Macintosh 128K/512K Troubleshooting
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Fault-finding on the Macintosh 128K and Macintosh 512K starts with Apple's own symptom charts, in the Service Source for these machines and the 1990 Apple Service Guide. Thomas Lee's and Larry Pina's repair references add board-level detail for the power/sweep board. Maintenance is on Macintosh 128K/512K General Maintenance and capacitor work on Macintosh 128K/512K Capacitor Replacement Guide.
 | The CRT holds about 12,000 V after the machine is unplugged. Discharge it to the metal ground lug before working inside the case, and never wear a grounding wrist strap while the machine is live or while discharging. See CRT Discharge Procedure.[1] |
Using Apple's symptom charts
[edit | edit source]Apple lists cures in order of likelihood. Try the first cure first and check whether the symptom persists; if you have swapped a module and the symptom remains, refit the original module before trying the next cure.[2] Apple works at module level: its cures replace the power/sweep board, the logic board, the drive or a cable, not individual components.
Power and start-up faults
[edit | edit source]| Symptom | Apple's cures, in order |
|---|---|
| No video, no audio tone and no drive operation | Connect the power cord; turn the power on; replace the power cord; check the fuse; replace the power supply, logic board and internal drive together (see below); replace the logic board |
| Continuous beeping, keeps trying to power up | Perform the voltage adjustment; disconnect the internal drive, and replace it if that clears the fault; replace the logic board; replace the power supply, logic board and internal drive together |
| Clicking or chirping | Connect the logic board cable; perform the voltage adjustment; replace the power supply board; replace the logic board |
| Smoke or a burning smell | Replace the power supply, logic board and internal drive together |
| Resets at random with a programmer's switch fitted | Remove the switch and file it down about 1/16 in |
Apple's caution, printed against several of these entries: "Replacing only power supply board may damage new power supply board. Replace power supply board, logic board, and internal drive, then turn on Macintosh."[2] A fault in the logic board or drive that brought down the old supply can damage a replacement. For both beeping and chirping, Apple puts the voltage adjustment before any replacement.[2]
Checking the supply
[edit | edit source]Measure at the external drive port with voltage test cable 077-0135: +12 V should read 11.90-12.75 V and +5 V 4.85-5.15 V. The regulator on the power/sweep board is adjusted only if the readings are out of tolerance, +12 V first; if both cannot be brought into tolerance, Apple replaces the board.[4] The procedure is set out on Macintosh 128K/512K General Maintenance#Voltage check.
The adjustment trimmer, R56, regulates the +12 V rail, and the other rails follow it. Thomas Lee reports that the "boots, bongs, resets, boots again" loop is usually dirty contacts on connector J4, the cable between the power/sweep board and the logic board, and that cleaning J4 should come before any adjustment: turning R56 up to correct a low +5 V reading caused by a bad contact drives +12 V over specification.[5]
A rhythmic "whup-whup-whup" is the supply's crowbar at work. A 12 A SCR, Q8, shorts the +12 V rail if it rises about 15% over specification; the supply current-limits and retries. To rule out a misadjusted R56, turn it fully clockwise (viewed from the insulator side of the board) for the lowest setting. If the noise continues, the fault is elsewhere on the board or in the load.[5]
Video faults
[edit | edit source]| Symptom | Apple's cures, in order |
|---|---|
| No video, but the audio tone is present and the drive works | Turn the contrast control fully clockwise; check the video cable; replace the neck cable; replace the power/sweep board; replace the logic board |
| Screen bright and sound present, but no video information | Replace the power/sweep board; replace the logic board |
Thomas Lee names cracked solder joints at the four-pin yoke connector J1 as the most common fault on the power/sweep board. They give an intermittent, wobbly, narrow or missing picture, or a single vertical line down the centre, and slapping the side of the case often makes the symptom come and go. Reflow the J1 pins with the connector halves mated.[5]
Capacitor C1 in the horizontal sweep circuit fails in a recognisable sequence: poor horizontal linearity (the left of the screen wider than the right), then bright diagonal lines a few minutes after switch-on, then a darkening picture and smoke. Switch off at the first sign and check J1, which is often burned as well.[6] Replacement parts for C1 are on the capacitor replacement guide.
