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signal, followed by sine wave frequencies of 0.5
MHz, 1.0 MHz, 2.0 MHz, 3.0 MHz, 4 . 0 MHz and
3.58 MHz. This sequence of frequencies is called
the "multi-burst".
This multi-burst portion of the V I T S is the portion
that can be most valuable to the technician. The
second frame of the V I T S (lines 18 and 2 8 0 ) , which
contains the sine-squared pulse, window pulse and
the staircase of 3.58 MHz bursts at progressively
lighter shading, are valuable to the network, but
have less value to the technician. A s seen on the
television screen, field 1 is interlaced with field 2 so
that line 17 is followed by line 2 7 9 and line 18 is
followed by line 2 8 0 . The V I T S appears at the end
of the vertical blanking interval and just before the
first line of video.
Now to analyze the waveform. All frequencies of
the multi-burst are transmitted at the same level,
but should not be equally coupled through the
receiver due to its response curve. Fig. 2 8 shows
the desired response for a good color television
receiver, identifying each frequency of the multi-
burst
and showing
the
attenuation for each.
equals half the reference voltage (the 2.0 MHz
modulation should be used for reference).
To localize trouble, start by observing the V I T S at
the video detector. This will localize trouble to a
point either before or after the detector.
multi-burst is normal at the detector, check the
V I T S on other channels. If some channels look
okay but others do not, you probably have tuner or
antenna-system
troubles.
V I D E O
V E R T I C A L
S Y N C
V E R T I C A L B L A N K I N G
P U L S E
I F
A M P
T E L E V I S I O N S E T
Fig. 18 Set-up for viewing fields 1 and 2 of V I T S information
sllowable amount
Remember that
— 6 d B
If the
Don't
overlook
the
V I D E O
D E T E C T O R
S O U N D
I F
F R E Q U E N C Y — M H z
Color T V I F amplifier response curve
chance of the antenna system causing " h o l e s " or
tilted response on some channels. If the V I T S is
of
abnormal at the video detector on all channels, the
trouble is probably in the IF amplifier stages.
A s another example, let us assume that w e have a
set on the bench with a very poor picture. Our os-
cilloscope s h o w s the V I T S at the video detector to
be about normal except that the burst at 2.0 MHz
is low compared to the bursts on either side. This
suggests an IF trap is detuned into the passband,
chopping out frequencies about 2 MHz below the
picture
carrier frequency.
channels, carrying V I T S . If the same thing is seen.
V I D E O
V I D E O
A M P
A M P
C O L O R
I F
Fig. 17
Switch to
another
P I C T U R E
T U B E
17
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