Apple Hard Disk SC General Maintenance
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The Apple Hard Disk 20SC, 40SC, 80SC and 160SC are Apple's external SCSI hard disks of 1986 to the early 1990s. All four use the same plastic case, metal mounting frame, power supply and fan, and Apple's service procedures treat them as one product with different drive mechanisms.[1] The newest of them is now over thirty years old. The care and take-apart below come from Apple's Technical Procedures and the 20SC owner's guide; faults are on Apple Hard Disk SC Troubleshooting Guide and the power supply recap on Apple Hard Disk SC Capacitor Replacement Guide.
Warning: the power supply runs from mains voltage. Unplug the drive before removing the case top. Apple's owner's guide warns that the drive contains high-voltage components that keep their charge after the unit is switched off.[2]
Handling
[edit | edit source]Apple's service procedures blame careless handling for more hard disk failures than all other causes combined.[1]
- Never move the drive while it is powering down. After switch-off the disks slow down and the heads park within 30 seconds; a jolt in that time can crash the heads.[1]
- Never drop the drive. Apple says even a slight drop can cause a failure.[1]
- Work on a soft, grounded surface with a grounding wrist strap, and keep the drive's board away from plastics, vinyl, polyester and polystyrene. Do not touch the circuit board unless you are grounded.[1]
- Keep a spare mechanism in its anti-static bag and shipping container until it is needed.[1]
Placement and cleaning
[edit | edit source]Keep the vents at the sides and back clear. Beside a Macintosh, put the drive on the right-hand side, away from the power supply and video circuitry on the left; do not stand a monitor on it. The drive runs at 50 to 104 °F (10 to 40 °C) and 20 to 80 % relative humidity. Keep liquids, solvent fumes and steam away from it.[2] Clean the outside with a soft, damp cloth, with a drop of liquid detergent for stains.[2]
Switching on and off
[edit | edit source]- Switch the drive on before the computer. It whirs and is ready within 20 seconds.[1] With two or more drives, switch them all on and wait at least 10 seconds before switching on the Macintosh.[2]
- In use, the front LED shows that the drive is being accessed. It is off when the drive is ready and idle.[1]
- Shut the Macintosh down from the Special menu, then switch off the drive. Never switch the drive off while the LED is on.[1]
SCSI ID and termination
[edit | edit source]Each device on the SCSI bus needs its own priority number (SCSI ID). On these drives it is set with the select switch at the back, below the SCSI ports, by pushing an opened paper clip into the hole and clicking until the number shows.[1] When testing a drive on a Macintosh SE or II, whose internal drive is ID 0, use a number from 1 to 6 for the external drive.[1]
Apple's support article on termination puts the terminator on the SCSI input of the first device and, if there is more than one device or more than 10 feet of cable, a second terminator on the last device.[3] The cables in Apple's parts list are the System Cable (658-8031), Terminator Cable (658-8032), Extender Cable (658-8033) and Peripheral Interface Cable (658-8034).[1]
Software checks
[edit | edit source]- Run Disk First Aid to repair the HFS file structure before reinitialising a drive that holds data. Disk First Aid will verify, but not repair, the current startup disk, so start from a utilities floppy.[1]
- Reinitialise with Apple HD SC Setup. Apple's 1989 procedures call for installer version 1.4 or later, or HD SC Setup 2.0.1 or later from the System Tools or Utilities disk. Reinitialising erases the whole disk.[1]
- On an Apple II, reinitialise with System Utilities 3.0 or later on a IIGS, an enhanced IIe or a IIe with a Rev C or later board, and System Utilities 2.1.1 on an unenhanced IIe.[1]
Take-apart
[edit | edit source]Apple's tools are a #2 jeweller's screwdriver, needle-nose pliers, a medium flat-blade screwdriver, a #2 Phillips screwdriver and, for the 160SC's end bracket, a small Torx screwdriver.[1]
Case top
[edit | edit source]- Find the two holding tabs at the back, inside the ventilation slots either side of the top SCSI connector.
- Press in gently on the lower end of each tab with a jeweller's screwdriver while pushing the case top up slightly.
