I recently got my hands on a 1986 Apple Macintosh Plus 1MB, which came with a 1988 Apple 20SC external 20MB SCSI HDD, both in unknown condition.
The Apple Hard Disk 20SC (not to be confused with the Apple Macintosh Hard Disk 20) was Apple’s first SCSI based hard drive for the Apple II family as well as the Macintosh and other third party computers using an industry standard SCSI interface. The drive allowed considerably faster data transfer rates (up to 1.25 megabytes per second) over its much slower predecessors, the Hard Disk 20 (500 Kilobits per second) and ProFile (which could only be used with the Macintosh XL).
On its arrival, it seemed to be in good physical condition, but appeared to have been dropped during shipping – on testing with the Mac Plus, the computer seemed to recognise the drive and tried to boot into the file system, however would fail to boot and would return very quickly to a floppy disk prompt – it also sounded like an aircraft taking off, but that’s probably normal for old 5.25″ hard drives.
The drive was therefore in need of repairs (my favourite part).
The first step was to disassemble the drive and check over everything inside.
Disassembling the 20SC
The 20SC is easy to dismantle with basic tools – the top plastic case piece is held in place with several clips around its perimeter that can be removed with a spudger.
The drive is really quite simple inside – it has an internal 5V/12V switch-mode power supply, a 5.25″ mechanical hard drive (a Seagate ST-225N), and a 12V cooling fan.
The internal SMPS and hard drive caddy are simply clipped into place, and can be removed without tools – the hard drive is plugged into the SCSI connector, power supply, and device select switch, and is held into its caddy with four screws.
Repairing the 20SC
As the Mac Plus has no space for an internal hard drive from the factory, and because the original ST-225N had died, I decided to fit an SD card adapter into the shell of this 20SC and continue to use it as an external drive.
From the factory, the Mac Plus also doesn’t provide 5V power over the external SCSI bus connector – it’s a simple modification to add this (the installation of a single diode), but I wanted to keep the Mac stock, so I also decided to rebuild the internal 20SC power supply. This also meant that the cooling fan would continue to work.
Power Supply Rebuild
I wanted to rebuild the internal PSU for the 20SC, which primarily involved the replacement of all of the aluminium electrolytic capacitors contained within.
The PSU is easy to dismantle: the lid is held in place with two screws; the board is held in place by its mains cable and four screws, two on the bottom and two on the side.
Aluminium electrolytic capacitors are commonly used for filtering, smoothing, and decoupling in both high- and low-voltage electronics. They typically comprise aluminium windings which are coated with a liquid electrolyte, which can dry out over time (negatively affecting the performance of the capacitor, often causing them to fail dead-short), or leak out and cause corrosion to the PCB and surrounding components.
The 20SC PSU uses through-hole electrolytic capacitors – being an established technology for the time, these capacitors are generally quite reliable. However, I decided to replace them all out of caution – including those on the daughtercard.
I usually remove each capacitor one-by-one using my desoldering station (a Duratool D00672) and immediately install a replacement, taking particular care to ensure that the value, voltage rating, and orientation of the new capacitor are correct – electrolytic capacitors are polarised, so must be installed the correct way around, else they’ll cause problems later (potentially even blowing up during use). I then clean up all the remaining flux residue or heat marks using 99.9% IPA.
You can’t always trust the markings on the PCB silkscreen, as sometimes mistakes were made in the design from the factory (take the PCB layout of the audio circuit on the Commodore CD32, for example), so care must be taken to match the orientation of the new capacitor with the original. Take lots of “before” pictures for reference.
I couldn’t find any commercially available capacitor packs for the 20SC, so I just made up my own by noting the specifications of all of the electrolytic capacitors on the board, and ordering a set of high-quality known-brand (i.e. Panasonic, Nichicon, etc) 105C-rated parts.
This was a total of:
- 1 x 4.7uF 350V (radial)
- 1 x 22uF 35V (radial)
- 1 x 22uF 100V (radial)
- 2 x 47uF 25V (radial)
- 2 x 100uF 10V (radial)
- 1 x 150uF 400V (radial snap-in)
- 1 x 330uF 16V (radial)
- 1 x 470uF 10V (radial)
- 2 x 2200uF 16V (radial)
The daughtercard also had two 3.3Ohm 1W power resistors onboard which seemed to have been damaged by overheating, so I also replaced these with suitable parts.
When substituting electrolytic capacitors, the capacitance needs to be the same, and the voltage rating can be the same or higher (within reason) – when you’re going through all this effort to recap something, be sure to use high-quality replacements.
SD Card Adapter Installation
For the SD card adapter, I decided to go with a BlueSCSI, for various reasons: they’re easy to find, cheap to build/buy, are easy to set up and use, and because I’d already made some for my Mac Classic and Mac Classic II, and had spare parts in stock.
The BlueSCSI fits into a 3D-printed 3.5″ drive caddy – this is available to buy from the creator, which I didn’t realise at the time, so I used the open-source design files and had several parts made by PCBWay, and bought some of the appropriate PCB mounting screws (M2.5 x 5mm).
The 3.5″ caddy can then be installed into the 5.25″ caddy using the original screws.
The drive caddy can then be fitted back into the 20SC case, connected to the PSU using an appropriate power adapter cable, connected to the rear SCSI bus port, and connected to the front LED using one of the onboard pin headers.
The BlueSCSI has separate pin headers for both power and drive activity – from the factory, the front LED of the 20SC is for drive activity, but given that the unit is now so quiet during operation, I decided to hook this up as a power LED.
The original power LED only had a very short cable, so I fitted an LED cable from a Commodore 64, modified with a modern ultra-bright red 5mm LED.
Reassembly & Testing
The reassembly of the drive is the opposite procedure to its disassembly.
After all of this work, I tested the drive with the Mac Plus again, and it now booted correctly into the operating system loaded onto the SD card.
- Hard drive reads and writes correctly over SCSI bus.
- Power LED works OK.
- Power switch works OK.
- Rear fan works OK.
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