The wobbly RAM pack

The ZX81 arrived in March 1981 with just 1 KB of memory. To do anything serious you needed the 16 KB RAM pack, a black box that plugged into the expansion connector at the back and cost £49.95, as much as the computer itself in kit form ^[1].

The problem was the way it was held in place. The RAM pack was top-heavy and held only by the board's edge connector, with no other support. A nudge, a key pressed a little too hard or a tug on the cable was enough for the connection to drop out for an instant, and in that instant the ZX81 crashed and the program in memory vanished ^[1].

One knock on the table was enough for the ZX81 to forget everything.

Inside, the expansion was more complicated than it looked: two boards joined by a ribbon cable, one with 74-series logic chips and the other with 4116 dynamic memories, which need +12 V and -5 V. The RAM pack generated those voltages itself, with a small single-transistor switching circuit. With so much hanging off one connector, the slightest disturbance reset the machine ^[2].

Owners came up with their own fixes: chewing gum, double-sided tape and, above all, Blu-Tack between the case and the computer ^[1]. Some also used rubber bands and made-to-measure wooden frames to hold the whole thing together ^[2].

The flaw was so well known that it created a small market. In April 1983, Sinclair User featured Cheetah Marketing's 16 KB RAM pack, with its front moulded to the curve of the ZX81's back, at £19.75: in testing, the magazine hammered the keyboard and the computer did not crash ^[3]. In September, the same section showed Kelwood's "Wobble Stopper", a bracket with a nylon screw that fixed the RAM pack to the ZX81, from £5.25 ^[4].

More than forty years on, the problem still turns up on the bench. The edge connector contacts wear and oxidise, and it takes very little to break one of the connections and crash the ZX81. One modern fix is to fit the 16 KB inside the computer itself and do away with the external box ^[5].