Floppy Drive Repair

Floppy disk drives

Floppy disk drives were the main removable storage for vintage computers from the 1970s to the early 2000s. This page covers the faults that are common across manufacturers and the procedures that apply to most 5.25-inch and 3.5-inch mechanisms. For drive-specific figures — spindle speed tolerances, alignment limits, belt part numbers — go to the machine's own service manual; the Tandon TM100-2 page is a worked example with the manufacturer's real numbers.

Formats you are likely to meet:

• 5.25-inch, 48 TPI, 40 tracks — IBM PC, Commodore, Apple II, Atari, Tandy. 250 KB unformatted per side; formatted as 160/180 KB single-sided or 320/360 KB double-sided on the PC.
• 5.25-inch, 96 TPI, 80 tracks — the 1.2 MB "HD" drives of the PC/AT era.
• 3.5-inch double density — 720 KB on the PC, 800 KB on the Macintosh and Amiga with different low-level formats.
• 3.5-inch high density — 1.44 MB.

The single most useful thing to understand before starting: the vast majority of "dead" drives are not electrically dead. They are drives whose rail lubricant has hardened, whose heads are caked in oxide from a perished diskette, or whose belt has perished — and whose owner is testing them with the diskette that caused the problem.

Before you start

• Power off and unplug the machine or enclosure.
• Wear an anti-static wrist strap with a current-limiting earth lead when handling the logic board — see Recommended Tools.
• Note the ribbon cable orientation and the terminator/jumper configuration before removing anything.
• Keep at least one known-good diskette that you have not used in the suspect drive. Testing a cleaned drive with the disk that fouled it is the classic trap.

Cleaning

The mechanism

• Blow out dust with a bulb blower or filtered compressed air.
• Clean hardened lubricant off the head carriage rails with isopropyl alcohol and a lint-free cloth.
• Inspect the belt; a belt that is glazed, slack or leaves black dust is finished.
• On 5.25-inch single-sided drives, check the pressure pad opposite the head. A hardened, compressed or missing pad gives weak, intermittent reads.

The heads

In practice:

1. Start with a cleaning diskette. This is the manufacturer-sanctioned route. Cleaning diskettes are mildly abrasive, so use them when there is a reason to, not on a schedule.
2. Heavy oxide contamination — where a perished diskette has wiped a thick brown layer onto the heads — is beyond what a cleaning diskette will shift in one pass. Restorers do use isopropyl alcohol on a cotton tip here, very carefully, and report that clearing a thick layer can take minutes of patient work. That is a considered departure from manufacturer guidance on a drive that is otherwise unusable. It is recorded so you know what you are trading; it is not this page's default advice.
3. Never use a dry swab, abrasive, or a solvent other than isopropyl alcohol near the head.
4. Throw the offending diskette away. A disk that sheds oxide will foul the heads again within seconds.

Connectors

Oxidised edge connectors and ribbon headers cause faults that look like drive faults. Clean the drive's edge connector, the controller card's edge connector and the ISA or expansion slot. A dielectric connector grease such as NyoGel 760G is designed for exactly this job — protecting contacts against fretting and corrosion, with a dielectric breakdown of 11.2 kV and a service range of −40 to 135 °C. It is not a mechanism lubricant, despite being frequently recommended as one.

Lubrication

What to use. For the head carriage rails, a silicone grease. That is the restoration-community consensus for the Tandon full-height drives and applies equally to most similar mechanisms. Apply a thin film; the carriage should slide with no more resistance than the stepper's own detent.

What not to use, and why:

• White lithium grease — widely repeated as the right choice for drive rails, but no drive manufacturer document consulted for this page specifies it.
• NyoGel 760G and other connector greases — designed for electrical contacts, not sliding mechanisms.
• WD-40 — a water-displacing penetrant, not a lubricant. It thins existing grease, migrates to places you did not intend, and leaves a sticky residue as the carrier evaporates.
• Oil on a stepper motor shaft — the shaft bearings are not a user-serviceable part and oil migrates onto the lead screw and into the optics.

No service interval. There is no manufacturer figure for re-greasing a floppy drive, and the honest answer is that a drive that is in regular use and moving freely does not need it. Re-lubricate when the carriage is stiff, when you have cleaned old lubricant off, or when the drive has come out of long storage — not on a calendar.

Alignment and adjustment

Rule out everything else first. Alignment is the diagnosis of last resort. True alignment drift is indicated when both of these are true:

1. Disks formatted in the drive read back correctly in that same drive, and
2. The drive cannot read good disks formatted on a known-good, correctly aligned drive.

Even then, the more likely causes are:

• A dirty track 0 sensor. Chuck(G), quoted on minuszerodegrees, notes that many apparent alignment faults on these drives turn out to be a dirty track 0 sensor. Clean the optics before touching anything adjustable.
• Rail drag — the carriage not reaching the commanded cylinder.
• Spindle speed out of tolerance.
• The media. A double-density disk written or formatted in a high-density 1.2 MB drive does not meet the double-density standard and often cannot be read in a DD drive.

Spindle speed

The one adjustment that is safe to make. Tandon's specification for the TM100 series is 300 RPM ± 1.5 % — 295.5 to 304.5 RPM — adjusted at R4 on the servo board. Most 300 RPM drives of the period share both the figure and the adjustment method. Three ways to measure:

1. The strobe rings moulded or printed on the spindle flywheel, one marked 50 Hz and one 60 Hz. Use the ring matching your mains frequency, under strong mains-powered fluorescent light. The pattern should stand still.
2. ImageDisk's Test RPM command, or an equivalent utility.
3. A digital oscilloscope with a frequency readout on the tachometer output of the speed-control IC.

