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3.10
3.11
Checks in the video amplifier
The
FG5
generates all patterns with equal video amplitude at a defined white
level. As a result all signals should be displayed by the TV receiver at approxi-
mately equal amplitude. Should the grey scale and the vertical lines, for example,
indicate different signal amplitudes, check the video amplifier bandwidth
and the
IF amplifier pass band.
The signal amplitude for the video stages is best set using the cross-hatch pattern
or the grey scale. Note that the VHF modulator of the FG5 delivers the best sig-
nals in channel 5 (channel 40 for UHF)
in respect to linearity and carrier sup-
pression.
Faults in the grey scale are best identified if the oscilloscope display of the video
output
signal
differs
greatly from
the required
display
(Fig. 4). The
distortion
could
originate
in the video
channel
or in the RF/IF stage,
perhaps
by over-
modulation when the control potential is absent.
Grey setting
To
ensure
perfect monochrome
reproduction
from
colour
receivers,
the beam
current
relationship
of all three guns
must
be set throughout the full system
characteristic.
With
correct setting no colouring
must
be observed
even
when
the tube
is modulated
from a very low to a very high level. This applies
to
cross-hatch and grey scale patterns. Likewise, there must be no colouration when
the contrast and brightness controls are turned from their high to their low set-
tings.
PAL-Switch
The 4-vector pattern is used to check the PAL switch. With a faulty PAL switch
there will be an incorrect colour rendering of the two central bars (green —
red).
To check for perfect synchronisation of the PAL switch, operate the PAL switch @)
several times.
Immediately afterwards the correct colour sequence must again appear.
Automatic
colour killer
With any colour pattern displayed, reduce
the VHF/UHF
signal until the display
becomes
excessively
noisly. This
is when
the colour
killer should
respond,
changing the display over to monochrome.
If required, reset the colour killer to
the threshold level so found.
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