1.4 Sub-Carrier Oscillator. The chrominance sub-carrier oscillator
frequency should be 4.433619 MHz ± 50 Hz. This frequency may be
checked/adjusted
by
usinq
one of two
methods,
listed
in (a) or (b)
below.
Apply power to the power socket usinq a bench power supply set at 9V.
Current consumption will be approximately 500 mA to 700 mA for the 16k
Spectrum and 700 mA to 900 mA for the 48k Spectrum.
(a) Feed the Spectrum modulator output via a co-axial cable into a
standard colour TV receiver. Measure the frequency of the locked
TV chroma sub-carrier.
(b)
Connect
pin 17 of
IC14
(LM1889)
via a 4.7 pF
capacitor
and a
lead
to the
frequency
meter.
It is
recommended
that
this
is
done
usinq
a jiq
made
up
from
an 1C
test
clip.
Pin 18 of
this clip
must be removed to minimise stray capacitance. Connect a 10k ohm
resistor between the input terminals of the frequency meter.
Measure the frequency and adjust trimmer TC2 if the measured
frequency is outside tolerance.
1.5 14 MHz Oscillator. This frequency is not readily set by workinq to a
specified frequency and tolerance. On Issue 2 boards trimmer TCI
should be adjusted to minimise the effects of beat frequencies which
cause waves of distortion to flow across the screen. These are
particularly apparent with certain colour combinations, e.g. red INK
or green PAPER, and the phenomenon is sometimes referred to as 'dot
crawl'.
It is
stressed
that
adjustment
of
this
frequency
is not
straight-forward, as deviations due to temperature chanqe are easily
visible;
tuninq
should
be
adjusted
to
minimise
rather
than
remove
this
effect. Proceed as follows:
(a) Set up the
Spectrum
to
display
qreen
PAPER
with
red
INK.
(b) Type-in three or four lines of characters and monitor the screen.
(c) Adjust trimmer TCI to minimise the effects of distortion.
3.2
Pot. IC14 Factory Settinq Optimum (mV) Overall Ranqe
Pin No. (mV) Voltage Tolerance (mV)
VR1 4 130 ± 20 +50 -5 +45 to
+150
VR2 2 75 ± 20 -50 -50 -45 to
-100
+5
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