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09.015 Can I replace a bombed Apple II 3.5" drive mechanism with one from a Mac?


This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.

09.015 Can I replace a bombed Apple II 3.5" drive mechanism with one from a Mac?

         Yes. 3.5" drive mechanisms are cheaply and abundantly available from the
Macintosh world. Although Apple II users have increased their interest in 3.5"
drives, these drives have declined in their utility for Mac users who have
shifted to hard drives, CD-ROM, and flopticals.  Few Mac owners have use for
the external 3.5" drive any more; if they have not yet discarded their external
3.5" drive, it is now in storage.


     The basic Sony 3.5" mechanism is shared by Mac and Apple II.  It stores
800K of data on a two-sided disk.  Unlike the IBM version with its constant
rotational speed, Mac/Apple II drives maintain constant head velocity by
varying the rotational speed as the head assembly approaches towards or moves
away from the disk hub.  Mac and Apple II drives differ in their track
sectoring arrangements, so disks cannot be read directly without special
translational software.  The opportunity for compatible disk sh

aring was lost during development, because rival teams working on Mac/Apple II
drives went their separate ways (Steve Weyhrich, APPLE II HISTORY, Pt. 9,

     The platinum 3.5" drive supplied with the IIgs is directly compatible with
the Mac, although the Mac ignores its front panel manual eject button; Mac disk
ejection is handled strictly by the desktop trash icon command.  An older
version of the external Mac 3.5" drive lacks the manual eject button and, in
its casing, is plug-incompatible with Apple II.  Thanks to advice provided by
Ken Watanabe, I learned that the inner mechanism is identical  among all
versions of 800K Mac and Apple II drives, includi

ng the internal drive mechanism in the Mac CPU.  This is good news for Apple II
users who wish to transplant the abundantly available Mac mechanism into their
platinum 3.5" drive casing.


     The classic white Unidisk drive was released in 1985 as a 3.5" platform
for the IIe and IIc.  This release date was 18 months prior to the introduction
of the IIgs.  Disks written by the Unidisk 3.5 and Platinum 3.5 drives are
fully interchangeable; the two models differed because the earlier Unidisk 3.5
used an intelligent microprocessor-controlled analog board to slow the data
transfer rate to match the IIe/IIc parameters.  This slowdown was not needed
for the popular platinum  3.5 drive used by the


     Can the Mac mechanism be transplanted to the Unidisk 3.5 casing? Probably
yes, but this has not yet been verified.  I am reluctant tohack with the
working Unidisk 3.5's attached to my IIC's.  I now seek a mechanically jammed
Unidisk 3.5 drive to verify whether its life can be resurrected with a Mac


     The internall DSDD 800k drive mechanism can be salvaged from any mid-
vintage Mac except for early models (Mac 128, Fat Mac 512) ---- those two
models used a quaint single-sided 400k drive.  Suitable models include the Mac
Plus, Mac SE, Mac II, or other Macs that have the standard DSDD 800K mechanism
--- newer Macs have incompatible high density drives.  Get a genuine Mac Sony
drive mechanism, not a clone; the suitability of non-Sony clones is uncertain.

     Salvaged internal drives must be removed from the Mac internal mounting
bracket --- take out the four side-mounted bolts, and slide the mechanism
forward.  The early version of the external Mac mechanism is mounted in a
plastic casing that resembles the Apple II platinum drive except that the
manual eject button is absent.

     Remove the mechanism from the casing, but save its round external cable
and db-19 plug --- that cable/plug can be used later to adapt flat-ribbon Apple
II drives for use with the IIgs or IIIc!  The Mac externaldrive's plastic
casing can be saved for use as a coin bank, or discarded.

     You should anticipate  that the older Mac drive has had plenty of use;
most Mac users have fewer drives attached to their computer than is common for
the Apple II.

     The 3.5" drives are sturdier than hard drives, but to protect the drive's
head assembly from damage during rough shipment, the seller should be asked to
ship the unit with a disk inserted.


     Use a well-light work area that gives you plenty of elbow room, with
containers to hold bolts and other small parts  All dimensions (left/right/top/
bottom/front/rear) refer to the unit's own dimensions, NOT to your own
egocentric viewpoint as the observer.  Standard precautions against static or
other electrical damage must be followed: Discharge static frequently by
touching grounded metal, wear a grounded wrist strap, hand all power  OFF when
attaching/removing drives, put insulating tape over the d

b-19 drive plug when not in use.

     Move slowly and patiently when removing or inserting the mechanism from
its housing ---- metal parts must not be forced or bent.  These tools
areneeded: (a) medium and small Phillips-head screwdrivers, (b) a small pliers,
and (c) a fine-tipped felt marker.  This procedure was outlined in an essay by
Lorne Walton (Apples BC, 1992), but many further details have been added here
to facilitate disassembly and drive replacement.

