The year was 1959. Batista was fleeing Cuba, the Soviet Union was advancing its Luna program, Ben-Hur was breaking box office records, and the Antarctic Treaty was being signed. On the computing side, IBM was offering its computers the IBM 729 tape storage unit—one of the company's most recognized and complex drives, featuring a notable vacuum system to reduce access times, reels that measured more than 700 meters long, and a maximum capacity of... three megabytes. Want to see how it works?
The first thing to note is that it has the size of a refrigerator (read 'fridge', 'icebox', etc.). In reality, that is not the computer. The actual system, an IBM 1401, appears in the opening seconds of the video. IBM announced the 1401 in September 1959, and despite developing half a dozen extra models under the 1400 series, the 1401 was so popular and in demand that IBM officially retired it in February 1971.
Now, back to that 'refrigerator'. The video published by CuriousMarc—the same channel that documented the restoration of a Xerox Alto—introduces the IBM 729 tape storage unit. It's an extraordinary piece of engineering, and one of the last high-end models available with seven-track storage. Technically, six tracks were for data, and the remaining one was used for parity.
Inside the IBM 729
Among the most notable aspects of the IBM 729 is its vacuum mechanism, which serves two purposes: reducing access times through greater tape acceleration, and absorbing any spontaneous increases in tension. The final capacity of the tape depended on the 'characters per inch' (CPI). If we combine the original value of 200 CPI with the 730-meter average tape length, the maximum was about three megabytes.
At least eight motors share the work of moving the tapes and generating the vacuum (the latter being larger and more powerful than the rest). But since they work constantly, a series of magnetic clutches is needed to respond to all changes in movement, and logically, to avoid blowing up in the process.
The unit in question was experiencing a technical problem. After so many years, wear on the contact points that supply electricity to the clutches had reached a critical point, and as expected, there are no new parts for a 1959 storage system. The solution? The magical path of DIY. As for the rest of the video... well, it's a real delight.
