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3.15
3.16
3.17
A number of incorrect adjustments will cause a number of different: fault indl-
cations without any form of interaction. This simplifies diagnosis and final rectifi-
cation.
on
os
If it is necessary to carry out this adjustment using an oscilloscope then it is
preferred to use the 4-vector display pattern.
With correctly adjusted PAL delay decoder the (R-Y) and (B-Y) input must only
produce a display.within the range of the two corresponding bars (Figs. 12 a, b).
With
incorrect adjustment a lesser signal will also appear
in the range
of the
two other bars (Figs. 13 a, b).
The
phase relationship for the (R-Y) and (B-Y) synchronous
demodulators
is set
by
aligning
for maximum
amplitude
across
the
demodulator
outputs
and
to
obtain the displays shown in Figs. 14 and 15.
The
oscilloscope
display of the demodulated
(R-Y)
and
(B-Y) signals
derived
from
the special
test pattern
also indicates
fault conditions
in the PAL
delay
decoder and the synchronous demodulators.
The
reference
line is displayed
for the grey
bars with a correctly aligned
re-
ceiver as shown in Fig. 16. Splitting of the reference
line will occur when the
PAL delay decoder is incorrectly set. This condition
is shown
in Fig. 17. Incor-
rectly aligned synchronising demodulators will produce an oscillogram as shown
in Fig. 18 whilst
a combination
of both faults will cause
a combination
of both
fault oscillograms.
4.433 MHz Trap
The video
channel
contains
a trap or absorption
circuit to suppress
rest com-
ponents
of the 4.433MHz
colour
sub-carrier.
Set
the
receiver
to maximum
contrast, minimum colour contrast. Connect the oscilloscope to the tube cathodes
in case
of colour difference
modulation,
to one
of the colour modulation
elec-
trodes
in case
of RGB
modulation.
Select
a colour
test pattern
and
set the
4.433 MHz trap or circuit to minimum superimposed
RF component.
Sound carrier interference
With the 6 MHz sound carrier added, receiver sound traps may be adjusted
in
similar manner. Again tune for minimum interference lines displayed on the screen
and caused by interference between the 4.433 MHz and 6 MHz frequencies.
Sound channel
The 1 kHz modulation signal may be added to the 6 MHz sound carrier frequency
and allows checks through the full sound channel.
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