Introduction
The Sinclair ZX 81 was the successor of the ZX80 and can be regarded as an evolution of it.
The ZX81 could now handle floating point numbers and data files stored on cassette.
It had an 8K ROM (compared to the 4K of the XZ80) with 30 additional functions and some instructions to drive the printer.
Technical Details
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 chip (Uncommitted Logic Array, basically the input/output chip) - 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.
The machine was assembled by the Timex Corporation in their Scottish plant.
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 1K of RAM built in) had to grip the board to make a connection.
Your carefully typed in program was often lost in an instant if a key was pressed too hard or the expansion pack knocked. These sudden crashes also led to the ULA chip burning out on occasions.
Although the Z80A CPU and the TV modulator are still available, the other chips are not, making repairs to the electronics of the ZX-81’s not really viable.
The keyboard membranes are as rare as rocking horse droppings and with them being very firmly glued to the top plate repairs are again a no no. The only realistic solution is to try and buy a complete top assembly which can occasionally be picked up from America.
© DataServe 2006
