Macintosh IIsi Troubleshooting

This guide provides solutions to common faults experienced in the Macintosh IIsi. From power issues to video problems and audio failures, these diagnostics will help identify, isolate, and resolve problems.

Preliminary Checks

Before detailed troubleshooting, verify the basics:

Check that the power cable is securely connected
Test with a known-good power outlet
Disconnect all external peripherals (SCSI devices, serial devices)
Remove any PDS or NuBus cards
Check for visible capacitor leakage or battery corrosion on the logic board

No Power (Unit Dead)

**Power Issues & Basic Startup Diagnostics**
Symptom	Possible Cause	Recommended Action
No signs of life (no fan, no LED)	Failed power supply, blown fuse, bad power cable	Test power outlet; check power supply fuse; test with known-good power supply
Clicking or ticking from power supply	Failed capacitors in PSU, shorted component	Recap power supply; check for shorts on logic board
Fan spins briefly then stops	Power supply overload, shorted logic board	Disconnect logic board and test PSU alone; check for capacitor leakage
Power LED on but no startup	Dead PRAM battery, failed logic board	Replace PRAM battery; check for battery corrosion damage

Power Supply Test

The IIsi uses a 160W Sony power supply. With the logic board disconnected, the PSU should still power on when the soft power switch is actuated. If the PSU fails to start with no load, it likely needs recapping or replacement.

Startup Failures

**Startup and Boot Problems**
Symptom	Possible Cause	Recommended Action
No startup chime, gray screen	RAM failure, ROM failure, capacitor damage	Reseat RAM SIMMs; check for capacitor leakage; test with minimal RAM config
Startup chime but no video	Video circuit failure, bad monitor connection	Check video cable; try different monitor; verify video port pins
Sad Mac error code	Hardware failure (RAM, ROM, or logic)	Decode error code (see below); test RAM; reseat ROM
Flashing question mark	No bootable drive found	Check SCSI drive; verify SCSI termination; test with boot floppy
Frozen gray screen	SCSI bus conflict, bad SCSI device	Disconnect all SCSI devices; check termination; verify SCSI ID settings

Sad Mac Error Codes

When a Sad Mac appears, the hexadecimal error code indicates the type of failure. Common codes for the IIsi:

**Common Sad Mac Error Codes**
Code Range	Meaning	Resolution
01XXXX	ROM test failure	Reseat or replace ROM SIMM (if equipped); check for ROM socket corrosion
02XXXX – 05XXXX	RAM test failure	Test with different SIMMs; clean SIMM slots; try minimal RAM config
0DXXXX	NuBus card failure	Remove NuBus adapter and any expansion cards
0EXXXX	SCSI chip failure	Check for capacitor damage near SCSI controller
0FXXXX	Data bus test failure	Indicates logic board failure; check for trace damage from capacitor leakage

See Sad Mac Error Codes for a complete reference.

Video Problems

**Display and Video Issues**
Symptom	Possible Cause	Recommended Action
No video but startup chime present	Video circuit failure, monitor issue	Test with different monitor; check video port pins; verify cable
Distorted or garbled display	RAM configuration issue, video RAM problem	Ensure all 4 SIMM slots have matching SIMMs; try lower RAM config
Slow video performance	Video using slow soldered RAM	Set disk cache to 768 KB or larger to force use of faster SIMM RAM
Wrong resolution or colors	Monitor sense pin issue	Check video adapter; verify monitor compatibility

Video RAM Note

The IIsi shares system RAM for video, using 64 KB to 320 KB depending on resolution. The slow 1 MB of soldered RAM is used first, which impacts video performance. Installing a dedicated video card in the PDS or NuBus slot will significantly improve video speed.

Audio Problems

**Sound Issues**
Symptom	Possible Cause	Recommended Action
No sound from internal speaker	Speaker contact failure, speaker wire disconnected	Clean speaker daughterboard contacts with pencil eraser; check speaker connections
Sound works through headphones only	Speaker contact corrosion	Remove logic board; clean spring contacts on speaker daughterboard
Intermittent audio dropouts	Speaker contact fretting	Clean and reseat speaker contacts; apply contact enhancer
Distorted or crackling audio	Failed capacitor near audio circuit	Recap logic board; check audio DAC
No sound at all (speaker or headphones)	Audio chip failure, capacitor damage	Check for capacitor leakage; may require logic board repair

SCSI and Storage Issues

**SCSI and Drive Problems**
Symptom	Possible Cause	Recommended Action
Flashing question mark at startup	No bootable system found	Test with boot floppy; check SCSI drive; verify System Folder is blessed
SCSI drive not recognized	Incorrect SCSI ID, termination issue	Verify unique SCSI IDs; ensure proper termination at end of chain
System hangs during SCSI scan	SCSI bus conflict, bad device	Disconnect all SCSI devices; add back one at a time
Floppy drive not reading disks	Dirty heads, failed drive	Clean heads with IPA; test with different disks; may need drive replacement
Floppy disks not ejecting	Mechanical failure, eject motor issue	Check eject mechanism; clean and lubricate; may need gear replacement

SCSI Termination

The IIsi's internal SCSI bus requires proper termination:

Internal drive should be terminated if it's the only SCSI device
If using external SCSI devices, terminate only the last device in the chain
The IIsi logic board does not provide internal termination power by default

Expansion Slot Issues

PDS Card Problems

The IIsi PDS runs at 20 MHz; some SE/30 PDS cards designed for 16 MHz may not work
Ensure card is fully seated in the slot
Check for bent pins in the PDS connector

NuBus Adapter Issues

The Apple IIsi NuBus Adapter converts PDS to NuBus
Some NuBus cards may not fit due to case height restrictions (7" cards recommended)
The adapter includes the 68882 FPU socket

Capacitor-Related Failures

Many IIsi problems stem from leaking electrolytic capacitors:

**Capacitor Failure Symptoms**
Symptom	Likely Cause	Solution
Intermittent startup	Leaking caps affecting reset circuit	Full logic board recap
Corrupted video	Leaking caps near video circuit	Recap and clean affected area
Random crashes	Voltage instability from failed caps	Recap logic board and power supply
No startup after long storage	Dried-out capacitors	Full system recap

See Macintosh IIsi Capacitor Replacement Guide for detailed recapping procedures.

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