Apple DuoDisk

Apple DuoDisk
DuoDisk on an Apple IIe, under the monitor
Manufacturer	Apple Computer, Inc.
Type	Dual 5.25-inch floppy disk drive
Release date	1984
Price	US$795 with the Apple II accessory kit (February 1984)
Interface	DB-25 on the drive; cable to a DB-19 on the DuoDisk controller card
Compatible	Apple II, Apple II Plus, Apple IIe, Apple IIGS
Model	A2P2029 (with accessory kit)
Media	Single-sided 5.25-inch disks, 143K formatted

The Apple DuoDisk is two 5.25-inch floppy disk drives in one case, for the Apple II family. Both drives run from a single analog card, and the left-hand drive is drive 1. It stores up to 143K per single-sided disk and works with any Apple II except the Apple IIc. Apple's February 1984 dealer price list gives US$795 for the DuoDisk with the Apple II accessory kit (A2P2029). Wikipedia dates its introduction to 1984 and notes that it was shaped to stand on the computer under the monitor.

The DuoDisk uses the same mechanical assembly (661-72128) as the Apple 5.25 Drive and the UniDisk, with its own sub-bezel. Its controller card is the same as the UniDisk and Apple 5.25 Drive card. Apple's support article says the Apple III's drive mechanism is not compatible with the DuoDisk.

Connecting

The DuoDisk connects through a controller card in a slot of an Apple II, II Plus, IIe or IIGS, or to the disk drive port of an Apple IIGS. One controller card runs one DuoDisk; a second DuoDisk needs a second card. Apple's owner's manual puts the first card in slot 6 and a second one in slot 4, with slot 5 left empty. The interface cable is round and shielded, with a 19-pin connector at the computer end and a 25-pin connector at the drive. Use the shielded cable supplied, because the drive was certified for radio interference with it.

DuoDisk cable 590-0114 (Apple Tech Info Library article 2396)

DB-25 (drive)	Signal	DB-19 (card)
2	Ground	1
4	Ground	2
9, 21	Ground	3
10, 14	Ground	4
23	−12 V	5
7	+5 V	6
19	+12 V	7
20	+12 V	8
8	Enable B	9
12	Write protect	10
15, 16, 17, 18	Phases 0 to 3	11 to 14
5	Write request	15
24	+5 V	16
11	Enable A	17
3	Read data	18
6	Write data	19
1, 13, 22, 25	Not connected

Apple IIGS

On the Apple IIGS disk drive port, up to four 5.25-inch and 3.5-inch drives can be chained, and a DuoDisk counts as two. The DuoDisk has no daisy-chain port, so it must be the last drive in a chain. Apple's 5.25 Drive owner's guide adds that to daisy-chain a DuoDisk with a serial number below 433754, its controller card must be in slot 6.

• An unmodified DuoDisk cannot be the last device on the IIGS disk port: a UniDisk 3.5 elsewhere in the chain stops working, although a DuoDisk on its own is fine. Apple's fix was to cut out one resistor on the analog board: R8 (location A2) on cards numbered 676-[ ]101 or 676-[ ]102, or R39 (location C3) on cards numbered 676-[ ]107.
• On an Apple IIGS with ROM 03, the DuoDisk cannot be used on the disk port at all. Apple found that code in that ROM does not handle the DuoDisk properly, and the only workaround is a DuoDisk interface card (service part 661-92131). The symptom is a IIGS that will not start, with drive 1's light on, until a write-protected disk is put in the drive or the write-protect switch is pressed. On ROM 01 machines the DuoDisk can go directly on the disk port or behind another drive.

Using it

Never remove a disk while the "in use" light is on. Opening the drive door lifts the head load arm off the disk, so Apple suggests opening the door when a disk will be left unused in a drive for several hours. Apple's owner's manual describes the drive as more delicate than the computer: dropping it, jarring it sharply or letting something heavy fall on it can cause a malfunction.

Analog card

Apple's drawings of the two DuoDisk analog card layouts. Top: cards 676-[ ]101 and 676-[ ]102, with C29 (4) and C30 (5). Bottom: card 676-[ ]107.

There are three versions of the analog card, identified by the engineering number printed at the lower left of the card: 676-[ ]101 and 676-[ ]102 share one layout, and 676-[ ]107 has another. Before replacing any chip, inspect the card for melted or broken parts, particularly the 74LS125 and capacitor C21. Apple's manual says fuzz on the card usually means C21 has exploded because the computer was switched on while the card was being installed; Apple's instruction was to return such a card.

The DuoDisk capacitor fix

A DuoDisk can destroy data on a disk when the Apple II is restarted with Open Apple-Control-Reset, or when it starts certain copy-protected software. If a drive does this but passes Apple's 5.25-inch drive diagnostic, check the analog card. On cards numbered 676-[ ]101 and 676-[ ]102, cut off capacitors C29 (zone A1) and C30 (zone B1) with small wire cutters, or work them back and forth until the leads snap. Do not replace them. Apple's support note named software from PFS and Electronic Arts as especially prone to the damage, and said Apple was modifying boards as they came in for repair.

Chip-swapping chart

Replace the chips for a symptom one at a time, in the order listed, and switch the computer on after each one. If the fault remains after all of them, go back to the flowchart below.

