Skip to content

Apple Macintosh 512K

In January 1984, Steve Jobs triumphantly unveiled Macintosh. An “insanely great” computer for the rest of us. And while the original Macintosh was amazing, it also struggled to complete basic tasks. Limited to 128K of RAM and a single 400K floppy drive, the computer was a contradiction, with state-of-the-art software and limited hardware.

The Apple IIe (with 64K of RAM) was Apple’s most popular product when Macintosh was released. But the Mac’s bitmapped graphics and robust operating system devoured memory. Jobs was committed to keeping the original Mac at 128K of RAM to control costs. Burrell Smith knew it was a trivial matter to replace the 4264 DRAM chips with 41256 chips instead, and he surreptitiously modified the logic board to permit this upgrade. Indeed, the impressive introductory demo staged by Jobs at the Flint Center in Cupertino ran on a souped-up Mac with 512K of RAM.

Shortly after the first Macs rolled off the Fremont, California assembly line, users wanted two things: a second floppy drive and more memory for larger files and sophisticated software. In May, an external 400K floppy drive was available for $495, and thanks to Smith’s clandestine improvements, Apple would be ready with more RAM. So, in September, less than nine months after the 128K Mac was released, they introduced the Macintosh 512K. The original Macintosh sold for $2,495 and the 512K version had a suggested retail price of $3,195 (when the 512K was announced, the 128K’s price was lowered to $2,195). Aside from the RAM upgrade and a new badge on the back of the computer, the two models were identical. (128K owners could upgrade to a new logic board with 512K of RAM for $995.)

Known affectionately as the “Fat Mac” due to its plumped-up RAM, the Macintosh 512K provided immediate relief for one of the 128K’s biggest problems: disk swaps. Without an external floppy drive, copying files or whole disks was a tedious process. The Mac would read a bit of the source disk into its limited memory, then eject that disk and ask for the destination disk. Then it would write what was saved in memory to the second disk, then ask for the first disk again to read more of its contents, and then eject it and ask for the second… over and over. It was not uncommon for a Mac user to swap disks half a dozen or more times to complete a single operation. The Fat Mac solved this by reading an entire 400K floppy into RAM for one seamless transfer.

The extra memory was accompanied by an updated version of MacPaint with smoother scrolling thanks to the ability to hold more of the image in memory. MacWrite was also improved to support larger documents, and MacDraw and MacProject were able to store more objects and information. Apple hoped the additional memory would prompt third-party developers to take the Mac seriously–and to Apple’s delight, they did.

I found this Macintosh 512K on Facebook Marketplace. It was listed for a reasonable price because the display was not working. I took a chance the problem was fixable–most likely on the analog board. When it arrived, I was very impressed with its condition. This computer was shipped in its original Apple-branded carry bag, and it came with the mouse and keyboard. Surprisingly, even the original manuals and disks were tucked in the bag. Finally, the seller included a third-party external floppy drive and an ImageWriter printer. I was pleased the original 4.5V battery had not leaked. Of course, the battery was long dead, but I was impressed with its sturdy construction. If other AA-style batteries were that well made, many electronic devices could avoid a corrosive death.

Given its condition, I suspect this computer had been snuggled in its padded bag for decades. When I powered it on, I heard the distinctive startup ding from the speaker, but as expected, the screen was dark. I inspected the analog and logic boards. The logic board looked great, and, as I suspected, I spotted a few dodgy connections on the analog board.

I decided to recap both boards and reflow the cold solder joints. The process was straightforward, and I was aided by the convenient capacitor kits from Console 5. My only complication was the logic board cable would not budge from its connector on the analog board. However, I could easily disconnect the other end, so it was not a problem.

Even though the logic board looked great, I replaced the three electrolytic capacitors while the computer was disassembled. I did find a crusty connection on the serial port, but after some scrubbing with isopropanol and additional solder, it was back in good condition.

After recapping the boards and fixing the bad solder points, I reassembled the Mac and expected to see the display light up when I flipped the power switch, but instead, I saw nothing. Perplexed, I poked around the computer, and I noticed if I flexed the analog board, a scratchy image would briefly appear. I was thrilled to know the flyback transformer was working and the tube was not dead! Apparently, I missed a bad connection somewhere. So I disassembled the computer and, this time used my microscope to check each connection on the analog board.

After running my iron over several additional solder connections, I reassembled the computer. Again the screen was dark, but then I remembered the basics and started adjusting the potentiometers on the analog board. After a bit of fiddling, an image appeared, and after more fiddling, it was properly adjusted and looking good.

One of the adjustments controlled the voltage. While the screen was working, the floppy drive was making strange sounds when the computer booted, so I pulled out my multimeter and readjusted the voltage potentiometer to produce a solid 5V, 12V, and -12V.

I then extracted the internal floppy drive from its mounting cage and saw the expected Sony branding. I lubricated all the moving parts and cleaned the single-sided magnetic head.

I next tackled the peripherals. The M0110 keyboard and M0100 mouse can be hard to find, so I was delighted they were included. I opened them up and gave them a good cleaning. I spotted the remnants of a long-ago spill on the side of the keyboard PCB, but it cleaned up quickly.

Before closing up the computer, I gave everything one last long look and then paid my respects to the Macintosh team, whose names are etched inside the back cover.

I finally turned my attention to the unusual third-party floppy drive that accompanied the computer. It was branded Sydewyndr, and after asking online, Ron from Ron’s Computer Videos discovered PKI, Inc. from Torrance, California, sold the drive. It used a Chinon mechanism, and I was surprised it had both upper and lower magnetic heads for double-sided disks. Compared to the Sony drive, there was more plastic used inside, and I performed the customary cleaning and lubrication, then reassembled the drive.

While this Macintosh 512K was in excellent cosmetic condition, someone, unfortunately, had trouble removing the case at some point. The seam between the front of the back case is scared with pry marks in several places where someone tried to manhandle them apart.

It’s worth pointing out that these early Macs have five screws holding them together: two on the bottom near the ports, two under the carry handle, and one hidden inside the battery compartment. Also, the plastic interrupt/reset switch can sometimes complicate case removal and should be detached first, if for no other reason than to keep it from snapping.

Once I assembled the computer, I inserted a disk with System 4.1, but the computer responded with a Sad Mac code. I decided to try the Floppy Emu, loading an image for System 2.0, and the Mac booted fine. I then reinserted the System 4.1 disk in the running computer, and the drive read the disk without issue. I then tried booting the Mac with the original System disk that accompanied the computer, and it failed too. This time with a different Sad Mac code indicating the wrong operating system. With the computer booting from the Floppy Emu, I decided to try formatting several disks in the internal drive, but each time it clicked through the tracks then reported an error at the end. Since the internal drive was being difficult, I checked out the Sydewyndr external drive. While it made noises once a disk was inserted, it promptly spits the disk out.

My bad luck with floppy drives still haunts me.

Floppy issues aside, I am pleased to add this well-preserved Macintosh 512K to my collection. It was the best Mac launched in 1984, with the simple (but expensive) addition of RAM giving software developers and businesses alike confidence in the Macintosh.

Published inMacintosh


    • The Mac 128K and 512K lack SCSI support found in the later compact Macs. Making them a floppy-only platform; though a FloppyEMU is a handy way to address the situation.

Leave a Reply

Your email address will not be published. Required fields are marked *

© 2023