6. POWER SUPPLIES
6.1 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 1C logic circuits, the ULA and the UHF
modulator,
(b) -5V and +12V for the standard 16k dynamic RAM,
(c) +12V for the colour modulator circuits.
6.2 The external power pack incorporates a mains transformer, full wave
rectifier and capacitive smoothing. A thermal fuse is fitted at the
transformer input.
6.3 The
on-board
power
supply
unit
incorporates
a
7805
regulator,
deriving
the +5V
power
rail,
and an
inverter
stage
TR4/TR5.
The
latter
raises
the level of the +9V unregulated supply to in excess of +12V. The
resultant sguare wave at the junction of TR4 collector and the
inverter
coil
is
subseguently
rectified
and
smoothed
by
D5/C44
producing the +12V output for the RAM. Additional smoothing, imparted
by
R62/C45,
produces
the
+12V supply
for the TV
circuits
free
from
noise generated by the RAM. The sguare wave at TR4 collector also
supplies
the
Zener/rectifier
diode combination
D16/D12
generating
-5V
for the RAM, and is available at the expansion port for use by
peripherals. The +12V, +5V and -5V are also made available.
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