Sinclair initially worked alone in the evenings in a room in London (he was still a technical journalist during the day), selling radio kits by mail order.

Radionics initially developed hi-fi equipment; it released its first product, the Sinclair Micro-amplifier, in December 1962.

In 1965 the "Micro-FM" debuted as "the world's first pocket-size FM tuner-receiver", but was unsuccessful due to technical difficulties.

In 1966, Sinclair Radionics re-entered the hi-fi market with the "Stereo 25", a low-cost pre-amp control system.

The amplifier used low-frequency germanium transistors as pulse width modulators and switches and wrongly relied on the loudspeaker's inductance to filter the class D signal into audio.

In 1972, Radionics launched its first electronic calculator, the Executive, which was considerably smaller than its competitors' by the use of hearing-aid-sized batteries.

What had made this possible was it had been discovered that there was considerable latency in the display and memory and that, with the addition of a timer, power could be withheld from these battery-draining components for much of the time, thus greatly extending battery life.

In marked contrast to the rest of the Sinclair range, the instruments gained a reputation for reliable conventionality rather than often unreliable idiosyncrasy.

[9] Including a five-digit LED display, it suffered from technical flaws related to the design of the case, the chip, the battery and accuracy.

The Black Watch fiasco had a devastating effect on Sinclair's finances, and the company would have gone bankrupt had not the Government, through the National Enterprise Board, stepped in to support it.

In April 1976, the National Enterprise Board bought a 43% stake in Sinclair Radionics for £650,000, and in October the National Research and Development Council agreed to provide £1 million for a revived portable TV project, which was finally launched in January 1977 as the Microvision TV1A and MON1A at £99.95.

[11] When it became clear that Radionics was failing, Clive Sinclair took steps to ensure that he would be able to continue to pursue his commercial goals: in February 1975, he changed the name of Ablesdeal Ltd. (an off-the-shelf company he bought in September 1973, for just such an eventuality) to Westminster Mail Order Ltd.; this was changed to Sinclair Instrument Ltd. in August 1975.

Sinclair Instrument developed the "Wrist Calculator" to generate cash, which soon became a commercial success selling in surprising figures.