Macintosh LC Troubleshooting

Macintosh LC (1990) at the Apple Museum, Prague. Photo: Benoît Prieur, Wikimedia Commons, CC0.

Fault-finding on the Macintosh LC (1990) 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 are the first suspects.

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 30-pin DRAM SIMMs, the VRAM SIMM and any processor-direct card. The LC has 2 MB on board and takes 100 ns or faster SIMMs up to 10 MB; it accepts 12 MB but recognises only 10 MB.
• Look for leaked electrolyte around the capacitors and the battery holder, on both sides of the board.

Startup sounds

Apple lists a separate error chord for the LC and the eight-tone chord for the later models.

Error chords (Service Source p. 20)

Symptom	Apple's cures, in order
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 6.0.7 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

PRAM battery

The LC will not start without a good PRAM battery: fan and hard drive run but there is no chime and no picture. PAPPP found exactly that after removing the cell for storage, and a fresh 3.6 V half-AA cell cured it. Apple's charts put "Check battery" into the no-video and no-power entries, and its test is to replace a cell below 3.0 V. A leaking cell corrodes the board around its holder; neutralise lithium leakage with sodium bicarbonate. See Battery Explosion, Capacitor or Corrosion Damage.

Capacitor leakage

The LC logic board, 630-0309, carries fifteen small electrolytics; Console5 notes that C9 is a large leaded 2200 µF part on some revisions. On PAPPP's machine the ADB port was dead, and one of the two 47 µF surface-mount capacitors nearest the ADB connector was visibly leaking, and the port worked again once he had replaced both. Recap and clean the board before condemning it, and check tracks and vias near each capacitor if a fault remains. See Macintosh LC Capacitor Replacement Guide and 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 the smaller capacitors, and PAPPP replaced the ELNA 56 µF and 270 µF capacitors in his clicking TDK-built supply. Test it under load: out of the machine his supply still clicked, and in the machine it ran with every rail stable. 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. Console5 lists six versions of the supply from three makers (ASTEC, Dyna Comp and TDK), with different capacitor sets.

Related pages

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