The machine was designed by English entrepreneur and inventor Sir Clive Sinclair and his small team in Cambridge, and was manufactured in Dundee, Scotland by Timex Corporation.

[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.

[26] Aside from a new crystal oscillator and extra chips to add additional kilobytes of memory,[27] the ZX Spectrum was intended to be, as quoted by Sinclair's marketing manager, essentially a "ZX81 with colour".

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.

The software architecture of the ZX80, however, had been tailored for a severely constrained memory system, and in Nine Tiles' opinion was unsuitable for the enhanced processing demands of the ZX Spectrum.

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.

[36] To increase the reliability, a leading period of constant tone was introduced, which allowed the cassette recorder's automatic gain control to settle itself down, eliminating hisses on the tape.

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".

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.

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".

[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.

[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.

Resourceful programmers swiftly devised workarounds; its rudimentary audio functionality compelled developers to explore unconventional methods such as programming the beeper to emit multiple pitches.

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.

An initial ULA design flaw occasionally led to incorrect keyboard scanning, which was resolved by adding a small circuit board mounted upside down next to the CPU in Issue 1 ZX Spectrums.

[75] In 1985, Sinclair developed the ZX Spectrum 128 (codenamed Derby) in conjunction with their Spanish distributor Investrónica (a subsidiary of El Corte Inglés department store group).

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.

[118] 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.

This variant features distinct buffers for both the ULA and the CPU, significantly enhancing compatibility with ZX Spectrum software compared to the American model.

[173] The Abbeydale Designers/Watford Electronics SPDOS and KDOS disk drive interfaces were bundled with office productivity software, including the Tasword word processor, Masterfile database, and Omnicalc spreadsheet.

[175] In the mid-1980s, Telemap Group launched a fee-based service allowing ZX Spectrum users to connect their machines to the Micronet 800 information provider via a Prism Micro Products VTX5000 modem.

[1] The ZX Spectrum had an extensive library of video games, including iconic titles such as Manic Miner, Jet Set Willy, Chuckie Egg, Elite, Sabre Wulf, Knight Lore, and The Hobbit.

Many of the popular ZX81 titles were rewritten for the Spectrum to take advantage of the newer machine's colour and sound capabilities (Psion's Flight Simulation being a notable example).

Critics in Britain welcomed the new machine as a worthy successor to the ZX81; Robin Bradbeer of Sinclair User praised the additional keyboard functions the Spectrum had to offer, and lauded the "strength" of its ergonomic and presentable design.

[197] Computer and Video Games' Terry Pratt compared the Spectrum's keyboard negatively to the typewriter-style used on the BBC Micro, opining that it was an improvement over the ZX81 but unsuited for "typists".

[199] Likewise, Gregg Williams from BYTE criticised the keyboard, declaring that despite the machine's attractive price the layout "is impossible to justify" and "poorly designed" in several respects.

[217] 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.

[224] The Vega, released in 2015, took the form of a handheld TV game[224][225] 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.

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".

[236] 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".