ZX Spectrum

Sir Clive Sinclair riding an X-Bike prototype in 1990.

The ZX Spectrum was conceived and designed by English entrepreneur and inventor Clive Sinclair, who was well known for his eccentricity and pioneering ethic.[9] On 25 July 1961, three years after passing his A-levels, he founded Sinclair Radionics Ltd as a vehicle to advertise his inventions and buy components.[10] In 1972, Sinclair had competed with Texas Instruments to produce the world's first pocket calculator, the Sinclair Executive.[11] By the mid 1970s, Sinclair Radionics was producing handheld electronic calculators, miniature televisions, and the ill-fated digital Black Watch wristwatch.[12] Due to financial losses, Sinclair sought investors from the National Enterprise Board (NEB), who had bought a 43% interest in the company and streamlined his product line. Sinclair's relationship with the NEB had worsened, however, and by 1979 it opted to break up Sinclair Radionics entirely,[13] selling off its television division to Binatone and its calculator division to ESL Bristol.[14]

After incurring a £7 million investment loss, Sinclair was given a golden handshake and an estimated £10,000 severance package.[11][15] He had a former employee, Christopher Curry, establish a "corporate lifeboat" company named Science of Cambridge Ltd, in July 1977, called such as they were located near the University of Cambridge.[16] By this time inexpensive microprocessors had started appearing on the market, which prompted Sinclair to start producing the MK14, a computer teaching kit which sold well at a very low price.[17] Encouraged by this success, Sinclair renamed his company to Sinclair Research, and started looking to manufacture personal computers. Keeping the cost low was essential for Sinclair to avoid his products from becoming outpriced by American or Japanese equivalents as had happened to several of the previous Sinclair Radionics products.[11] On 29 January 1980, the ZX80 home computer was launched to immediate popularity; notable for being one of the first computers available in the United Kingdom for less than £100.[18][19] The company conducted no market research whatsoever prior to the launch of the ZX80; according to Sinclair, he "simply had a hunch" that the public was sufficiently interested to make such a project feasible and went ahead with ordering 100,000 sets of parts so that he could launch at high volume.[20]

On 5 March 1981, the ZX81 was launched worldwide to immense success with more than 1.5 million units sold,[21] 60% of which was outside Britain.[22] According to Ben Rosen, by pricing the ZX81 so low, the company had "opened up a completely new market among people who had never previously considered owning a computer".[23] Its release heralded the point when computing in Britain became an activity for the general public rather than the preserve of businessmen and hobbyists. The ZX81's commercial success made Sinclair Research one of Britain's leading computer manufacturers, with Sinclair himself reportedly "amused and gratified" by the attention the machine received.[24]

An original concept sketch of the ZX Spectrum, depicting an angular and wedge-like form similar to its predecessor, the ZX81.

Development of the ZX Spectrum began in September 1981, a few months after the release of the ZX81. Sinclair resolved to make his own products obsolete before his rivals developed the products that would do so, thus seeking to make the technology as cheap as possible. Architecture from the ZX80 and ZX81 were recycled to ensure a speedy and cost-effective manufacturing process. The team consisted of 20 engineers housed in a small office at 6 King's Parade, Cambridge.[25] During early production, the machine was known as the ZX81 Colour or the ZX82 to highlight the machine's colour display, which differed from the black and white of its predecessors. The addendum 'Spectrum' was added later on, to emphasise its 15-colour palette.[26] According to Sinclair, the team had the concept of using a single bank of random access memory as opposed to having separate data areas for audio and video. Much of the code was written by computer scientist Steve Vickers from Nine Tiles,[27] who compiled all control routines on Sinclair BASIC, a custom interpreter of the general purpose BASIC programming language. The interpreter enabled all subroutines to be loaded onto a very small amount of read-only memory (ROM). By taking advantage of pre-existing television sets and stereo speakers people "would already have" in their homes, Sinclair Research did not need to manufacture additional audiovisual equipment. Aside from a new crystal oscillator and extra chips to add additional kilobytes of memory,[28] the ZX Spectrum was, as quoted by Sinclair's marketing manager, essentially a "ZX81 with colour".[25]

Chief engineer Richard Altwasser was responsible for the ZX Spectrum's processor design. His main contribution was the design of the graphics mode using less than 7 kilobytes of memory, and together with Vickers, wrote most of the ROM code. Lengthy discussions between Altwasser and Sinclair engineers resulted in a broad agreement that the ZX Spectrum must have high-resolution graphics, 16 kilobytes of memory, an improved cassette interface, and an impressive colour palette.[29] To achieve this, the team had to divorce the central processing unit (CPU) away from the main display to enable it to work at full efficiency – a method which contrasted with the ZX81's integrated CPU.[29] The inclusion of colour to the display proved a major obstacle to the engineers. A Teletext-like approach was briefly considered, in which each line of text would have colour-change codes inserted into it. However this was ruled out as it was deemed unsuitable for high-resolution graphs or diagrams that involved multiple colour changes. Altwasser devised the idea of allocating a colour attribute to each character position on the screen. This ultimately used six bits of memory; three bits to provide any one of eight foreground colours and three bits for the eight background colours, for each character position. Overall, the system took up slightly less than 7 kilobytes of memory, leaving an additional 9 kilobytes to write programs — a figure the team was pleased with.[30]

