic board 661-1830 for the LC 550 (661-0812 LC 520, 661-0867 LC 575), fan 922-1803, internal microphone 922-1802.

Capacitors

Logic board

Nine surface-mount electrolytics: two 100 µF 6.3 V, six 47 µF 16 V and one 10 µF 16 V.

MacDat carries that list and has a reference photograph for this board; neither Console5 nor Recap-a-Mac covers the LC 550. Apple published no component-level capacitor data for any Macintosh of this era, so this is a community reading of physical boards rather than a manufacturer specification. Read the value and working voltage off each can before you lift it.

MacDat gives the single small part as 10 µF 16 V here and 10 µF 50 V on the LC 520. Whether that is a real board difference or a transcription slip cannot be settled from the published sources. Read the can. A 50 V part in a 16 V position is harmless; a 16 V part in a 50 V position is not.

The analog board

No component-level capacitor list has been published for this chassis's analog board. That is the honest gap on these machines, and it is worth saying what was searched:

• Apple's Service Source for the LC 520/550/575 — no capacitor values in any chapter. Apple serviced this board by replacement.
• Apple's Macintosh LC 520 Developer Note — architectural and electrical detail, but no analog-board parts list.
• Console5 — no LC 520 or LC 550 entry at all.
• MacDat — logic board only, though it does have a reference photograph for the LC 550 board.
• Recap-a-Mac — has guides for the LC 575, LC 580 and LC 630 but none for the LC 520 or LC 550.

So read the values off your own board. Every can has its capacitance and working voltage printed on it and every position is silk-screened. Photograph the board first, list what is actually there, and work from that list. Ranges such as "main filter 1000–3300 µF, 16–25 V" are not a specification; on a board carrying a mains-derived supply and a horizontal deflection stage, guessing a working voltage is exactly the mistake that destroys boards.

What is safe to say without a list: the usual failures on an analog board of this generation are dry joints around the flyback transformer, the horizontal output transistor and the heavy-current connector pins, plus tired electrolytics in the low-voltage supply. Reflow the high-stress joints with fresh leaded solder before condemning a component.

Connectors and sockets

Oxidised contacts cause more intermittent faults on a machine this age than failed silicon does, and they are the cheapest thing to put right.

• SIMM sockets. Clean with isopropyl alcohol on a swab and reseat. Do not take an abrasive to a module'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.
• NuBus and PDS slots. Inspect for bent contacts and debris before a card goes home; check the card's edge fingers too.
• ROM SIMM socket, where one is fitted. A poorly seated ROM gives a machine that chimes and does nothing else.
• Logic-board power connector. Look for browning or a distorted housing. A discoloured power connector is a warning, not a cosmetic problem.
• ADB, serial, SCSI and floppy connectors. Reseat; check ribbon cables for cracked conductors where they fold.

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.

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 — including to the auto-inject version — 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. On manual-inject drives a worn eject gear is a genuine and common failure.

• 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.
• If a disc will not eject, hold the mouse button down while switching the machine on to run a full eject cycle before taking anything apart.

See Macintosh Floppy Drive Maintenance.

The LC 550's drive is the manual-inject SuperDrive and it needs the keyhole-shaped bezel. Apple's instruction is to replace drives like for like: fitting an auto-inject drive behind a keyhole bezel, or the reverse, does not work.

Hard disk

The original SCSI mechanisms are past 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.

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

Automatic SCSI termination

This chassis terminates its own SCSI bus, and that is worth knowing before you add a terminator to it. Apple's developer note describes the arrangement: when no external device is connected, circuitry on the logic board connects the bus to an on-board terminator near the external connector; when one or more external devices are connected, the circuitry detects the external termination during system reset and disconnects the on-board one.

Two practical consequences:

• Do not add a terminator on the assumption one is missing. The machine has already dealt with it.
• The detection happens at reset. Hot-plugging an external SCSI device and expecting the termination to re-negotiate will not work — restart the machine.

Apple groups the LC 520 with the IIvx and the Centris 650 as using this third arrangement, against the older scheme on machines such as the LC II and Quadra 700 where a terminator block at the last external device does the job.

Apple documents that arrangement for the LC 520; the LC 550 shares the chassis and the SCSI cabling, and its manual describes no separate scheme.

Things that are not faults

• A thin grey horizontal line across the lower third of a lit screen. Apple prints this explicitly: it is inherent in the design of Trinitron monitors — it is the shadow cast by the aperture grille's damper wire — and Apple's instruction is do not replace modules.
• Raster width varying slightly between resolutions. Apple documents up to 3/16 in at each side.
• A tray-loading CD-ROM drive. The CD 300+ in the LC 550 is caddyless by design; the caddy-loading AppleCD 300i belongs to the LC 520.

Upgrades, and one that is not documented

The LC 550's logic board is frequently transplanted into a Color Classic chassis, and the LC 520's board is frequently described as convertible to 33 MHz by removing particular resistors.

Neither modification is documented by Apple, and the resistor designators quoted for the clock change vary between sources. Removing the wrong resistor on a logic board of this age is not recoverable, so no designator list appears on this page. If you intend to try it, work from a source that shows a photograph of the board it was done on, and check the designators against your own board first.

What Apple does document is the hardware difference underneath: the LC 550 runs at 33 MHz where the LC 520 runs at 25 MHz, on logic boards Apple sells as separate service parts (661-1830 and 661-0812).

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 520 — the 25 MHz original
• Macintosh LC 575 — 68LC040 version of the same chassis
• Macintosh LC 580
• Macintosh TV — the same chassis in black, with a tuner
• Macintosh Color Classic II
• Macintosh LC 520/550/575 Service Source
• Macintosh LC 520 Developer Note
• CRT Discharge Procedure
• Recommended Tools