1986 Apple Macintosh Plus 1MB Repair & Restoration

I recently got my hands on a 1986 Apple Macintosh Plus 1MB, which came with a keyboard, mouse, and a 1988 Apple 20SC external 20MB SCSI HDD, all in unknown condition.

The Macintosh Plus was a personal computer designed, manufactured, and sold by Apple Computer, Inc. It was the third model in the Macintosh line, introduced on January 16 1986, two years after the original Macintosh and a little more than a year after the Macintosh 512K, with a price tag of $2599. As an evolutionary improvement over the 512K, it introduced RAM expansion from 1 MB to 4 MB, and the SCSI peripheral bus, among other smaller improvements. Its relative rarity makes it quite collectible.

On its arrival, it seemed to be in good physical condition, but appeared to have been dropped during shipping as the keyboard case had been damaged and a key had snapped off – on testing, it did seem to successfully boot to the floppy prompt.

I wanted to perform some preventative maintenance on the computer, service the internal 3.5″ 1.4MB floppy disk drive, install a RAM upgrade, fit a new PRAM battery, and repair the keyboard – then, test everything thoroughly.

The first step was to disassemble the computer and check over everything inside.

CRT Display Safety

Before we start, a word of warning: like all other mains-voltage electronics, CRTs can be dangerous to work on, so it is important to take care and use the proper precautions – if you don’t know what you’re doing, it’s probably best to leave it to someone who does.

The high voltage side of CRTs can contain extremely high voltages (up to 30 KV for colour displays, less for monochrome), and the “low” voltage side has mains voltage and large line filter capacitors, all of which can give you a nasty shock – most CRTs contain safety features to automatically discharge any stored charge (i.e. bleeder resistors), but even if these exist they can fail, so it’s always best to err on the side of caution.

If you work on a CRT, you do so at your own risk – keep one arm behind your back so as to minimise shock across you torso; if possible, work on the CRT when it is unplugged and powered down, and safely discharge the CRT anode cap before starting work (preferably using a high-impedance lead so as not to damage the CRT interior coating).

Due to a phenomenon known as dielectric absorption, capacitors (including the CRT itself) can self-charge again even after being discharged – if you’re worried about this, keep any potential hazards shorted out while you’re working.

If you do need to work on the CRT while it’s powered up (for example, whilst measuring voltages or making control adjustments), use insulated tools and an isolation transformer.

Disassembling the Mac Plus

The Mac Plus is easy to dismantle with basic tools: if a programmers switch is fitted, this should be pulled out before attempting to open the case, else it may be damaged; there are five torx T15 screws holding the rear case in place, two near the mainboard ports at the bottom rear, two inside the carry handle recess at the top rear (which will need a long screwdriver bit to access), and one under the battery compartment cover.

This is a good time to check the PRAM battery – the original PX-21 4.5V alkaline battery has a habit of leaking corrosive alkali, which can cause damage to the battery terminals and analogue board, so should be removed and disposed of.

Luckily, this battery hadn’t leaked too badly, and the contacts only had minor corrosion – this cleaned up nicely using white vinegar.

With the case opened, you get your first look inside the computer – this Mac Plus seemed complete and all original, just very dusty.

The analogue board is held in place with four cross-head screws (one with an earth strap to the chassis), a connector to the digital board, a connector to the CRT neckboard, and a connector to the CRT yoke – these connectors can be very stiff, and need to be removed very carefully as to prevent damage to the CRT neck and cabling.

With the analogue board removed, the digital board can be disconnected from the internal floppy drive, then it simply pulls out the back of the chassis.

Analogue Board Rebuild

I wanted to rebuild the analogue board for the Mac Plus, which generates the main power supplies for the digital board as well as the high-voltage and vertical/horizontal deflection for the CRT display – this primarily involved reflowing any bad solder joints, replacing all of the aluminium electrolytic capacitors on the board, and replacing the two mains filter capacitors which have a habit of failing and releasing acrid smoke.

The insulation on the back of the analogue board is held in place with three foam adhesive pads, which have usually deteriorated with age.

Cracked and cold solder joints are common in equipment of this era, especially on joints which are under physical stress, such as on connectors and under heavy components like the flyback transformer – it’s a good idea to check over the boards looking for bad joints, as these can cause all kinds of weird and intermittent issues. Ideally the original solder should be removed, and replaced with new solder.

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 Mac Plus analogue board uses through-hole electrolytic capacitors – being an established technology for the time, these capacitors are generally quite reliable. However, the bipolar electrolytic capacitor at C1 is known as a common failure point, so I decided to replace them all out of caution.

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.

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.

I couldn’t find any commercially available capacitor packs for the international version of the Mac Plus analogue board, 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:

I also replaced the two paper mains filter capacitors with suitable 4.7nF safety capacitors.

Instead of using adhesive foam to reattach the rear insulation, I used four R3055 plastic pop-rivets like those used on the later Mac Classic analogue boards.

The digital board has a handful of axial electrolytic capacitors, but these are reliable low-voltage parts, so I left the originals in place; similar to the analogue board, I reflowed the solder joints on all the connectors, cleaned all of the ports and IC sockets with contact cleaner, and gave the board a thorough clean.

Disk Drive Servicing

The 3.5″ 800KB floppy disk drive in the Mac Plus usually requires a service due to its old age – the eject mechanism gets gummed up and doesn’t work properly any more, the eject motor gear has a habit of cracking and failing, the drive internals get dusty, and the read/write heads will probably need a good clean.

The FDD needs to be removed for servicing – the drive caddy is held into place on the chassis with four cross-head screws, and the drive itself is held into the caddy with another four cross-head screws.