A divergence of perspectives between Nine Tiles and Sinclair Research marred the development process, leading to disagreements on the optimal course of action. Notably, Vickers highlighted Sinclair's emphasis on expediting the release of the Spectrum, primarily by minimising alterations from the ZX81. The software architecture of the ZX80, tailored for a severely constrained memory system, proved unsuitable for the enhanced processing demands of the ZX Spectrum. Sinclair's precedent with the ZX81 indicated a reluctance to overhaul the ZX80 code, favouring instead the incorporation of expansion modules onto the existing framework. However, this approach, deemed acceptable for the ZX81, raised concerns within Nine Tiles regarding its potentially detrimental impact on the more sophisticated Spectrum.[31]

Designer Rick Dickinson pictured in Sinclair Research's Cambridge office in 1983.

The distinctive wedge-shaped case and colourful design of the ZX Spectrum was the brainchild of Rick Dickinson, a young British industrial designer who had been hired by Sinclair to design the ZX81. Dickinson was tasked to design a sleeker and more "marketable" appearance to the new machine, whilst ensuring all 192 BASIC functions could fit onto 40 physical keys.[25] Early sketches from August 1981 showed the case was to be more angular and wedge-like, in similar vein to an upgraded ZX81 model. Dickinson later settled on a flatter design with a raised rear section and rounded sides in order to depict the machine as "more advanced" as opposed to a mere upgrade. In drawing up potential logos, Dickinson proposed a series of different logotypes which all featured rainbow slashes across the keyboard.[32]

Designing the Spectrum's rubber keyboard was minimalised from several hundred components to a conventional moving keyboard down to "four to five" moving parts using a new technology.[33] The keyboard was still undergoing changes as late as February 1982; some sketches included a roundel-on-square key design which was later featured on the later Spectrum+ model.[32] Dickinson recalled in 2007 that "everything was cost driven" and that the minimalist, Bauhaus approach to the Spectrum gave it an elegant yet "[non] revolutionary" form.[3] The drawing board on which Dickinson designed the ZX Spectrum is now on display in the Science Museum in London.[34][35]

The need for an improved cassette interface was apparent from the number of complaints received by ZX81 users who encountered problems when trying to save and load programs. To remedy this, the team increased the number of cycles of which the machine read cassettes, and implemented periods of constant tones which allowed the cassette recorder's automatic gain control to settle itself down, eliminating hisses on the tape.[29] Altwasser considered the improved tape mechanism to be his proudest achievement; originally the team aimed for the getting the ZX Spectrum to operate at 1000 baud, but succeeded in getting it to work at a considerably faster 1500 baud.[29]

As with its predecessors, the ZX Spectrum was manufactured in Dundee, Scotland by Timex Corporation at the company's Dryburgh factory.[36][37] Timex had not been an obvious choice of manufacturing subcontractor, as prior to the manufacture of the ZX80, the company had little experience in assembling electronics. It was a well-established manufacturer of mechanical watches but was facing a crisis at the beginning of the 1980s. Profits had dwindled to virtually zero as the market for watches stagnated in the face of competition from the digital and quartz watches. Recognising the trend, Timex's director, Fred Olsen, determined that the company would diversify into other areas and signed a contract with Sinclair.[38]

The ZX Spectrum was officially revealed before journalists by Sinclair at the Churchill Hotel in Marylebone, London on 23 April 1982.[32][39] Later that week, the machine was officially presented in a "blaze of publicity" at the Earl's Court Computer Show in London,[40][41] and the ZX Microfair in Manchester.[42] The ZX Spectrum was launched with two models; a 16KB 'basic' version, and an enhanced 48KB variant.[2] The former model had an undercutting price of £125, significantly lower than its main competitor the BBC Micro, whilst the latter model's price of £175 was comparable to a third of an Apple II computer.[43][44] Upon release, the keyboard surprised many users due to its use of rubber keys, described as offering the feel of "dead flesh".[32][45] Sinclair himself remarked that the keyboard's rubber mould was "unusual", but consumers were undeterred.[46]

90 year-old Harold Macmillan using the ZX Spectrum with Sinclair looking on (1984).

