Tuesday, November 1, 2011

Macintosh Autopsy

This is what science fiction used to look like:

...but then again, so was this.

Anyway, it's a Macintosh Plus 1MB, 1986 vintage. Dave had it set aside for scrapping in the warehouse but I scooped it temporarily as I've never actually been inside one of these before.

Here it is with the plastic backing removed. As you can see, it's a pretty simple machine -- two circuit boards, a CRT monitor, and a floppy drive.

The circuit board on the right-hand side is the so-called "analog board", primarily the power supply for both the computer itself and the screen. The computer itself is the other circuit board at the bottom of the case.

There are a few obvious weaknesses here; there's no cooling fan, and between the computer itself, the CRT and especially the power supply, there are a lot of sources of heat. Second, power supplies in general and particularly power supplies for CRT monitors generate a lot of interference, and there's almost no shielding here beyond the metal structure of the chassis itself.

But it's a simple computer, certainly no fire-breathing speed demon, and this line of computers made a tremendous impact on the industry, doing well in home, educational, and business markets. It's the longest-produced Macintosh ever made.

But I came here to tear the machine down, not massage its ego. Let's continue. Here's the main circuit board for the computer:

The big horizontal chip is the CPU -- a Motorola MC68000, running at a blistering 8MHz:

Also known as the M68k, this is a titan among simple CPUs. Production began in 1979 and the platform is still in use today -- at its peak in the home computer revolution in the 1980s, it was the engine powering Macintosh, Amiga, and Atari ST home computers, VAXStation and IRIS workstations, and a host of other devices like laser printers. It lives on in numerous close relatives in the world of embedded systems.

Right next to it are the two ROM chips containing the system BIOS:

And up here we have the 4 RAM slots:

...and ye gods, it looks like someone dropped a grenade! If you've got sharp eyes you might have already spotted this in the first picture of the motherboard. One of the RAM chips is burned to a crisp, the top blown right off. Have a look at the sockets:

Now *that* is what a computer looks like when it's really had the magic smoke let out of it. Strangely, nothing else seems to be damaged -- I would have expected to see burnt traces on the board itself or damage to the nearby components. This kind of RAM is called a 30pin SIMM (Single Inline Memory Module) -- and given that these take only a 5 volt supply, typically low current and very well protected from volatility, either the chip was slow-cooked like a Louisiana barbecue or it somehow found itself a higher voltage.

Here's my hypothesis: the SIMM slots are located just underneath the floppy drive. I'll bet someone poured a little coffee in.

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