Trio CS-1566A, Инструкция по эксплуатации

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This multi-burst portion of the V I T S is the portion
that can be most valuable to the technician. The
second line of Field 1 and the second line of Field 2
(lines 18 and 2 8 0 ) may contain the sine-squared
pulse, window pulse and the staircase of 3.58 MHz
burst at progressively lighter shading. These 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 fol-
lowed by line 2 7 9 and line 18 is followed by line
2 8 0 . The entire V I T S appears at the bottom of the
vertical blanking pulse 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. F i g . 2 4 shows
the desired response for a good color television
receiver, identifying each frequency of the
multi-burst and showing the allowable amount of
attenuation for each. Remember that — 6 d B
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. If the
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. Don't overlook the
chance of the antenna system causing " h o l e s " or
tilted response on some channels. If the V I T S is
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
oscilloscope shown 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 I F trap is detuned into the
passband, chopping out frequencies about 2 MHz
below the picture carrier frequency. Switch to
another channel carrying V I T S . If the same thing is
seen, then our reasoning is right, and the I F
amplifier requires realignment.
If the poor response at 2 MHz is not seen on other
channels, may be an FM trap at the tuner input is
misadjusted, causing a bite on only one channel.
Other traps at the input of the set could similarly be
misadjusted or faulty.
If the V I T S response at the detector output is
normal for all channels, the trouble will be in the
video amplifier.
Look for open peaking coils, off-value resistors,
solder bridges across foil patterns, etc.
With dual-trace oscilloscope operation, the signal
information on each vertical blanking interval can
be viewed separately without trace overlapping,
although the information alternates with each
field. F i g . 1 8 indicates the oscilloscope control
setting for viewing the alternate vertical blanking
intervals.
1. Set a color T V receiver to the station trans-
mitting color signals containing V I T S .
2. The method shown in F i g . 18 is used to obtain
Field 2 vertical sign on C H 1 .
3. S e t the oscilloscope and the receiver for
operation. Connect the C H 1 probe to the test
point of video detector or other desired test
point in the video section of the television
receiver.
4. Set the S Y N C switche as follows:
A. If the sync and blanking pulses of the
observed video signal are positive, use the
V I D E O + position.
B. If the sync and blanking pulses are
negative, use the V I D E O — position.
5. Adjust the sweep time V A R I A B L E control so
that 2 vertical fields are displayed on the os-
cilloscope screen.
6. Connect the C H 2 probe to the same test point
as does the CH 1 probe.
7. Set the M O D E switch to A L T position.
8. Place the sweep time V A R I A B L E in the C A L
position.
9. Set the S W E E P T I M E / D I V control to the 0 . 1
ms position to expand the display by
increasing the s w e e p speed. T h e V I T S
18
RESPONSE-D
B
F R E Q U E N C Y — M H z
Fig. 17
Color T V IF amplifier response curve