Despite very high demand, Sinclair Research was "notoriously late" in delivering the ZX Spectrum. Their practice of offering mail-order sales before units were ready ensured a constant cash flow, but meant a lacking distribution.[26] Nigel Searle, the newly-appointed chief of Sinclair's computer division, said in June 1982 the company had no plans to stock the new machine in WHSmith, which was at the time Sinclair's only retailer.[47] Searle explained that the mail-order system was in place due to there being no "obvious" retail outlets in the United Kingdom which could sell personal computers, and it made "better sense" financially to continue selling through mail-order.[48] The company's conservative approach to distributing the machine was criticised,[26] with disillusioned customers telephoning and writing letters.[49][50] Demand sky-rocketed beyond Sinclair's planned 20,000 monthly unit output to a backlog of 30,000 orders by July 1982. Due to a scheduled holiday at the Timex factory that summer, the backlog had risen to 40,000 units. Sinclair issued a public apology in September that year,[32] and promised that the backlog would be cleared by the end of that month.[49] Supply did not return to normal until the 1982 Christmas season, however.[51]

With the arrival of the more inexpensive Issue 2 motherboard, production rapidly increased. A further 500,000 units shipped throughout the remainder of 1982 and into 1983, with the vast majority of them being sold directly by Sinclair itself. The introduction of the Issue 3 motherboard bolstered the number of sold units to a total of three million by the end of 1983. By this point the exclusive distribution deal with WHSmith expired,[52] opening the ZX Spectrum to prominent High Street chains such as Boots, Currys, and John Menzies.[32] Sales of the ZX Spectrum reached 200,000 in its first nine months[53] rising to 300,000 for the whole of the first year.[54] By August 1983 total sales in the UK and Europe had exceeded 500,000[55] with the millionth Spectrum manufactured on 9 December 1983.[56]

The machine was also backed by a successful albeit "unusual" marketing campaign as the company refused to set a pre-determined budget on advertising. Searle recalled that they were prepared to spend "as much as possible" on marketing as long as the outcome produced profitable sales. In 1981, Sinclair Research had spent just over £5 on advertising, whereas after the launch of the ZX Spectrum it was projected that they would invest more than £10 million.[48]

In July 1983, the ZX Spectrum was launched in the United States as the more enhanced Timex Sinclair 2068. Advertisements described it as offering 72 kilobytes of memory, having a full range of colour and sound for a price under $200.[57] Sales proved poor and Timex Sinclair collapsed the following year.[32][58]

A woman is loaned a ZX Spectrum from a library in the Netherlands, 1984.

A crucial part of the company's marketing strategy was to implement regular price-cutting at strategic intervals to maintain market share. Ian Adamson and Richard Kennedy noted that Sinclair's method was driven by securing his leading position through "panicking" the competition. While most companies at the time reduced prices of their products while their market share was dwindling, Sinclair Research discounted theirs shortly after sales had peaked, throwing the competition into "utter disarray".[59] Sinclair Research made a profit of £14 million in 1983, compared to £8.5 million the previous year. Turnover doubled from £22 million to £54.5 million, which equated to roughly £1 million for each person employed directly by the company.[60]

Clive Sinclair became a focal point during the ZX Spectrum's marketing campaign by putting a human face onto the business. Sinclair Research was portrayed in the media as a "plucky" British challenger taking on the technical and marketing might of giant American and Japanese corporations. As David O'Reilly noted in 1986, "by astute use of public relations, particularly playing up his image of a Briton taking on the world, Sinclair has become the best-known name in micros."[61] The media latched onto Sinclair's image; his "Uncle Clive" persona is said to have been created by the gossip columnist for Personal Computer World.[62] The press praised Sinclair as a visionary genius, with The Sun lauding him as "the most prodigious inventor since Leonardo da Vinci". Adamson and Kennedy wrote that Sinclair outgrew the role of microcomputer manufacturer and "accepted the mantle of pioneering boffin leading Britain into a technological utopia".[63] Sinclair's contribution to the technology sector resulted in him being knighted upon the recommendation of Margaret Thatcher in the Queen's 1983 Birthday Honours List.[64][65][66]

The United Kingdom was largely immunised from the effects of the video game crash of 1983, in due part to saturation of home computers such as the ZX Spectrum.[67][68][69]The microcomputer market continued to grow and game development was unhindered despite the turbulence in the North American and Japanese markets. Indeed, computer games remained the dominant sector of the British home video game market up until they were surpassed by Sega and Nintendo consoles in 1991.[70] By the end of 1983 there were more than 450 companies in Britain selling video games on cassette, compared to 95 the year before.[71] An estimated 10,000 to 50,000 people, mostly young men, were developing games out of their homes based on advertisements in popular magazines. The growth of video games during this period was comparable to the punk subculture, fuelled by young people making money from their games.[72]

Sinclair's claims that his technology would surpass IBM's personal computers were taken seriously. In an interview with Practical Computing magazine, Sinclair was confident that the ZX Spectrum's compact size and fewer chips would set the precedent for future computers around the world.[73] By the mid 1980s, Sinclair Research's share of the British home computer market had climbed to a high of 40 per cent.[74] Sales in the 1984 Christmas season were described as "extremely good" with the ZX Spectrum outselling every other microcomputer, including the Commodore 64, its fiercest rival.[75] In early 1985 the British press reported the home computer boom to have ended,[76] leaving many companies slashing prices of their hardware to anticipate lower sales.[75] Despite this, celebration of Sinclair's success in the computing market continued at the Which Computer? show in Birmingham, where the fifth-million ZX Spectrum was issued as a prize.[75]

Sinclair shaking hands with Lord Sugar, to mark the conclusion of the sale to Amstrad in 1986.

