Sinclair ZX81

Introduction

The Sinclair ZX81 was the successor to the ZX80, but view this as an evolution rather than a revolution.

Many improvements were made including a more robust case, new colours (and matching peripherals) and the elimination of the ZX80's infamous screen flicker.

Now with an 8K ROM (compared to the 4K of the XZ80), the ZX81 could now handle floating point numbers and data files on cassette, plus it had an additional 30 functions in BASIC and some instructions to drive the Sinclair thermal printer.

A much higher level of integration meant lower production costs and a reduction of chips on the main board from 21 to just 4 – the ROM, Z80A CPU, 1K RAM and the Ferranti custom-made Uncommitted Logic Array (ULA) - it was as simple as that.

The keyboard was still formed by an under printed plastic, but this one was made of non-reflective material. Even with this slight improvement it was quite horrible to use, with some of the keys - in true Sinclair fashion - sporting up to 5 functions.

With only 1K of RAM, the 16K RAM pack quickly became an essential purchase.

The US company Timex were contracted to produce the ZX81 at its Dundee factory in Scotland and Sinclair used its relationship with Timex to sell a re-branded ZX81 as the Timex-1000 in the US.

A new version of the ZX81 with a built-in 16K RAM pack, fitted into what would be a Spectrum style case was also sold (only in the US) as the Timex Sinclair TS-1500.
With a better keyboard (just) and the extra RAM it was surprising that this model was not very successful.

Like the ZX80, the ZX81 it was still available as a kit, or ready assembled.
The machine was a big success in Europe - even without colour, sound or even a joystick!

Technical Details:

  • CPU: Z80A
  • Speed: 3.25MHz
  • RAM: 1KB
  • Membrane Keyboard
  • Optional 16K RAM Pack
  •  Local price at launch: £49.95(kit-form) - £69.95 (ready-built).

Problems and Issues
 
To save money, the computers did not have a socket for expansion.
 Copper tracks led to the edge of the circuit board and memory expansion packs (very necessary with only the 1K of RAM built in) had to grip the board to make a connection. 
To the infuriation of users, the slightest knock, or even a key press made too hard would crash the system losing hours of carefully typed in program!

The knock-on effect of the crashes nowadays though means that most ZX81's now have damaged ULA's and replacements are becoming very difficult to find.

The symptoms of are a very dark screen and barely visible flashing cursor.
Often though, with both the brightness and contrast turned up to full levels on the TV, the ZX81 is still usable.

Repairs
The Z80A CPU, the TV modulator, some versions of the ROM and (sometimes) the ULA are still available. but the other chips are not, making repairs to the electronics of the ZX-81’s almost impossible.

Keyboard membranes often fail, but spares are again available from the website shop.

The cassette lead points are also often damaged, or worn, so connection with the jack plugs is poor. These are also available in the shop and are the same type as the ZX80 and Spectrum.

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