Sinclair ZX Spectrum 128 Service Manual

Sinclair ZX Spectrum Service Manual
Spectrum For Everyone https://spectrumforeveryone.com/
20
1.10 POWER SUPPLIES
The on-board power supply unit receives a 9V unregulated supply from the external Sinclair ZX power
pack and derives the following internal supply rails:
a) Regulated +5V for the IC logic circuits, the ULA and the sound/UHF modulators
b) -5V for the expansion port
c) +12V for the RS232 driver IC33 and the keypad
d) -12V for the RS232 driver IC33 (unregulated -5V to -12V).
The external power pack incorporates a mains transformer, full wave rectifier and capacitive smoothing.
A thermal fuse is fitted at the transformer input.
The on-board power supply unit (Figure 1.5) incorporates a 7805 regulator, deriving the +5V power rail,
and an input supply for the inverter stage TR4/TR5. The latter raises the level of the +9V unregulated
supply above +12V. The resultant square wave at the junction of TR4 collector and the inverter coil is
subsequently rectified and smoothed by D15/C44 producing the +12V output. The square wave at TR4
collector also supplies a charge pump C111/112 and D28/29 which derives the -12V rail. The -5V supply
is taken from this rail via a Zener D19.
The following supplies are available on the expansion connector:
a) +5V (pin 3A)
b) Pulsed +12V (pin 23B)
c) +12V (pin 23A)
d) -5V (pin 20B)
e) +9V unregulated (pin 4A)
f) Ground (pins 6A, 7A, 14A)

Sinclair ZX Spectrum 128 Specifications

General

Brand	Sinclair
Model	ZX Spectrum 128
Category	Desktop
Language	English