The ZX Spectrum's successor, the Sinclair QL, was officially announced on 12 January 1984, shortly before the Apple Macintosh went on sale.[77] Contrasting with its predecessors, the QL was aimed at more serious, professional home users.[78] It suffered from several design flaws; fully operational QLs were not available until the late summer, and complaints against Sinclair concerning delays were upheld by the Advertising Standards Authority (ASA) in May of that year. Particularly serious were allegations that Sinclair was cashing cheques months before machines were shipped. By autumn 1984, Sinclair was still publicly forecasting that it would be a "million seller" and that 250,000 units would be sold by the end of the year.[79] QL production was suspended in February 1985, and the price was halved by the end of the year.[80] It ultimately flopped, selling a mere 150,000 units.[81]

The ZX Spectrum+, a rebranded ZX Spectrum featuring a QL-like keyboard, was introduced in October 1984 and made available in WHSmith's stores the day after its launch. Retailers stocked the device in high quantities, anticipating robust Christmas sales. Nevertheless, the product did not perform as well as projected, leading to a significant drop in Sinclair's income from orders in January, as retailers were left with surplus stock. The Spectrum+ retained the identical technical specifications as the original Spectrum. Subsequently, an upgraded model, the ZX Spectrum 128, was released in Spain in September 1985, with development financed by the Spanish distributor Investrónica.[82] The launch of this model in the UK was postponed until January 1986 due to the substantial leftover inventory of the prior model.[83]

Despite the continued domination of home computer market with the ZX Spectrum, Sinclair hoped to repeat his success in the fledgling electric vehicle market, which he saw as ripe for a new approach. On 10 January 1985, Sinclair unveiled the Sinclair C5, a small one-person battery electric recumbent tricycle. It marked the culmination of Sir Clive's long-running interest in electric vehicles.[84] The C5 turned out to be a significant commercial failure, selling only 17,000 units and losing Sinclair £7 million. It has since been described as "one of the great marketing bombs of postwar British industry".[85] The ASA ordered Sinclair to withdraw advertisements for the C5 after finding that the company's claims about its safety could not be proved or justified.[86]

The combined failures of the C5 and QL caused investors to lose confidence in Sinclair's judgement. In May 1985, Sinclair Research announced their intention to raise an additional £10 to £15 million to restructure the organisation. Given the loss of confidence in the company, securing the funds proved to be a challenging task. In June 1985, business magnate Robert Maxwell disclosed a takeover bid for Sinclair Research through Hollis Brothers, a subsidiary of his Pergamon Press.[87] However, the deal was terminated in August 1985.[74] The future of Sinclair Research remained uncertain until 7 April 1986, when the company sold their entire computer product range, along with the "Sinclair" brand name, to Alan Sugar's Amstrad for £5 million.[88] The takeover sent ripples through the London Stock Exchange, but Amstrad's shares soon recovered, with one stock broker affirming that "the City appears to have taken the news in its stride".[89] Amstrad's acquisition of the brand name saw the release of three ZX Spectrum models throughout the late 1980s, each with varying improvements.[90]

By 1990, Sinclair Research consisted of Sinclair and two other employees down from 130 employees at its peak in 1985.[91] The ZX Spectrum was officially discontinued in 1992, after ten years on the market.[51][3] Sinclair Research thereafter continued to exist as a one-man company, marketing Sir Clive Sinclair's inventions until his death in September 2021.

ZX Spectrum 48K motherboard (Issue 3B: 1983, heat sink removed)

The main microprocessor is the Zilog Z80, an 8-bit central processing unit (CPU) with a clock rate of 3.5 MHz. The original model features 16 KB (16,384 bytes) of ROM and either 16 KB or 48 KB of RAM.[5]

Video output is channelled through an RF modulator, intended for use with contemporary television sets, to provide a simple colour graphic display. Text is displayed using a grid of 32 columns × 24 rows of characters from the ZX Spectrum character set or from a set provided within an application. The machine features a colour palette of 15 shades, comprising seven colours at two levels of brightness each, along with black.[92] The image resolution is 256×192 pixels, subject to the same colour limitations.[93] To optimise memory usage, colour is stored separately from the pixel bitmap in a low resolution, 32×24 grid overlay, corresponding to the character cells. In practical terms, this means that all pixels within an 8x8 character block share one foreground colour and one background colour. Altwasser received a patent for this design.[94]

An "attribute" consists of a foreground and a background colour, a brightness level (normal or bright) and a flashing "flag" which, when set, causes the two colours to swap at regular intervals.[93] This scheme leads to what was dubbed "colour clash" or attribute clash, where a desired colour of a specific pixel could not necessarily be selected. This became a distinctive feature of the Spectrum, requiring programs, especially games, to be designed with this limitation in mind. In contrast, other machines available at the same time, such as the Amstrad CPC or the Commodore 64, did not suffer from this limitation. While the Commodore 64 also employed colour attributes, it utilised a special multicolour mode and hardware sprites to circumvent attribute clash.[95]