After a power/sweep board or CRT is replaced, Apple's video adjustment sets contrast, brightness, raster size (7 × 4.7 in) and focus.[7]
Floppy drive faults
[edit | edit source]| Symptom | Apple's cures, in order |
|---|---|
| Disk ejects and the screen shows a disk icon with a blinking "X" | Try another disk; replace the drive cable; replace the drive; replace the logic board |
| Disk will not go in all the way | Switch off, then hold the mouse button down while switching on so that the eject cycle completes; insert the disk metal end first, write-protect tab underneath; replace the drive |
| Disk will not eject | Shift-Command-1 (internal) or Shift-Command-2 (external); Eject from the File menu, two or three times; a straightened paper clip in the hole beside the slot; replace the drive |
| Will not read disks | Try another disk; replace the drive cable; replace the Mylar RFI shield; replace the drive; check the ROMs on the logic board; replace the logic board |
| Drive runs continuously | Try another disk; replace the drive cable; replace the drive; replace the logic board |
| 400K drive will not boot | If the logic board has Rev. A ROMs and the drive's stepper motor has serial number F518 or higher, fit Rev. B ROMs; otherwise replace the drive |
The Rev. B ROMs are 342-0220-B (ROM HI) at D5 and 342-0221-B (ROM LOW) at D8, fitted with the notches towards the front of the machine.[8] See also Macintosh Floppy Drive Maintenance.
Keyboard, mouse and printer faults
[edit | edit source]| Symptom | Apple's cures, in order |
|---|---|
| Cursor does not move | Connect the mouse; replace the mouse; replace the logic board |
| Cursor moves but clicking does nothing | Replace the mouse; replace the logic board |
| No key works | Replace the keyboard cable; replace the keyboard; replace the logic board |
| One key does not work | Replace the keyswitch; replace the keyboard; replace the logic board |
| Known-good ImageWriter will not print | Check the Chooser and Control Panel settings; use known-good system software; reset PRAM; replace the interface cable; replace the logic board |
| Known-good LaserWriter will not print | Check the Chooser and Control Panel settings; use known-good system software; reset PRAM |
Start-up error codes
[edit | edit source]For the Sad Mac codes shown by these machines, see Sad Mac Error Codes.
Related pages
[edit | edit source]- Macintosh 128K
- Macintosh 128K/512K General Maintenance
- Macintosh 128K/512K Capacitor Replacement Guide
- Macintosh 128K/512K Service Source
- Apple Macintosh Service Guide Volume 1 1990
- CRT Discharge Procedure
- Capacitor Failure Symptoms
References
[edit | edit source]- ↑ Apple Computer, Inc., Apple Service Guide: Macintosh Computers (1990), Safety, pp. 1-4 (PDF pp. 5-8). Hosted as File:APPLE MACINTOSH SERVICE GUIDE VOL1 1990.pdf; see Apple Macintosh Service Guide Volume 1 1990.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Apple Computer, Macintosh (Service Source for the Macintosh 128K and 512K), Troubleshooting, pp. 1-9 (PDF pp. 14-22). Hosted as File:Macintosh 128k.512k.pdf; see Macintosh 128K/512K Service Source.
- ↑ 3.0 3.1 3.2 3.3 Apple Service Guide: Macintosh Computers (1990), Tab 1, Symptom/Cure Chart, pp. 11-13 (PDF pp. 21-23).
- ↑ Macintosh Service Source, Adjustments, Voltage, pp. 1-6 (PDF pp. 61-66).
- ↑ 5.0 5.1 5.2 Thomas H. Lee, Classic Mac Repair Notes, revision of 31 May 2007 (©2000 Thomas H. Lee; not re-hosted here). https://archive.org/details/Classic_Mac_Repair_Notes Sections 4.1-4.2 (yoke connector J1, capacitor C1, power-supply faults), 5.1 (voltage adjustment and external floppy connector pinout) and 7.4 (regulator, R56, crowbar Q8).
- ↑ Larry Pina, The Dead Mac Scrolls (1992). Hosted as File:The Dead Mac Scrolls 1992.pdf; see The Dead Mac Scrolls.
- ↑ Macintosh Service Source, Adjustments, Video, pp. 14-17 (PDF pp. 74-77).
- ↑ Apple Service Guide: Macintosh Computers (1990), Tab 1, ROM Upgrade & ROM Compatibility, p. 16 (PDF p. 26).