Apple Macintosh 512K

Released 1984: The Macintosh 512K is a personal computer that was designed, manufactured and sold by Apple Computer, Inc. from September 1984 to April 1986. It is the first update to the original Macintosh 128K. It was almost identical to the previous Macintosh, differing primarily in the amount of onboard memory. The increased memory gained the ability to run more advanced software and turned the Macintosh into a more business-capable computer. It’s the earliest Macintosh model that can be used as an AppleShare server.

Facts

  • Type: All-in-one
  • Manufacturer: Apple Computer
  • Product family: Compact Macintosh
  • Released: September 10, 1984
  • Discontinued: April 14, 1986
  • OS: System 1.1 – 4.1
  • Introductory price: $2,795
  • CPU: 68000 @ ~¨8 MHz (effectively 6 MHz)
  • Memory: 512 KB RAM
  • Graphics: 512 x 342 pixel, 1 bit
  • Audio: 8-bit mono 22 kHz
  • Display: 9″ black and white CRT
  • Storage: 3.5″ FDD, single side (400 KB)
  • ROM 64 KB
  • Predecessor: Macintosh 128K
  • Successor: Macintosh Plus, Macintosh 512Ke

The Mac 512K originally shipped with Macintosh System Software 1.1 but was able to run all versions of MacOS up to System 4.1. It was replaced by the Macintosh 512Ke and the Macintosh Plus. Support for the Mac 512K was discontinued in 1998.

Features
Like the Macintosh 128K before it, the Macintosh 512K contains a Motorola 68000 running at 7.8 MHz and has a 16-bit data bus. Though the memory had been quadrupled, it could not be upgraded. The large increase earned it the nickname Fat Mac. The video controller used 22 KB of the system RAM and shares the data bus. This design reduces CPU performance up to 35%.

It shared a revised main board with the re-labeled Macintosh 128K, previously just called the Macintosh. This streamlined manufacturing.

Software
The applications MacPaint and MacWrite were still bundled with the Mac. Soon after this model was released, several other applications became available, including MacDraw, MacProject, Macintosh Pascal. Microsoft Excel was first written specifically for the Macintosh and required at least 512 KB RAM, but helped the Macintosh being seen as a serious business computer.

Apple Macintosh with Steve Jobs
Steve Jobs with an Apple Macintosh 128K or 512K in his lap.

Macintosh 512K enhanced
An updated version replaced the Macintosh 512K and debuted as the Macintosh 512K enhanced in April 1986. It had an 800 KB floppy disk drive and the same improved ROM as the Macintosh Plus. With the exception of the new model number “M0001E”, they were otherwise cosmetically identical. Apple offered an upgrade kit which replaced the floppy disk drive and ROMs, essentially turning it into a 512Ke.

Macintosh 512K
Macintosh 512K

My Macintosh 512K and repair

This Macintosh was completely dead and wouldn’t power on. The previous owner had found that “horizontal output” transistor “BU406” had failed on the analog board and replaced that one without any success to get machine to life.

Macintosh 512k - split open
My Macintosh 512k – split open
  • Replacing broken resistor: By measuring no voltages in the DC part I guessed there must be a  failure close to the AC input part of the board, I then found a dead resistor R55 (seems like 33K ohm, 0.5 watt) and replaced that one with two 0,25 resistors 68K ohm in parallel as I didn’t had any 0,5 watt resistors laying around.
    Machine could then be turned on but BU406 transistor got toasted with smoke coming up and the screen blacked out with a whining sound from the flyback transformator.
  • Recap analog board: I then decided I had to replace all the electrolytic capacitors on the board, more than 20 of them in total. I also replaced a Rifa AC filter capacitor that is know to “explode” with age, and also replaced the toasted BU406 transistor. 
    After this, machine would turn on without any smoke. But horizontal sync on screen was off. 
  • Non-polar capacitor: I had already replaced C1: non-polar 3.9uf 25v capacitor with 4 x 1uf ceramic non-polar capacitors in parallel, but I decided to try replacing them with a large 3.9uf AC filter 250 VAC capacitor instead.
    Success! Now the computer got a nice display without any distortion. Except the image was not centered accurate in the middle horizontally.
    Macintosh 512k - new capacitors
    Macintosh 512k – new capacitors
  • Adjusting horizonal centering: I’ve adjusted the horizontal centering by moving the centering rings on the CRT yoke directly. I first loosened the screw that holds it in place with the computer turned off. I then used a wooden stick to adjust the centering ring very carefully as there is high voltages in that area and screen has to be adjusted when machine is turned on.
  • Adjusting voltages: I measured the +5 amd +12 voltages on the external floppy connector and adjusted using the adjustment potentiometer on the analog board, the 12 volt was a little high so I adjusted slightly down so both 5 volt and 12 volt as about spot on.
  • Lubricate and clean floppy drive: When I first had the machine in parts I also cleaned and lubricated the floppy drive for smooth operation. 

Macintosh 512K

Remi Jakobsen


I'm collecting classic computers and video games, stretching from the 70's into the 90's. Restoration, history, usage ...


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