The effect of attribute clash on a MSX, similar to a ZX Spectrum

Sound output is produced through a built-in beeper capable of generating a single channel with ten octaves.[96] It is controlled by a single EAR bit, located on port 0xFE.[97] By toggling the EAR bit (0xFE port) on and off, simple sounds are generated. Later software became available that allowed for two-channel sound playback. The machine includes an expansion bus edge connector and 3.5 mm audio in/out ports, facilitating the connection of a cassette recorder for loading and saving programs and data. The EAR port has a higher output than the MIC and is recommended for headphones, while the MIC port is intended for attachment to other audio devices as a line-in source.[98]

The ZX Spectrum integrated various design elements from the ZX81. The ROM code, responsible for tasks such as floating point calculations and expression parsing, exhibited significant similarities, although a few outdated ZX81 routines remained in the Spectrum ROM.[99] The keyboard decoding and cassette interfaces were nearly identical, although the latter was programmed for higher-speed loading and saving. The central Uncommitted Logic Array (ULA) integrated circuit shares some resemblance with the ZX81's, however it features a fully hardware-based television raster generator with colour support. This enhancement indirectly provided the new machine with roughly four times the processing power of the ZX81, as the Z80 was relieved of video generation tasks. An initial ULA design flaw occasionally led to incorrect keyboard scanning, which was resolved by adding a "dead cockroach" (a small circuit board mounted upside down next to the CPU) in Issue 1 ZX Spectrums.[44]

The ZX Spectrum's full colour palette

The machine's Sinclair BASIC interpreter is stored in read-only memory (along with essential system routines) and was authored by Steve Vickers under a contract with Nine Tiles Ltd. The keyboard is imprinted with BASIC keywords. For instance, when in programming mode, pressing the letter "G" inserts the BASIC command GO TO.[100]

The BASIC interpreter is derived from the one used on the ZX81, and a ZX81 BASIC program can be entered into a ZX Spectrum with minimal modifications. However, Spectrum BASIC introduced numerous additional features, enhancing its usability. The ZX Spectrum character set was expanded compared to that of the ZX81, which lacked lowercase letters. Spectrum BASIC incorporated extra keywords for advanced display and sound functionality, and it supported multi-statement lines. The cassette interface is significantly more advanced than the ZX81, enabling savings and loadings at approximately five times the speed (1500 bits per second as opposed to 307 bits per second).[101] Unlike the ZX81, the Spectrum can maintain the TV display during tape storage and retrieval operations. In addition to program storage, the Spectrum had the capability to save the contents of arrays, screen memory, and any specified range of memory addresses.[102]

ZX Spectrum 16K/48K (Dimensions (mm): 233×144×30 (W×H×D) @ ≈552 grams).[103]

The original ZX Spectrum is remembered for its rubber chiclet keyboard, diminutive size and distinctive rainbow motif. It was originally released on 23 April 1982 with 16 KB of RAM for £125 (equivalent to £469 in 2021) or with 48 KB for £175 (equivalent to £657 in 2021);[104] these prices were reduced to £99 (equivalent to £355 in 2021) and £129 (equivalent to £463 in 2021) respectively in 1983.[105] Owners of the 16 KB model could purchase an internal 32 KB RAM upgrade, which for early "Issue 1" machines consisted of a daughterboard. Later issue machines required the fitting of 8 dynamic RAM chips and a few TTL chips. Users could mail their 16K Spectrums to Sinclair to be upgraded to 48 KB versions. Later revisions contained 64 KB of memory but were configured such that only 48 KB were usable.[106] External 32 KB RAM packs that mounted in the rear expansion slot were available from third parties. Both machines had 16 KB of onboard ROM.[26]

An "Issue 1" ZX Spectrum can be distinguished from later models by the colour of the keys – light grey for Issue 1, blue-grey for later machines.[107] Although the official service manual states that approximately 26,000 of these original boards were manufactured,[108] subsequent serial number analysis shows that only 16,000 were produced, almost all of which fell in the serial number range 001-000001 to 001-016000.[109] An online tool now exists to allow users to ascertain the likely issue number of their ZX Spectrum by inputting the serial number.[110]

Within the original iterations of the 16 and 48K models, an internal speaker with severely restricted capabilities served as the audio output. This speaker, capable of producing just one note at a time, was governed by the BASIC command 'BEEP', where programmers could manipulate parameters for pitch and duration.[111] Furthermore, the processor remained occupied exclusively with the BASIC BEEPs until their completion, limiting concurrent operations. Despite these constraints, it marked a significant step forward from the ZX81, which lacked any sound capabilities. Resourceful programmers swiftly devised workarounds; its rudimentary audio functionality compelled developers to explore unconventional methods such as programming the beeper to emit multiple pitches.[112]

These models experienced numerous changes to its motherboard design throughout its life; mainly to improve manufacturing efficiencies, but also to correct bugs from previous boards. Another issue was with the Spectrum's power supply. In March 1983, Sinclair issued an urgent recall warning for all owners of models bought after 1 January 1983.[113] Plugs with a plain (rather than textured) surface were at risk of causing shock, and were asked to be sent back to a warehouse in Cambridgeshire which would supply a replacement within 48 hours.[114][115]

ZX Spectrum+ (Dimensions (mm): 319×149×38 (W×H×D))[103]

