Introduction

After the success of his two previous computers, the ZX-80 and particularly the ZX-81, in April 1982 Sir Clive Sinclair presented the ZX-Spectrum. This time the succession was a revelation!

It was originally to be called the ZX82 (as shown in this design prototype) , but Sinclair particularly wanted a name to reflect the new colour capabilities.

 The Spectrum name came from a suggestion from division head Nigel Searle's girlfriend!

For the first time, a computer with high-resolution graphics, colour, sound and 48k RAM was sold for under £200. Once more Sinclair revolutionised the microcomputer industry with new standards at affordable prices.

A much improved keyboard over the ZX81, it was still a low cost, multi-function affair, but with a rubber key mat rather than a flat plate. It probably held the world record for the number of functions on one key - as many as six!

Again based on the Z80A CPU, two models were launched: one with 16K of RAM and one with 48K. The ROM size doubled again to 16K.

There were 9 of the motherboard, each fixing bugs and improving reliability. The final revision was issue 6A.

Revision 1 machines are much sought after now by collectors as only around 60,000 were sold.
The original issue 1 machine had light grey/fawn coloured keys.  This was changed fairly quickly to the blue keys most people will know and recognise which were easier to see under fluorescent lights. Only a few issue 1 spectrums had the blue keys as the issue 2 boards appeared very soon after the colour change.

All the other versions had the familiar darkish blue/grey keys which made the letters/keywords easier to see under electric lighting.

The best feature of the Spectrum was its graphic possibilities. With 256 x 192 pixels, it was one of the highest resolutions for a microcomputer of its range in 1982. Unfortunately, to minimise memory requirements, colours were limited to 1 ink colour and 1 paper colour which was applied to an entire per 8 x 8 pixel character block.

Although the addition of sound was a step forward after the silence of the ZX80 and ZX81, it was also to be a major disappointment - a poor quality (low cost) speaker controlled by the simple BEEP statement – that was it!

To make matters worse, as there was no dedicated sound chip, the Spectrum could not do anything else whilst it was "beeping", so games could stutter badly (unless programmed cleverly in machine code) meaning background music was virtually out of the question.

Release versions, Problems and Issues

Issue 1
The Issue 1 Spectrums had problems with the ULA so that INKEY$ (gives the currently pressed key) only worked in 50% of the cases. This was solved by adding a small circuit board (the "spider", or "dead cockroach"), piggybacked on the ULA. On some models, rather than the spider, there was just a mess of soldered wires and components, looking like a not very neat home repair. I sold an Issue 1 spectrum some years ago, only for the buyer to return it to me because of my "shoddy repair work"! This was genuinely how they shipped, so if you are lucky enough to come across an Issue 1, don't be alarmed with what you may find inside...

Issue 2
Introduced late in 1982, Issue 2 boards had all the memory mounted on the main board in response to the success of the 48K version, effectively making the 48K machine the Spectrum of choice. The internals were tidied up somewhat as the chip error which the spider fixed was sorted, so it was no longer required. There were still some problems with the ULA, but these were improved by adding a transistor on top of the CPU.

There was also a new colour for the keys which were now blue (rather than grey/brown) to improve clarity under electric lighting.

Around 500,000 Issue 2 Spectrums were sold in 1982-83.

Tuning & Colour Issues
Both Issue 1 & 2 boards are famous for poor background colour and poor tuning. Yellows which should be white, dot crawl and difficulties getting the colour to “lock” are very common. On some TV’s it is impossible to even achieve colour.

There are two pre-set capacitors and two variable resistors all in a row which can be adjusted to help with these difficulties - this is covered in the Repairs & Servicing pages of the Sinclair section of the website. Click here for the specific link.

Issue 3
The issue 3 boards were a complete redesign and have a much improved, lower power ULA which is self adjusting, so the preset capacitors were not required. There was also an uprated speaker (!) to make the BEEP louder.

This ULA upgrade caused significant compatibility problems as, to reduce power consumption, the value of some resistors was altered meaning a binary value on the EAR (mic in) socket previously present on Issue 1 & 2 Spectrums, was not present until the ULA had fully warmed up. The keyboard input port reads in this value as well as the keyboard bits, but lazy programmers were just checking for the value on the EAR socket.
This would not have been a problem if programmers had checked the whole keyboard input byte as was supposed to happen.

Issue 4 & 4B
With the Issue 4A and 4B boards, the only real new addition was the 6C001-7 version of the ULA which gave a much cleaner and brighter output to the screen.

For reasons unknown to me, this board version seems to give the most problems with add ons such as the Interface 1. When connected along with a Sinclair Microdrive, the "CAT 1" command (or any other microdrive command) fails with the message "CAT ?".

 The Spectrum works perfectly in every other way, but I have never been able to fix this problem, although I do suspect it is a compatibility issue between the new ULA and certain brands of Z80 CPU...

Also, if the Spectrum displays this problem, it will not work with the new divIDE interfaces which are becoming very popular.

Issue 5
The issue 5 board was a major tidying up exercise as six decoder/multiplexor chips were replaced with a Mullard ULA (ZX8401). A 74LS04 hex inverter chip completed the redesign.

Issue 6
The final version of the Spectrum 48K was the Issue 6 which differed in only very minor ways - certain capacitors and resistors were changed and some models had a ULA provided by Saga rather than Ferranti.