Macintosh LC III Troubleshooting

Macintosh LC III. Photo: Stephen Edmonds, Wikimedia Commons, CC BY-SA 2.5.

Fault-finding on the Macintosh LC III and its consumer version, the Performa 450, follows Apple's symptom charts in the Macintosh LC Series/Quadra 605 Service Source, which list cures "in the order of most likely solution"; try the first cure first and refit the original part before moving on if a swap does not help. Apple's charts stop at module level (logic board, supply, drive). On a machine this age, leaking surface-mount capacitors and the PRAM battery cause most logic board faults, so recap and clean the board before condemning it.

First checks

• Fit a good PRAM battery. Apple's charts put "Check battery" into the no-video and intermittent no-power entries. Apple's test is to replace a cell that reads below 3.0 V.
• Reseat the 72-pin DRAM SIMM, the VRAM SIMM and any processor-direct card.
• Look for leaked electrolyte around the capacitors and the battery holder, on both sides of the board.
• If the board has been recapped, check that C22 was fitted opposite to its silkscreen (see below).

Startup sounds

Error chords (Service Source pp. 20–21)

Symptom	Apple's cures, in order
Eight-tone error chord at startup	Replace SIMMs; check the SIMMs on a replacement logic board; replace the logic board

A Sad Mac icon with a code identifies the failing test; decode it with Sad Mac Error Codes.

Video

Built-in video (pp. 22–24)

Symptom	Apple's cures, in order
Screen dark; sound and at least one drive work; fan runs; LED lit	Adjust monitor brightness; try the monitor on another Mac; check battery; replace monitor; replace video cable; replace VRAM SIMM; replace logic board; replace power supply
Screen dark; no sound or drive activity; fan runs; LED lit	Remove any expansion card; remove peripherals; disconnect hard and floppy drives; replace SIMMs; replace logic board; replace power supply
Screen bright with sound, but no picture	Try the monitor on another Mac; replace video cable; replace monitor; replace logic board
Screen dark, fan not running, LED off	Check the monitor has power; remove expansion card; remove peripherals; disconnect drives; replace power supply; replace logic board
Lines or snow, or dark screen, with a normal startup tone	Try the monitor on another Mac; replace video cable; replace monitor; replace VRAM SIMM; replace logic board; replace power supply

Drives

Floppy and hard drive (pp. 25–27)

Symptom	Apple's cures, in order
Internal floppy drive does not work	Replace floppy cable; replace drive; replace logic board
Disk ejects; Mac icon with blinking X	Try a known-good system disk; replace cable; replace drive; replace logic board
Disk does not eject	Hold the mouse button down while switching on; push a straightened paper clip into the hole beside the slot; replace cable; replace drive
Hard drive runs continuously	Check the system is 7.1 or later; replace hard drive cable; replace drive; replace logic board
Hard drive does not work	Replace data cable; replace power cable; replace drive; replace logic board
Internal and external SCSI devices do not work together	Set different SCSI IDs; terminate both ends of the external chain; replace the external terminator; check the internal drive carries its terminator; replace the SCSI select cable

Peripherals and miscellaneous

Peripherals and miscellaneous (pp. 28–36)

Symptom	Apple's cures, in order
Cursor does not move	Restart; check the mouse connection; move the mouse to the rear ADB port without the keyboard; replace mouse; remove PDS cards; reset PRAM; replace logic board
No response from keyboard	Check the system version; check the ADB connection; replace keyboard cable; replace keyboard; remove the battery for 15 to 30 minutes to reset the logic board; replace logic board
Clicking, chirping or thumping	Disconnect the hard drive and replace it if the noise stops; replace power supply; replace logic board
Shuts down or restarts at random	Clear the vents (thermal protection can shut the machine down; it recovers after 30 to 40 minutes); replace power cable; fit the standoffs on an LC Ethernet card; replace power supply; replace SIMMs; replace logic board
Intermittently will not power on	Check cables; check the monitor has power; try a known-good keyboard and ADB cable; replace power cord; check the battery; replace logic board
No sound from the speaker	Check the volume setting is 1 or higher; reset PRAM; check nothing is plugged into the sound-out jack; replace speaker; replace logic board
Clock does not keep time	Replace battery; reset PRAM; check nothing is plugged into sound-out; replace logic board

To reset PRAM under System 7, hold Command-Option-P-R during startup, before "Welcome to Macintosh" appears.

Board-level faults

C22 and the serial ports

C22, the 47 µF bulk capacitor on the −5 V rail, is silk-screened and fitted backwards on the LC III board. The original aluminium part tolerated this. A tantalum fitted the printed way round has been measured pulling the −5 V rail to −2.3 V while drawing 1.3 A; the −5 V rail mainly feeds the RS-422 serial ports, so serial faults after a recap point at C22. Measure the rail at the power connector; the fix and the continuity check are on Macintosh LC III Capacitor Replacement Guide.

Capacitor leakage

Recap-a-Mac says the surface-mount electrolytics on an LC III that has not been recapped are "virtually guaranteed" to have leaked. Doug Brown found C21, the +12 V bulk capacitor, leaked badly enough to damage the solder mask and copper under it. After recapping, check continuity on tracks and vias near each capacitor if a fault remains. See Capacitor Failure Symptoms.

Power supply

The 661-1733 supply is shared with the rest of the LC family. Recap-a-Mac describes it as prone to leakage and failures in its smaller capacitors, and Doug Brown says the supply in all the LC models tends to go bad from capacitor leakage. Apple publishes no rail tolerances for it. Measure +5 V, +12 V and −5 V at the logic board connector against their nominal values, and look for a rail that sags under load. The supply's capacitor lists are on Macintosh LC Capacitor Replacement Guide.

PRAM battery

A leaking cell corrodes the board around its holder. Remove it from any machine going into storage, and neutralise lithium leakage with sodium bicarbonate. See Battery Explosion, Capacitor or Corrosion Damage.

Related pages

• Macintosh LC III
• Macintosh LC III General Maintenance
• Macintosh LC III Capacitor Replacement Guide
• Sad Mac Error Codes