- Free the four side tabs, rear holes first, by pushing the screwdriver straight into each tab hole and nudging the top upwards. Apple says prying is not needed and can damage the case.
- Lift the case top off. To refit it, fit the front under the lip of the case bottom, tuck the metal shields in, and press down until the tabs click.[1]
Power supply
[edit | edit source]- Push back the power supply tab, lift the front edge of the supply and rest it on the front of the case.
- Disconnect the power connector from the hard disk assembly. On some mechanisms it is on the other side of the drive.
- Release the clip on the fan cable connector and disconnect it, then lift the supply out.
- To refit, connect the fan cable and the power connector (each fits only one way), slide the supply towards the tabs at the back of the case bottom until they enter the holes in its metal case, and settle it under the front tab.[1]
The power supply (661-0343) opens with two screws on its lid. The board is held by its mains lead and four screws, two underneath and two at the side.[1][4]
Hard disk assembly
[edit | edit source]- Push back the large tab, slide the hard disk assembly up and rest it on the front edge of the case.
- Disconnect the power connector, open the end tabs (where fitted) on the HDA-to-case cable connector and disconnect it, then disconnect the SCSI select switch connector.
- Lift the assembly out. The 3.5-inch drives sit on four screws near the centre of the metal frame; the 5.25-inch drives fill the frame.
- To move a mechanism to another frame, remove the four screws on the bottom edge of the frame, then disconnect the LED cable and SCSI select cable.[1]
When refitting, reconnect the SCSI select switch, the HDA-to-case cable (close its end tabs) and the power connector, then slide the assembly under the small tabs on the case bottom and settle it under the holding tab.[1]
Fitting a replacement mechanism
[edit | edit source]Apple shipped most replacement mechanisms in their internal configuration, which has to be changed before the drive goes into an external case. The exceptions are the external 20 MB and 5.25-inch 160 MB service modules, which come in a frame that fits the external case.[1]
- Remove the drive from its internal shipping frame (four screws).
- Pull the three terminating resistor packs straight up off the controller board with needle-nose pliers. If they are left in, the drive will not work properly in the external case.
- Connect the case's SCSI select cable to the select pins on the controller board. On 5.25-inch drives this is an unkeyed double row of 16 pins: use the three pairs nearest the centre of the board with the tab towards the 50-pin connector, and leave the small jumpers on the other pins. On 3.5-inch drives the position and orientation depend on the board; Apple shows two layouts.
- Connect the LED cable (tab up, wires down), fit the drive to the customer's frame with four screws, and refit it.[1]
Apple's later Service Source volumes describe the same steps for mechanisms of other sizes, with the SCSI ID jumper tables, in Apple External Hard Drive SC Service Source and Apple Hard Drives Service Source.
LED, fan frame, fan and select switch
[edit | edit source]- LED cable: on the 160SC, remove four Torx screws and the end bracket first. Prise the plastic retaining ring off the LED holder, press the LED inwards while easing the holder apart until it snaps free, and unplug it from the controller board.[1]
- Fan frame: disconnect the HDA-to-case cable, the fan cable and the select switch cable, push back the plastic tab with a flat-blade screwdriver, and lift out the metal fan frame with the fan, HDA-to-case cable and select switch attached.[1]
- Fan: two screws hold it to the frame. When refitting, route the fan cable behind the HDA-to-case cable and between the connectors.[1]
- HDA-to-case cable: four screws hold it to the frame.[1]