Development of the ZX Spectrum+ began in June 1984,[116] and was released on 15 October that year.[117][118] This 48 KB Spectrum (development code-name TB[116]) introduced a new QL-style case with an injection-moulded keyboard and a reset button that functions as a switch shorting across the CPU reset capacitor. Electronically, it was identical to the previous 48 KB model. It was possible to swap the system boards between the original case and the Spectrum+ case. It retailed for £179.[119] A DIY conversion-kit for older machines was available. Initially, the machine outsold the rubber-key model 2:1;[116] however, some retailers reported a failure rate of up to 30%, compared with a more typical 5–6% for the older model.[118] In early 1985, the original Spectrum was officially discontinued, and the ZX Spectrum+ was reduced in price to £129.[75]

ZX Spectrum 128

In 1985, Sinclair developed the ZX Spectrum 128 (code-named Derby) in conjunction with their Spanish distributor Investrónica (a subsidiary of El Corte Inglés department store group).[120][121][122] Investrónica had helped adapt the ZX Spectrum+ to the Spanish market after their government introduced a special tax on all computers with 64 KB RAM or less,[123] and a law which obliged all computers sold in Spain to support the Spanish alphabet and show messages in Spanish.[124]

The appearance of the ZX Spectrum 128 is similar to the ZX Spectrum+, with the exception of a large external heatsink for the internal 7805 voltage regulator added to the right hand end of the case, replacing the internal heatsink in previous versions. This external heatsink led to the system's nickname, "The Toast Rack".[125] New features included 128 KB RAM with RAM disc commands 'save !"name"', three-channel audio via the AY-3-8912 chip, MIDI compatibility, an RS-232 serial port, an RGB monitor port, 32 KB of ROM including an improved BASIC editor, and an external keypad.[112]

The machine was simultaneously presented for the first time and launched in September 1985 at the SIMO '85 trade show in Spain, with a price of 44,250 pesetas. Sinclair first unveiled the ZX Spectrum 128 at the The May Fair Hotel's Crystal Rooms in London, where he acknowledged that entertainment was the most common use of home computers. Due to the large number of unsold Spectrum+ models, Sinclair decided not to start it selling in the United Kingdom until January 1986 at a price of £179.[126][69]

The Zilog Z80 processor used in the Spectrum has a 16-bit address bus, which means only 64 KB of memory can be directly addressed. To facilitate the extra 80 KB of RAM the designers used bank switching so the new memory would be available as eight pages of 16 KB at the top of the address space. The same technique was used to page between the new 16 KB editor ROM and the original 16 KB BASIC ROM at the bottom of the address space.[127]

The new sound chip and MIDI out abilities were exposed to the BASIC programming language with the command PLAY and a new command SPECTRUM was added to switch the machine into 48K mode, keeping the current BASIC program intact (although there is no command to switch back to 128K mode). To enable BASIC programmers to access the additional memory, a RAM disk was created where files could be stored in the additional 80 KB of RAM. The new commands took the place of two existing user-defined-character spaces causing compatibility problems with certain BASIC programs.[128] Unlike its predecessors, it has no internal speaker, being produced from the television speaker instead.[129]

ZX Spectrum +2

The ZX Spectrum +2 marked Amstrad's entry into the Spectrum market shortly after their acquisition of the Spectrum range and "Sinclair" brand in 1986. This machine featured a brand-new grey case with a spring-loaded keyboard, dual joystick ports, and an integrated cassette recorder known as the "Datacorder," akin to the Amstrad CPC 464. However, it was largely identical to the ZX Spectrum 128 in most aspects. The main menu screen did not include the "Tape Test" option found in the Spectrum 128, and the ROM was adjusted to incorporate a new 1986 Amstrad copyright message. Production costs were reduced, leading to a retail price drop to £139–£149.[90]

The new keyboard did not feature the BASIC keyword markings seen on earlier Spectrums, except for the keywords LOAD, CODE, and RUN, which were useful for loading software. Instead, the +2 introduced a menu system, almost identical to that of the ZX Spectrum 128, allowing users to switch between 48K BASIC programming with keywords and 128K BASIC programming, where all words, both keywords and others, needed to be typed out in full (though keywords were still stored internally as one character each). Despite these changes, the layout remained identical to that of the 128.[130]

The ZX Spectrum +2 used a power supply similar to the grey ZX Spectrum+ and 128 power supply.[131]

ZX Spectrum +3

The ZX Spectrum +3, which was launched in 1987, bore a resemblance to the +2 model but introduced a built-in 3-inch floppy disk drive instead of the tape drive. Initially priced at £249[132], it later retailed for £199.[133] It was the only Spectrum model capable of running the CP/M operating system without additional hardware.

The +3 model saw the addition of two more 16 KB ROMs, with one accommodating the second part of the reorganised 128 ROM and the other hosting the +3's disk operating system, a modified version of Amstrad's PCWDOS known as +3DOS.

