Editing Sinclair ZX Spectrum 128 (section)

== Power supply ==

The 128 takes an external unregulated supply and generates the rest of its rails on board:<ref name="sm128" />

* '''+5 V''' from a 7805 regulator.
* '''+12 V''' from an inverter around TR4/TR5, whose square wave is rectified and smoothed by '''D15/C44'''.
* '''−12 V''' from a charge pump ('''C111/C112''' with D28/D29) working off the same square wave, feeding the MC1488 RS-232 driver and the keypad.
* '''−5 V''' derived from the −12 V rail through zener '''D19''', for the expansion port.

The external pack contains a mains transformer, full-wave rectifier and capacitive smoothing, with a '''thermal fuse at the transformer input'''. It should not sag below '''7.0 V''' at 1.4 A with low mains (215 V AC), nor peak above '''13 V''' at 600 mA with high mains (265 V AC).<ref name="sm128" />

The expansion connector carries +5 V (pin 3A), pulsed +12 V (23B), +12 V (23A), −5 V (20B), +9 V unregulated (4A) and ground (6A, 7A, 14A).<ref name="sm128" />