- SCSI select switch: push it out through the fan frame from inside, pressing both plastic tabs at once with a jeweller's screwdriver. Refit it from outside until it snaps in, with its connector aligned as Apple shows.[1]
Parts list
[edit | edit source]| Item | Part number | Description |
|---|---|---|
| 1 | 630-5258 / 630-5288 / 630-5802 | Case top: beige / platinum / 160SC platinum |
| 2 | 590-0237 | LED cable |
| 3 | 076-0380 | End bracket and LED cable, 160SC |
| 4 | 805-0376 / 805-5081 | HDA frame / HDA frame, 160SC |
| 5 | 661-0343 | Power supply |
| 6 | 815-0974 | Lens |
| 7 | 630-5257 / 630-5287 | Case bottom, 20SC: beige / platinum |
| 7 | 630-5335 / 630-5337 / 630-5647 | Case bottom: 40SC / 80SC / 160SC |
| 8 | 865-0024 | Foot, platinum |
| 9 | 590-0260 / 590-0380 | Power cable: beige / smoke |
| 10 | 815-0975 / 815-0985 | Security D-ring: beige / platinum |
| 11 | 590-0238 | SCSI device select cable |
| 12 | 705-0045 | SCSI select switch assembly |
| 13 | 462-3100 | Screw, M3 × 0.5 × 6 |
| 14 | 805-0375 | Fan bracket |
| 15 | 699-0469 / 699-5017 | Fan / fan, 160SC |
| 16 | 590-0235 / 590-4037 | SCSI device-to-case cable / 160SC |
| - | 341-0768 | PROM TA.2 01 for some 3.5-inch Quantum 40 MB and 80 MB drives |
| Part number | Description |
|---|---|
| 661-0342 | HDA, external 20 MB, SCSI, Rev. A |
| 661-0612 | HDA, external 20 MB, SCSI, Rev. B |
| 661-0584 | HDA, external 160 MB, SCSI |
| 661-0391 | HDA, 5.25-inch, 40 MB, SCSI |
| 661-0411 | HDA, 5.25-inch, 80 MB, SCSI |
| 661-0601 | HDA, 5.25-inch, 160 MB, SCSI |
| 661-0625 | HDA, 3.5-inch, 160 MB, SCSI (internal only) |
Apple's documents give module and part numbers only. There is no schematic and no component list for the power supply or the drive boards.[1]
Related pages
[edit | edit source]- Apple Hard Disk SC Troubleshooting Guide
- Apple Hard Disk SC Capacitor Replacement Guide
- Apple Hard Disk 20SC
- Apple Hard Disk 40SC
- Apple Hard Disk 80SC
- Apple Hard Disk 160SC
- Hard Drive Maintenance and Repair
- Recommended Tools
References
[edit | edit source]- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 1.27 1.28 1.29 1.30 Apple Computer, Apple Service Technical Procedures, Cross Family Peripherals, Volume One (PN 072-0230, 1991), SCSI Hard Disk Drives chapter, hosted as File:Apple Service Technical Procedures Cross Family Peripherals Volume One Apr 1991.pdf and described at Apple Service Technical Procedures Cross Family Peripherals Volume One. Basics, pp. 1.3-1.11 (PDF pp. 178-186): drive matrix, handling, ESD, priority numbers, switching on and off, reinitialising, Disk First Aid, Apple II reinitialising; Take-Apart, pp. 2.2-2.27 (PDF pp. 188-213): one case and frame for all external drives, case top, power supply, hard disk assembly, reconfiguring service modules, SCSI select switch cable, LED cable, fan frame, fan, HDA-to-case cable, SCSI select switch; Diagnostics, p. 3.3 (PDF p. 216): SCSI IDs on the SE and II; Illustrated Parts List, IPL.3-IPL.11 (PDF pp. 256-264).
- ↑ 2.0 2.1 2.2 2.3 Apple Computer, Apple Hard Disk 20SC Owner's Guide (030-3138-A, 1986), hosted as File:Apple Hard Disk 20SC Owners Guide 1986.pdf and described at Apple Hard Disk 20SC Owner's Guide. Printed p. 4 (PDF p. 14): placement and ventilation; p. 22 (PDF p. 32): operating environment, cleaning and the high-voltage warning; p. 23 (PDF p. 33): switching on two or more drives.
- ↑ Apple Computer, "Hard Disk 20SC: Connecting a terminator", Tech Info Library article 662, revised 29 September 1990, in the archive.org collection Apple's Technical Information Library Articles, https://archive.org/details/APPLE_TIL_ARTICLES, file TIL00001_to_TIL60496.zip.
- ↑ Adam Wilson, "1988 Apple 20SC External HDD Repair & Restoration", Adam's Vintage Computer Restorations, 25 January 2023, retrieved 2026-10-04. Source for the power supply lid and board fixings, the 5 V/12 V switch-mode supply, the 12 V fan and the tool-free clip mounting of the supply and drive caddy.