These significant alterations caused a series of incompatibilities, such as the removal of several lines on the expansion bus edge connector (video, power, and IORQGE), resulting in complications for various external devices like the VTX5000 modem, which could be used with the aid of the "FixIt" device. Additionally, changes in memory timing led to certain RAM banks being contended, causing failures in high-speed colour-changing effects. The keypad scanning routines from the ROM were also eliminated, rendering some older 48K and 128K games incompatible with the machine. The ZX Interface 1 was also rendered incompatible due to disparities in ROM and expansion connectors, making it impossible to connect and use the Microdrive units.[134]

Unlike its predecessors, the ZX Spectrum +3 power supply utilised a DIN connector and featured "Sinclair +3" branding on the case.[135]

Production of the +3 was discontinued in December 1990, reportedly in response to Amstrad's relaunch of their CPC range, with an estimated 15% of ZX Spectrums sold being +3 models at the time. The +2B model, the only other model still in production, continued to be manufactured, as it was believed not to be in direct competition with other computers in Amstrad's product range.[136][137]

ZX Spectrum +2A

The ZX Spectrum +2A was a new version of the Spectrum +2[138] using the same circuit board as the Spectrum +3.[138][139] It was sold from late 1988 and unlike the original grey +2 was housed inside a black case.[138][139] The Spectrum +2A/+3 motherboard (AMSTRAD part number Z70830) was designed so that it could be assembled with a +2 style "datacorder" connected instead of the floppy disk controller.[140] The power supply of the ZX Spectrum +2A used the same pinout as the +3 and has "Sinclair +2" written on the case.[141]

1989 saw the release of the ZX Spectrum +2B and ZX Spectrum +3B. They are functionally similar in design to the Spectrum +2A and +3,[142] though changes to the generation of the audio output signal were made to resolve problems with clipping.[143] The +2B board has no provision for floppy disk controller circuitry, while the +3B motherboard has no provision for connecting an internal tape drive. Production of all Amstrad Spectrum models ended in 1992.[3]

Timex Sinclair 2068, released in 1983 for the American market.

Sinclair Research granted a licence for the ZX Spectrum design to the Timex Corporation in the United States. Timex marketed several computer models under the Timex Sinclair brand. They introduced an enhanced variant of the original Spectrum in the US, known as the Timex Sinclair 2068. This upgraded model features improvements in sound, graphics, and various other aspects. However, Timex's versions were generally not compatible with Sinclair systems.

Following Amstrad's acquisition of Sinclair Research's computer business, Sir Clive Sinclair retained the rights to the Pandora project. This project eventually evolved into the Cambridge Z88, which was launched in 1987.[144] The ill-fated Pandora portable Spectrum incorporated a ULA that introduced the high-resolution video mode originally pioneered in the Timex Sinclair 2068. The Pandora was designed with a flat-screen monitor and Microdrives, intended as Sinclair's solution for portable business computing.

Timex Computer 2048, highly compatible with the ZX Spectrum 48K, was very successful in both Portugal and Poland.

Starting in 1984, Timex of Portugal developed and produced several Timex branded computers, including the Timex Computer 2048, which was highly compatible with the Sinclair ZX Spectrum 48K. This model saw significant success in both Portugal and Poland.[145] While an NTSC version was initially intended for release in the United States, it was ultimately only sold in Chile, Ecuador, and Argentina.

Timex of Portugal produced a PAL region-compatible version of the Timex Sinclair 2068, known as the Timex Computer 2068. This variant features distinct buffers for both the ULA and the CPU, significantly enhancing compatibility with ZX Spectrum software compared to the North American model. The modification of the expansion port ensured 100% compatibility with ZX Spectrum's, eliminating the need for a "Twister Board" expansion required by the Timex Sinclair 2068 to support ZX Spectrum expansion hardware. Additionally, the AY sound output in the TC 2068 was directed to the monitor/TV speakers rather than the internal tweeter.

Software developed for the Portuguese-made 2068 remained fully compatible with its North American variant, as the ROMs were left unaltered. Timex of Portugal further created a ZX Spectrum "emulator" in cartridge form that precisely mapped the first 16 KB, akin to the earlier 2048 computer. Several other upgrades were introduced, including a BASIC64 cartridge enabling the TC 2068 to utilise high-resolution (512x192) modes.

Timex's Portuguese division was also in the process of developing a successor to the 2068 named the Timex Computer 3256, featuring a Z80A CPU and 256 KB of RAM.[146] The planned model was intended to have both ZX Spectrum BASIC and a CP/M operating modes. However, the company terminated its development as the 8-bit market ceased to be profitable by the end of 1989. Only one fully functional prototype of the 3256 was completed.[147]

The Indian-made deciBells dB Spectrum+ and PSU

In India, Deci Bells Electronics Limited[148] introduced a licensed version of the Spectrum+ in 1988.[149][150][151] Dubbed the "dB Spectrum+", it performed well in the Indian market, selling over 50,000 units and achieving an 80% market share.[152]

Didaktik Gama

Numerous unofficial Spectrum clones were produced, especially in the Eastern and Central European countries (e.g. in USSR (called Russian: Байт, Ленинград, Балтика), Romania, and Czechoslovakia) where several models were produced (such as the Tim-S, HC85, HC91, Cobra, Junior, CIP, CIP 3, Jet, Didaktik Gama), some featuring CP/M and a 5.25"/3.5" floppy disk.

