Sinclair's long-standing enthusiasm for hi-tech names revealed itself in the X-10, an audio amplifier designed by Gordon Edge which used a new technique called pulse width modulation. It aroused a lot of interest, as nothing of similar size, price or performance had been seen before.

Unfortunately, Sinclair's claims were a little too extravagant and the amplifier ran into the first advertising standards problem of Sinclair’s career. Wireless World reportedly refused to take Sinclair’s subsequent advertising for the X-10 because of complaints over the performance claimed for the amp. Its stated output was 10 watts R.M.S., but in reality it was capable of only a quarter of that, and was temperamental to boot.

The debut of the X-10 was also fraught with problems. The original circuit design had been delivered to Sinclair Radionics accompanied by a working prototype; Jim Westwood took the prototype and engineered it into a marketable form which was sent to the firm in Hampshire which manufactured Sinclair's circuit boards. They promised that the first batch of boards would be delivered within a few days, and the first advertisement for the X-10 was planned to coincide with promised deliveries.

The boards arrived exactly on time. They looked all right at first, and Westwood set about building the first real X-10 amplifier. But disaster struck - the board was a mirror image of what should have been and the whole batch was useless. By the time the correct boards had been made and an amplifier built, the X-10 had been appearing in advertisements for some time, with many customers wondering when their amplifiers would arrive. This was the first but by no means the last time that technical problems were to cause major delays to deliveries of a Sinclair product.