I won’t go into detail about the drive service procedure, as Adrian Black has an excellent video on the subject – it basically involves dismantling the drive, cleaning the old grease and any grime, adding new lubricant to the stepper rails and screw, cleaning the read/write heads with IPA, and cleaning the microswitches with contact cleaner.

I also decided to replace the original brittle eject gear inside the eject mechanism, using a suitable 3D-printed replacement which will last much longer.

Analogue Board Repair

The Mac Plus worked fine for a while, then failed suddenly one day – during use, the screen went blank and the computer started making a “ticking” noise.

When turning the computer on from that point, there was no video output, no boot “bong”, and the ticking continued – it sounded like the low-voltage side of the internal PSU was cutting itself out due to overvoltage.

C24 and C32 (new aluminium electrolytic capacitors), which are across the 5Vdc rail, were getting extremely hot – in case of 5Vdc overvoltage, I replaced the 5V rectifier CR20 (MBR1035) with an uprated MBR1045, and replaced C24 and C32 in case they had been damaged. After this, I was still getting the same symptoms, but C24 and C32 were no longer running hot.

Unloaded, the +5Vdc rail was still varying between 4V and 5V, +12Vdc was around 2.6V, and -12Vdc was around -13V. As 12Vdc seemed to be so far off, I replaced the 12V rectifier CR21 (IR31D8-06-6G) with an uprated MUR410.

While I was at it, I also replaced the vertical transistor at Q2 (BU406), which is apparently a common failure, replaced the vertical rectifier CR5 (GI854) with an uprated MR824, and replaced CR1 (GI854) with an uprated MR824 too.

I adjusted the voltage rails accordingly while monitoring the them all on the connector to the digital board – this is done using the 12Vdc adjustment potentiometer on the back of the analogue board, from which the others rails are derived.

Analogue board connector pinouts.

With the computer reassembled, it would boot correctly and give good-quality video with the proper vertical deflection – however, the horizonal deflection was acting weirdly, and the image was slightly wrapping round on the left-hand-side of the screen.

The horizontal adjustments would move the display from side-to-side, however it was constantly offset to the left, and I couldn’t centre the image using the controls. Something was therefore still not working quite right on the analogue board.

The problem could have been the flyback transformer, but its coil resistances checked out OK, or the CRT yoke, but its coil resistances also checked out OK; another potential problem would be the bipolar capacitor at C1, which had been replaced, or the fixed inductor coil at L3 and/or the adjustable inductor coil at L2.

I removed L1 and L2 from circuit to test using an LCR meter, and they seemed to test OK (L3 was 10.8uH, L2 was 12.1uH with the slug out and 44.2uH with the slug in).

After a lot of time spent hitting my head against the wall, and checking and re-checking my previous work, it turned out that the parts at CR1 and CR5 had been replaced with MR754s, not MR854s – the seller had sent the wrong parts, and I hadn’t noticed! With the original GI854s fitted instead, the horizontal issues went away.

RAM Upgrade

I wanted to upgrade the memory on this Mac Plus from 1MB to 4MB, which is a simple process – the original RAM SIMMs need to be removed, four suitable 1MB RAM SIMMs need to be installed in their place, and the size select jumpers need to be set correctly.

Care must be taken when removing and installing RAM SIMMs on the Mac Plus digital board, as the plastic clips on the sockets are very stiff and brittle.

On this mainboard, the 256K jumper at R8 needed to be removed.

The new RAM SIMMs can then be installed.

With that done, the RAM upgrade seemed to be working well.

New PRAM Battery

After cleaning up the battery holder previously, I wanted to fit a suitable replacement for the original PRAM battery, so that the battery-backed RAM and RTC would work properly.

The original PX21 4.5V alkaline battery (which is just three LR50s in series, internally) is difficult to come by nowadays and can be quite expensive. I managed to find an adapter designed for the PX21 footprint which uses three 1.5V LR44 alkaline cells in series.

The adapter fits the battery holder perfectly, and the onboard RTC works OK.

Keyboard & Mouse Refurbishment

The mouse was in good physical condition and worked fine, but was very dirty – all it needed was a good clean with degreaser, and the removal of a sticker on the underside.

The keyboard had been damaged during shipping – the case had been cracked, one of the standoffs that holds the case together had snapped off, and one of the key stalks had been broken (but luckily the lost key was still in the package).

The keyboard is held together with six cross-head screws.

The damaged key switch can be desoldered and removed – I was very kindly given a couple of suitable working ALPS key switches by a friend on Twitter, and the replacement was easy to install. The remaining broken key stem was pulled out of the underside of the broken off key cap, and the key cap refitted.

The broken case standoff was reattached to the upper case piece using superglue.

After cleaning and reassembly, the keyboard looked and worked great.

The M0010A keyboard cable looks like a standard telephone cable (RJ11 connectors on both sides). However, the pinout is proprietary – if an incorrect cable is used to connect the keyboard to the computer, the keyboard controller could be irreparably damaged.

Reassembly & Testing

The reassembly of the computer is the opposite procedure to its disassembly.

After all of this work, I tested the Mac Plus with the modified 20SC external hard drive again, and it now booted correctly into the operating system loaded onto the SD card.

  • All keys register correctly.
  • All mouse inputs work OK.
  • Internal FDD works OK.
  • External SCSI interface works OK.
  • Monochrome video on internal CRT works OK.
  • Internal speaker works OK.
  • Audio output works OK.
  • 4MB RAM registers OK.
  • NMI and reset buttons work OK.

Published by themightymadman

My name is Adam Wilson - I'm an electronics engineer based in the North East of England, UK, and I like tinkering with old junk. In my spare time, I collect, repair, refurbish, and (sometimes) sell vintage computer systems and peripherals, typically from the 1980s (the likes of Commodore, Sinclair, Acorn, Apple, Amstrad, and Atari).

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