Editing Sinclair ZX Spectrum 128 (section)

=== The Issue 6K keypad defect ===

The 128 was advertised with a separate numeric keypad, and one was bundled with the Spanish machines — but it was never sold in the UK, and only a couple of UK units are known to exist.

The reason is a hardware fault. On the '''Issue 6K''' board an incorrect resistor value in the circuit supplying the −Vcc line to the '''MC1488''' line driver — which handles both the serial and keypad ports — meant the MC1488 never received a proper supply, so a keypad could not communicate with the machine. It was corrected on the Issue 6U. Rather than recall every 6K machine, Sinclair simply did not offer the keypad in the UK.<ref name="keypad">[https://spectrumforeveryone.com/technical/fixing-keypad-issues-on-an-issue-6k-128k-toastrack/ "Fixing Keypad Issues on an Issue 6K 128K Toastrack"], Spectrum for Everyone. Fault identified and fix derived by Ian Gledhill.</ref>

The fix on a 6K board is to '''replace R137 with a 15 Ω resistor''', after which a Spanish keypad works normally.<ref name="keypad" />