The Sinclair ZX80 was advertised as the first personal computer for under £100, and it got there with 21 chips: a Z80, a 4 KB ROM, two 1K x 4 RAM chips and seventeen ordinary 74-series logic chips.
Why it matters
Jim Westwood’s board had no video chip at all. The Z80 draws the picture itself by running NOP instructions through the display data, which is why the screen blanks whenever a key is pressed or a BASIC program is running. It was a price-driven compromise, and a clever one. Production reached about 9,000 a month by the end of 1980, and Sinclair put total sales at 100,000 within 18 months, although a Computer History Museum label gives about 50,000. The 4 KB ROM held Nine Tiles’ integer-only BASIC, so there were no decimals and no trig. When the ZX81 arrived in March 1981, ZX80 owners could prise out the old ROM, fit the 8 KB ZX81 ROM and stick a thin keyboard overlay over the original keys. The result behaves like a ZX81 stuck permanently in FAST mode, because the ZX80 hardware has no NMI generator for SLOW mode.
What to check before you buy
- Find out which ROM is inside. A 4K machine has the original keyboard legends and integer-only BASIC; an 8K machine has the ZX81 overlay and floating-point BASIC. The two BASICs differ, so 4K programs may not run on an upgraded machine.
- Look at the case. Issue 1 cases, in thin vacuum-formed plastic, crack and warp easily. Later Issue 2 ABS cases are much sturdier.
- Test every key on the membrane, and check the ribbon tail for cracks. Replacements exist, but fitting one is fiddly.
- The machine has no power switch and runs from a 9 V unregulated supply with a positive tip on a 3.5 mm jack. Confirm the polarity of any supply before plugging it in, and check the regulator does not get alarmingly hot.
- Check the rear edge connector and any RAM pack for wobble resets. The original RAM chips are socketed 2114s and easy to swap if a bank is faulty.
- The TV output is UHF, so you need a TV with an analogue tuner or a modulator. The screen blanking on keypresses is normal behaviour.