There were also clones produced in South America (e.g. Microdigital TK90X and TK95, made in Brazil and the Czerweny CZ, made in Argentina). In the Soviet Union, ZX Spectrum clones were assembled by thousands of small start-ups and distributed through poster ads and street stalls. Over 50 such clone models existed.[153] Some of them are still being produced, such as the Pentagon and ATM Turbo.

In the UK, Spectrum peripheral vendor Miles Gordon Technology (MGT) released the SAM Coupé as a potential successor with some Spectrum compatibility. By this point, the Amiga and Atari ST had taken hold of the market, leaving MGT in eventual receivership.[154]

In 2013, an FPGA-based redesign of the original ZX Spectrum known as the ZX Uno, was formally announced. All of its hardware, firmware and software are open source,[155] released as Creative Commons licence Share-alike. The use of a Spartan FPGA allows the system to not only re-implement the ZX Spectrum, but many other 8 bit computers and games consoles[156] The device can also run modern open FPGA machines such as the Chloe 280SE.[157] The Uno was successfully crowdfunded in 2016 and the first boards went on sale during the same year.[158]

In January 2014, Elite Systems, who produced a successful range of software for the original ZX Spectrum in the 1980s, announced plans for a Spectrum-themed bluetooth keyboard that would attach to mobile devices.[159][160] The company used a crowdfunding campaign to fund the Recreated ZX Spectrum, which would be compatible with games the company had already released on iTunes and Google Play.[161] Elite Systems took down its Spectrum Collection application the following month, due to complaints from authors of the original software that they had not been paid for the content.[162] Wired UK described the finished device, which was styled as an original Spectrum 48k keyboard, as "absolutely gorgeous" but said it was ultimately more of an expensive novelty than an actual Spectrum.[163] In July 2019, Eurogamer reported that many of the orders had yet to be delivered due to a dispute between Elite Systems and their manufacturer, Eurotech.[164]

The ZX Spectrum Vega TV Game Console, made by Retro Computers.

Later in 2014, the Sinclair ZX Spectrum Vega retro video game console was announced by Retro Computers Ltd and crowdfunded on Indiegogo with the backing of Clive Sinclair.[165] The Vega, released in 2015, took the form of a handheld TV game[165][166] but the lack of a full keyboard[46] led to criticism from reviewers due to the large number of text adventures supplied with the device.[167][168] Most reviewers branded the device cheap and uncomfortable to use.[169][163]

The follow-up, the ZX Spectrum Vega+ was designed as a handheld game console. Despite reaching its crowdfunding target in March 2016,[170] the company failed to fulfil the majority of orders. On 30 July 2018, Eurogamer reported that one backer had received a ZX Vega+ console and quoted them as being "quite disappointed" that "the few supplied sample games don't work" and that the "build quality's not the greatest".[171] Reviewing the Vega+, The Register criticised numerous aspects and features of the machine, including its design and build quality and summed up by saying that the "entire feel is plasticky and inconsequential".[172] Retro Computers Ltd was wound up on 1 February 2019.[173]

The ZX Spectrum Next is an expanded and updated version of the ZX Spectrum computer implemented with FPGA technology[174] funded by a Kickstarter campaign in April 2017,[175] with the board-only computer delivered to backers later that year.[176] The finished machine, including a case designed[177] by Rick Dickinson who died during the development of the project, was released to backers in February 2020.[178] MagPi called it "a lovely piece of kit", noting that it is "well-designed and well-built: authentic to the original, and with technology that nods to the past while remaining functional and relevant in the modern age".[179] PC Pro magazine called the Next "undeniably impressive" while noting that the printed manual lacked an index, and that some features are "not quite ready".[180] A further Kickstarter for an improved revision of the hardware was funded in August 2020.[181]

The ZX Spectrum enjoyed a very strong community early on. Several commercially published print magazines were dedicated to covering the home computer family and its offshoots including Sinclair User (1982), Your Spectrum (1983) – rebranded as Your Sinclair in 1986, and CRASH (1984). In the early years, the magazines were focused on programming for the system, and carried many articles containing type-in programs and machine code tutorials. Later on they became almost completely game-oriented, starting many of the writing-styles, trends and tropes found in later video-game publications and reviews.

Several other contemporary computer magazines covered the ZX Spectrum as part of their regular coverage of the home computer industry at that time. These included Computer Gamer, Computer and Video Games, Computing Today, Popular Computing Weekly, Your Computer and The Games Machine.[218]

The Spectrum is affectionately known as the Speccy by elements of its fan following.[219]

More than 80 electronic magazines existed, many in Russian. Most notable of them were AlchNews (UK), Enigma Tape Magazine (UK), 16/48 (UK), ZX-Format (Russia), Adventurer (Russia), Microhobby (Spain) and Spectrofon (Russia). These frequently included games, demos, and utilities alongside the magazine content (much like a covertape on a paper magazine).