Check the spindle flywheel's securing screw is tight before adjusting anything — a loose one causes a speed error that no adjustment will fix.

Track 0

Track 0 detection is an optical or mechanical switch plus a mechanical stop. Clean it first. Adjust it only per the drive's own service procedure — Tandon's is Sections 5.9 and 5.10, with a defined switch position and a separate stop adjustment.

Common faults

Drive will not read anything

In rough order of likelihood: fouled heads; perished or slipping belt; hardened rail lubricant; dirty track 0 sensor; spindle speed out of tolerance; bad media; and only then head or electronics failure.

Tandon's own checklist for intermittent ("soft") errors is worth following before condemning a drive: ensure proper earthing between power supply, drive and host; check for conducted and radiated electrical noise; confirm the ambient temperature is within specification; and verify the fault on a second diskette.

Drive pulls the machine's power supply down

Check the drive's own tantalum decoupling capacitors before suspecting the supply. On the Tandon TM100-2A these are C36 and C43, 4.7 µF / 16 V, and they fail short. Tantalums fail this way generally — see Capacitor Failure Symptoms.

Eject and loading mechanism

• 3.5-inch Sony and Sony-derived mechanisms are known for hardened grease in the eject assembly and for cracked eject gears. The gear is a moulded plastic part that splits along the hub.
• Clean the old grease out completely before re-greasing; adding new grease on top of hardened old grease achieves nothing.
• Check the disk-present and write-protect microswitches or photo-interrupters. A drive that reports every disk as write-protected almost always has a dirty or failed write-protect sensor, not a fault in the host.

Erratic stepping

Symptom: read errors that vary with track number, or a carriage that moves unevenly. Cause: rail drag in almost every case. Clean and re-lubricate the rails before measuring the stepper. Measure the stepper's coil resistances only after that, and compare the phases against each other rather than against a remembered figure.

Termination and drive select

On Shugart-bus drives the terminator resistor pack goes in the last drive on the cable only — every other drive must have it removed. Incorrect termination gives erratic behaviour rather than a clean failure, which makes it easy to misdiagnose. Drive-select jumpering varies by machine; read the drive's own service manual rather than a remembered convention.

Media

The drive is only half the problem.

• Use the right disk. Double-density drives need double-density media; the IBM controller additionally requires soft-sectored disks.
• A disk that sheds oxide is finished and will foul the heads of every drive you put it in. Brown residue on the head, or a visible ring worn into the disk surface, is the sign.
• Never store a disk in the drive. On drives with a mechanical head-load or clamp this leaves the mechanism under load indefinitely.
• Image before you rely on it. Flux-level imaging hardware such as Greaseweazle or Kryoflux reads the raw transitions, which is both the best way to preserve original media and a good way to test a repaired drive against known-good images — see Recommended Tools.

Storage

Store drives dry, dust-free and at stable temperature. There is no evidence that periodically powering a drive "keeps the lubricant distributed": lubricant hardens with time and oxidation, not with disuse, and running a drive with hardened lubricant on its rails wears the rails.

Manufacturer manuals on this wiki

• Service Support Tranning Programs Laserwriter Technical Procedures Jan 1985 — despite its

 title, this Apple binder contains the DuoDisk and UniDisk Technical Procedures, including
 the DuoDisk Analog Card Chip Swapping Chart (symptom → chip by grid position) and both drives'
 illustrated parts lists with Apple part numbers. It also carries Apple's warning that a DuoDisk
 analogue card connected to a computer with the power on will show "fuzz" and an exploded
 C21.

• Macintosh HDI-20 1.4 MB Drive Service Source — Apple's 18-page volume for the HDI-20

 external floppy drive. It is Troubleshooting and Take Apart only: there is no Basics and no
 Specifications chapter, so it gives no capacity, data rate, current draw or mechanism part number.
 What it does give is the take-apart for the cable interface board and the HDI-20 external
 cable, which is the part that actually fails on these because the connector takes the strain.

• The Dead Mac Scrolls — Larry Pina's chapter 4 covers floppy drives, including

 replacing drive heads without test instruments (p. 265), which is the only published
 procedure of its kind held here.

For Apple's own external drives, the manufacturer's symptom chart is on this wiki: Apple SuperDrive/HDI-20 Service Source covers the external 1.4 MB SuperDrive and the HDI-20, and it is worth reading before condemning a mechanism because two of its entries are normal behaviour rather than faults. The specifications for the 800K mechanism are in Apple 3.5 Drive Service Source, and the 5.25-inch drive's 300 rpm and current figures in Apple PC 5.25 Drive Service Source.

Related pages

• Tandon TM100-1 and Tandon TM100-2 — worked examples with manufacturer figures
• IBM PS2 FDD Technical Reference (S42G-2194) — IBM's own 3.5-inch diskette drive technical reference, hosted here: the three interface pinouts, and a plain statement that IBM published no mechanical service data for these drives
• Hard Drive Maintenance and Repair
• Capacitor Failure Symptoms
• Category:Capacitor Replacement Guides
• CRT Discharge Procedure — before opening any machine with a picture tube
• Recommended Tools