Macintosh LC III Troubleshooting

This guide covers common issues and diagnostic procedures for the Macintosh LC III.

Preliminary Checks

Before troubleshooting, verify:

**No Power Diagnostics**
Symptom	Possible Cause	Action
Completely dead, no fan	Power supply failure	Check power supply capacitors; test with known-good PSU
Clicks or ticks repeatedly	Short circuit or capacitor failure	Inspect logic board for leaked capacitors; check for shorts
Fan spins briefly, then stops	Thermal protection or PSU fault	Check PSU output voltages; inspect for overheating
LED on but no startup	Logic board issue	Reset PRAM (Cmd+Opt+P+R); inspect for capacitor damage

Measure at the logic board power connector:

Out-of-spec voltages indicate power supply problems.

**Chime But No Video**
Symptom	Possible Cause	Action
Normal chime, black screen	VRAM failure or video circuit issue	Reseat VRAM SIMM; test with different monitor
Chime with garbage display	VRAM or logic board damage	Check VRAM; inspect for capacitor leakage near video circuit
Distorted or shifted image	Video timing issue	Check video output connector; try different monitor cable

**Error Chime Codes**
Chime Pattern	Meaning	Action
No chime	CPU or ROM failure	Inspect logic board; check ROM
Single tone (not normal chime)	RAM failure	Reseat 72-pin SIMM; test with different SIMM
Four tones (ascending)	RAM failure	Replace RAM SIMM

A Sad Mac icon with error codes indicates hardware failure:

For a complete list of Sad Mac codes, see Sad Mac Error Codes.

**Intermittent Issues**
Symptom	Possible Cause	Action
Random freezes	Capacitor failure or bad RAM	Recap logic board; test RAM
Works when cold, fails when warm	Thermal-related component failure	Inspect capacitors; check solder joints
Spontaneous restarts	Power supply instability	Check PSU capacitors and output voltages
Corrupted graphics after warmup	VRAM or video circuit failure	Reseat VRAM; check for capacitor damage

**SCSI Troubleshooting**
Symptom	Possible Cause	Action
Hard drive not recognized	SCSI ID conflict or cable issue	Check SCSI ID (drive should be ID 0); reseat cables
Flashing question mark	No bootable system found	Boot from floppy; check hard drive
External devices not detected	Termination or ID issue	Verify termination; check all SCSI IDs are unique

**Floppy Drive Problems**
Symptom	Possible Cause	Action
Disk not ejecting	Mechanical jam or failed eject motor	Manually eject with paperclip; check mechanism
Cannot read disks	Dirty heads or misalignment	Clean heads with IPA; try different disks
Disks not recognized	Drive failure or controller issue	Test with known-good drive; check ribbon cable

If you have installed an optional 68882 FPU:

**FPU Problems**
Symptom	Possible Cause	Action
FPU not detected by software	Poor socket contact	Reseat FPU; check for bent pins
System unstable after FPU install	Wrong FPU speed	Use 25 MHz or faster 68882
Crashes in FPU-intensive apps	Defective FPU	Test with different 68882

**Audio Problems**
Symptom	Possible Cause	Action
No sound output	Capacitor failure in audio circuit	Recap logic board
Distorted audio	Failing capacitors	Replace capacitors
Crackling or popping	Bad solder joints or capacitors	Reflow solder joints; replace capacitors