DuoDisk analog card chip-swapping chart (Technical Procedures p. 4.6, rev. Mar 88)

Symptom	Location and part, in order
Motor and LED on, but the disk will not boot	B1 CA3141 (cards 101 and 102 only); B3 74LS125; C3 74LS125 (card 107 only); C1 MC3470
Drive reads but does not write	B1 CA3141 (cards 101 and 102 only); B3 74LS125
Data damaged when using Open Apple-Control-Reset or some copy-protected software, on a 101 or 102 card that passes the diagnostic	Capacitors C29 (A1) and C30 (B1): remove both, do not replace

The 1985 edition of the DuoDisk procedures has a longer chart with more chips; it is reproduced on the Disk II page.

Take-apart

Apple's tool is a medium Phillips screwdriver.

1. Remove the two screws at the back and lift off the top cover.
2. Facing the back, remove the screw on top of the right-hand shield, which also holds one end of the ground strap, and lift the shield off.
3. Remove the two screws holding the left-hand shield to the left drive and the screw bracing it to the right drive, and lift it off.
4. Analog board: disconnect the read/write head cables (J1, J2), the stepper motor cables (J3, J4) and the LED cables (J6, J7), pulling on the connectors and not the wires, remove the two screws and slide the board out between its guides.
5. When refitting the board, route the cables through their cutouts. If the stepper motor connector for the right drive (J3) has no key, fit it with the side marked "A2" upwards. The head cable connector fits either way round, so line each pin up with a metal tab inside the connector.
6. Mechanical assembly: remove the cover, shield, cable and analog board, the black insulation paper, the ground strap screw and the metal shield, then the four screws under the case that hold the metal chassis. Slide the chassis out, remove two screws on each side and slide the mechanism out of its housing.

The first 5,000 DuoDisks had threaded screw holes in the bottom of the mechanism casing, used to fix the mechanisms to the bottom case; later mechanisms do not. Fitting a later mechanism to an early unit needs four 400-1604 screws, four 415-1410 screws, a 805-5000 shield, four 860-0053 lock washers and four 860-0242 washers.

Loose analog boards

Some DuoDisks with serial numbers below 325000 have loose analog boards that can slip out of their guide tracks onto the disk mechanism and short-circuit. Apple's repair bends the top of the shield inward by about 1 mm with needle-nose pliers so that the guides grip the board. Do not bend it further: stress on the mechanism can upset head radial alignment. Run the drive diagnostic afterwards, and bend the shield back if alignment errors appear.

Adjustment and maintenance

For the DSPEED adjustment, connect a working drive to the analog card as drive 1 and the drive to be adjusted as drive 2, with Apple's 5.25-inch drive diagnostic running. The adjustment screw is inside a small hole near the front of the underside of the drive being adjusted. Keep the DuoDisk flat, turn the screw very slowly until the indicator is in the "good" range as close to 0 as possible, and let the test run for 30 seconds before repeating it. A drive that will not adjust goes back as a mechanism exchange.

The read/write head cleaning (80 % isopropyl alcohol, 20 % water, no grease on the guide rails), head load button and drive belt procedures are the same as for the Apple 5.25 Drive.

Troubleshooting

Connect the customer's drive and controller card (in slot 6) to a known-good Apple II, II Plus, IIe or IIGS, put a system disk in drive 1 and switch on. At each answer box, swap modules one at a time in the order given and start again; once it boots, run the drive diagnostic and refit the customer's modules one by one.

DuoDisk troubleshooting flowchart (Technical Procedures p. 4.3)

Symptom	Modules to swap, in order
Drive motor does not run	Interface card; interface cable; analog board; mechanical assembly; return the unit to Apple
Red "in use" LED does not light	Interface card; LED; analog board; return the unit to Apple
Motor and LED work, but the screen does not show the right disk information	Interface card; interface cable; DSPEED adjustment; chip-swapping chart; analog board; mechanical assembly

Apple's service note for the DuoDisk adds these checks, using the Drive Acceptance Program (077-8101) and the Digital Diagnostic Disk (689-8024) on a known-good Apple IIe with a known-good controller card in slot 6:

• Will not boot, motor runs, I/O errors: try a second copy of the test disk; try reading from the other drive, and if that also fails replace the analog card; adjust DSPEED; clean the head with 80 % alcohol and 20 % distilled water, and replace the mechanism if the head is scored; then replace the cable, the mechanism and the analog card in turn.
• Will not boot, motor does not run: boot from the other drive (if that fails, replace the analog card), then replace the cable, the mechanism and the analog card.
• Unacceptable noise from a drive: replace its mechanism.
• A brand-new disk will not format and the system reports "Volume Not On Line": use System Utilities 2.1 or later; if formatting still fails, swap the analog card.

Parts

Selected DuoDisk parts (Illustrated Parts List, IPL.3-IPL.7)

Part number	Description
661-72128	Uni/DuoDisk disk mechanical assembly
661-92130	DuoDisk analog card
590-0114	DuoDisk cable
970-1258	Sub-bezel, Uni/DuoDisk, beige
815-0637	Door, beige
805-5000	DuoDisk shield
805-5001 / 805-5002	Drive 1 / drive 2 top shield
590-0140 / 590-0223	LED cable assembly, drive 1 / drive 2
U880-0002	Disk drive belt
U815-0064	Load button
355-3470	IC, MC3470 floppy disk read amplifier
306-0125	IC, 74LS125

Documentation

• Apple DuoDisk Technical Procedures: Apple's service section, with take-apart, DSPEED, troubleshooting, the capacitor fix, chip swapping, preventive maintenance, loose analog boards and the parts list.
• Apple DuoDisk Owner's Manual: Apple's 1983 owner's manual, with installation and care.
• Disk II: the original drive in this family, with the 1985 DuoDisk chip chart.

Related pages

• Apple 5.25 Drive
• Apple Disk IIc
• Disk II
• Apple IIGS
• Floppy Drive Repair