Macintosh Floppy Drive Maintenance

The internal floppy drives in vintage Macintosh computers are 3.5-inch mechanisms, from the Macintosh 128K of 1984 onwards. There are three generations:

• 400K, single-sided (Macintosh 128K, Macintosh 512K).
• 800K, double-sided (Macintosh Plus, Macintosh SE and others). The 800K mechanism is a Sony drive, labelled MFD-51W on Apple units and referred to by repairers as the MP-F51W.
• 1.4 MB, the Apple SuperDrive (FDHD). Apple part 661-0474 is a Sony MP-F75W-21G or MFD-75W-01G.

The usual faults are dust, lubricant that has hardened, a crumbled eject gear, and dirty heads. Cleaning, fresh lubricant and a new eject gear recover most drives. Head alignment is not a user adjustment on these mechanisms (see What cannot be adjusted).

These drives have no belt

The 3.5-inch mechanisms of this period drive the spindle directly from a brushless DC motor, with no belt or pulley. Mitsubishi's MF355C specification, for example, says the drive "uses a DC brushless, direct-drive motor so that no maintenance is required". None of Apple's service manuals for its 400K, 800K, 3.5 Drive, SuperDrive or HDI-20 drives mentions a belt. Belt-driven floppy drives are the 5.25-inch full-height type (Tandon TM100, CDC/MPI and similar); see Floppy Drive Repair.

Apple 3.5 Drive specifications

Apple publishes a specification for the 800K Apple 3.5 Drive, which can be used as a bench reference.

Apple 3.5 Drive (800K)

Parameter	Apple's figure
Tracks per inch	80
Recording surfaces	2
Seek time, track to track	6 ms maximum
Settle time	30 ms maximum
Drive start-up time	600 ms maximum
Head position accuracy	±0.035 mm
Formatted capacity	800K (certified double-sided media)
Unformatted capacity	1,246K
Operating temperature	5-50 °C (40-122 °F)
Relative humidity	5-90%, no condensation

Apple 3.5 Drive current draw

Rail	Standby	Typical	Peak
+12 V	10 µA	120 mA	600 mA, during eject only, 2 seconds maximum
+5 V	10 mA	360 mA	-

Disassembly and cleaning

Before you start

• Power off and unplug the Macintosh.
• Work on a static-safe surface with an earthed wrist strap.

To remove the drive:

1. Open the case (see the machine's Service Source for the order).
2. Disconnect the ribbon cable and, on drives that have one, the power connector.
3. Remove the mounting screws and slide the drive out.

Floppy drive head

Cleaning the mechanism

• Blow out dust with compressed air, or lift it with a soft anti-static brush.
• Remove old grease from the carriage rails and gears with isopropyl alcohol and a lint-free cloth.
• Clean the heads with a cotton bud moistened with isopropyl alcohol. Support the upper head arm so it does not snap down onto the lower head, and do not bend the head assembly.
• Apple's first cure for "drive is unable to read; many file transfer errors occur" and for "drive fails to format" on the SuperDrive and the HDI-20 is "clean drive", then replace the mechanism.

Lubrication

Re-lubricate the carriage rails, the eject gears and the eject cams sparingly with a plastic-safe synthetic grease. TinkerDifferent's teardown compares Molykote, which is close to the stock lubricant, with SuperLube. Do not use WD-40 or other penetrating oils. Keep grease off the heads and the disk path.

Common failures and repairs

Eject mechanism

The 800K and 1.4 MB Sony drives use an Omron eject motor with a small plastic gear that becomes brittle and crumbles; when it goes, the drive stops ejecting. Replacement gears are sold moulded (made from the original design) and 3D-printed; the Japanese Tune-O-Gear set fits Omron motors only, not Sanwa.

If a new gear does not cure it, check the eject-motor driver transistor on the drive's own logic board. A TinkerDifferent repairer found Q7 (a 2SB624, marked BV3 or BV4) shorted on two MP-F51W-23 drives and suspected that a broken gear had left the eject motor running until the transistor failed.

In Apple's charts, a drive that will not eject goes to "replace drive mechanism" first, then the eject switch cable assembly.

Drive will not read

• Clean the heads and inspect them under magnification for wear or scoring.
• Check that the spindle turns freely and the stepper moves the carriage its full travel.
• Try known-good media. Old disks shed oxide and can contaminate a freshly cleaned head.

Drive reads but will not write

On the SuperDrive, Apple's chart first asks the technician to verify that the drive really is a 1.4 MB SuperDrive, then to replace the mechanism, then the external cable. Check the write-protect shutter on the disk and the write-protect sensor in the drive before replacing anything.

Drive dead, or ejects continuously

An 800K mechanism fitted as an internal drive needs the right cable: Apple's chart for a dead drive with an LED that flashes once or not at all starts with "verify that logic board-to-drive cable is yellow, not red", and an internal drive on a Macintosh Plus that ejects continuously has the red-striped cable. Herb Johnson notes that the black-lettered MFD-51W drives often came with red-striped cables (590-0167) and the red-lettered MFD-51W-03/-23 with yellow-striped ones (590-0437).

What cannot be adjusted

Apple's service documentation for these drives has no adjustment procedure: every symptom ends at cleaning the drive, a cable, the LED or eject-switch assembly, or a new mechanism. iFixit's teardown of the MP-F75W warns against loosening the head alignment screws.

• There is no felt pressure pad. Both heads face each other on gimbal springs and follow the disk surface. The felt pad belongs to single-sided 5.25-inch drives. Bending a 3.5-inch drive's head arm or gimbal changes the head loading.
• Head alignment needs an alignment disk and test equipment. Moving the stepper by trial and error against known-good disks leaves the drive unable to read disks written elsewhere.

If a cleaned, lubricated drive still misreads disks from other machines, replace the mechanism with a known-good one.

Things that are not faults

• An MS-DOS drive will not read a disk formatted on a 1.4 MB SuperDrive. Apple's instruction is to format all disks on the MS-DOS drive first; both drives can then read and write them.
• A drive that reads and writes 800K disks but not high-density ones may be an 800K drive. Apple's chart asks the technician to confirm it is a 1.4 MB SuperDrive before replacing the mechanism.

Apple HD SC Setup is a hard disk utility for SCSI and IDE drives and has no floppy test function.

Apple drive manuals on this wiki

• Apple 3.5 Drive Service Source: the one Apple floppy volume with a Specifications chapter (the figures above), plus a symptom chart covering cable colour and the Apple IIGS daisy-chain board (CXD1085A to CXD1085B).
• Apple SuperDrive/HDI-20 Service Source: the external 1.4 MB drive's symptom chart and take-apart.
• Macintosh HDI-20 1.4 MB Drive Service Source: 18 pages, symptom chart, take-apart (including the cable interface board and HDI-20 external cable) and an exploded view without part numbers; no specifications chapter.
• Apple 400K Drive Service Source and Apple 800K Drive Service Source: take-apart procedures; the 800K volume also has Apple's Restoring Mouse Eject procedure.
• Apple PC 5.25 Drive Service Source: the 360K 5.25-inch drive, with its 300 rpm and current figures.
• Service Support Training Programs LaserWriter Technical Procedures Jan 1985: includes Apple's DuoDisk and UniDisk technical procedures, the DuoDisk chip-swapping chart and both illustrated parts lists.
• Macintosh II Repair and Upgrade Secrets: Larry Pina's chapter on 400K, 800K and 1.4 MB drives. A secondary source.

Related pages

• Floppy Drive Repair
• Macintosh HDD Maintenance
• Macintosh SE General Maintenance
• Macintosh SE/30 Troubleshooting
• Apple Service Source