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3.12
4.433 MHz oscillator
3.13
3.14
The 'colour synchronisation will disappear if the frequency of the reference Os-
cillator. differs greatly from the standard frequency of 4. 433618 MHz.
'No concrete rules can be quoted for the readjustnent of the reference esciistor
because of the many possible circuit variations. The service details should there-
fore be consulted.
It must be noted, however, that the accuracy of the FG 5 is greatest in an ambient
temperature of 20° C and that at least 15 minutes should elapse after switching
on
before accurate
measurements
are made.
Tube drive
The
characteristic
oscillogram
display can be obtained
from
the corresponding
control electrodes,
depending
on the form of tube drive (R-, B- or G-, or colour
difference). The displays to expect are shown in Figs. 5a, b, c, and 6a, b and they
allow conclusions to be drawn if the results deviate from those shown.
PAL delay decoder and synchronous demodulators
The FG5 can produce a special test pattern to align the PAL delay decoder in
respect of amplitude and phase and to align the phase relationship between the
two
synchronous
demodulators
without
the use
of additional
test instruments,
using only the tube screen as indicator.
The screen
display shown
in Fig. 7 is produced
with a correctly aligned
PAL
delay decoder
and with correctly set synchronous
demodulators,
4 colour
dif-
ference vectors
will be displayed
in the sequence
yellow, green,
red, blue and
with a uniformly grey centre bar.
With incorrect amplitude setting of the PAL delay decoder a coloured line struc-
ture will appear as background to the central bar (Fig. 8). Correct alignment will
cause this "venetian blind" effect to disappear. Incorrect phase alignment causes
coloured "venetian blind" effects in the upper and lower picture. The green bar
will show yellow lines, the red bar will show blue lines (Fig. 9). Discolouration of
the central
bar without
additional
line structures
indicates
incorrect
phase
re-
lationships in the synchronous demodulators.
The
phase
relationship
of the
(B-Y)
synchronising
demodulator
must
be
re-
adjusted
if the two
outer sections
of the grey bar show
any discolouration.
Similarly, the (R-Y) synchronising demodulator must be readjusted if the two inner
grey bars show any discolouration.
If the whole grey bar shows discolouration, both synchronous demodulators
must
be reset, whereby the general phase of the reference
oscillator should
be set
first, the 90° phase shift network next.
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