     The first step is to remove the worn/defective mechanism from its Apple II
platinum casing.  Flip the casing on its back and rest it on soft cloth. Re,ove
the four shiny bolts from the bottom of the casing. With the unit inverted,
slowly lift the bottom half-shell of the plastic casing upward and push the
external cable's attached grommet towards the upper casing.  The unit's bottom
casing should come off cleany.

     Use the felt marker to label the unit's own main dimensions, writing on
the metal internal shroud: front-bottom, rear-bottom, left side, right side.
Examine the metal innards as they lie upside down in the upper casing. Note
that a red and black wire pair are tucked on the inner edge of each side --
- These two wires go to the eject switch (right front) and to the red in-use
LED lamp (left front).

     At the unit's rear, observe that the wire pairs terminate in RED and BLACK
plugs.  Use the felt-tipped marker to write "R" and "B" on nearby metal
surfaces to identifythe positions of these two plugs. These letters will help
during reassembly when reinsterting the two plugs onto their proper pins. Next,
use the small pliers to grasp each plug, slowly and carefully pulling it
backwards to remove it from its mounting pins.

     With the black and red plugs each removed, slide the top plastic cover in
a rear-to-front direction, past the metal-enshrouded mechanism. The wire-pairs
from the eject-button and also from the in-use LED lamp should remain tucked
into their plastic side-braces.

     You now hold the mechanism, enshrouded in its grey metal shielding, with
the external db-19 cable protruding from the rear. Remove the two medium
Phillips mounting bolts (with flat washers) from each side.  Remove the single
medium Phillips bolt/washer that is centered on the upper-rear metal shroud.
Then lift off the upper-rear should and look inside.

     Note that the round external cable terminates in a familiar IDE-20 flat-
ribbon connector that plugs into the inner mechanism.  Unplug that inner
connector --- the small pliers can be used to rock and pull the connector
towards the rear.  With the IDE-20ribbon connector unplugged, the inner
mechanism can be slid forward and out.

     As you hold the inner mechanism in your hand, observe that a shiny thin
metal shroud covers its to and sides.  Use the felt-tipped pen to label this
shrou's dimension: TOP-FRONT and TOP-REAR.  This thin shroud should be removed
by rocking it and spreading its thin side-tabs.  At this point, you have the
bare mechanism in your hand, with heads visible from its top perspective, and
with pancake motor visible underneath.

     You are now ready to begin reassembly, but pause to appreciate what is
before you.  Hold the old mechanism and its Mac replacement side-by-side --
- they should appear identical.  The date of manufacture is coded on a sticker
on the pancake motor (e.g., 8809 = September 1989).  Apply rubbing alcohol with
a cotton swab to clean the surfaces of both read/write heads.


     Remount the innermost top-and-sides metal shroud, taking care that its
"fingers" have clicked into place on the mechanism's sides. When properly
fited, both bolt-holes on each side will be visible through the shroud's thin
metal. If the shroud does not fit, or if the bolt-holes are not seen, check
with your dimensional labels to verify that the front and rear have not been

     With the top/side inner shroud correctly in place, then reverse the
disassembly steps: Slide the mechanism through the front of the metal shroud.
Reattach the IDE-20 internal ribbon connector, align the two bolt holes on each
side of the outer shroud with the mechanism and reinstall the four medium
Phillips bolts and their washers.  Then reattach the rear-upper shroud with its
centered bolt and washer. Reassembly of the outer metal shroud is now finished!

     The final reassembly task is to refit the enshrouded mechanism into the
plastic outer casing.  Lay the inverted UPPER plastic half-shell on the
bench,with its front facing away from you.

     Observe the small red or black wires tucked along the sides of the upper
plastic half-shell. With the metal enshrouded drive mechanism upside down, it
should be slid into the plastic top-shell, from  its rear to its front.  Check
that the red-black wires remain tucked along the inner edge between the plastic
casing and the metal shroud.  Insert the black and red plugs into their
respective connectors.  Note the "R" and "B" markings you wrote on the metal
shroud; those markings will guide the plugs' in

sertion into their proper connectors.

     The oblong-shaped grommet attached to the round external cable should be
fitted first to the bottom plastic half-shell casing, which is then mated to
the top casing.  Reattach the four small shiny Phillips bolts through the
bottom plastic half-shell, and you're done!

By:  David Empson


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previous page: 09.014 Recently I picked up two 3.5" drives at a swap meet-- a Uni-disk and a Laser 128 drive. I've tried cleaning the heads; but, neither works with my IIc. What's the problem?
page up: Apple II Csa2 FAQs
next page: 09.016 I have one 5.25" drive connected to my GS, but the System 6.0.1 Finder display shows two 5.25" icons!? How can I fix this?