To localize trouble, start by observing the VITS at the
video detector. This will localize trouble to a point either
before or after the detector. If the picture quality of each
channel is different, the trouble is in the tuner or antenna
system. If the picture quality is the same for all channels but
the multi-burst is abnormal, then the trouble may be in the
IF stage.
As another example, let us assume that we have a set
on the bench with a very poor picture. Our oscilloscope
shows the VITS at the video detector to be about normal ex-
cept that the burst at 2.0MHz is low compared to other
burst signals. This suggests the IF trap is detuned into the
passband, chopping out frequencies about 2MHz below the
picture carrier frequency. Switch to another channel. If the
amplifier requires realignment. If the picture quality is the
same, then our reasoning is right, and if the picture quality of
another channel is normal, the FM trap at the tuner input is
misadjusted. Fig. 15 shows the method of viewing the VITS
waveforms.
V I D E O
S T A R T WITH
2mS
See text)
P I C T U R E
T U B E
Fig. 15. Set-up for viewing fields 1 and 2 of V I T S information
1. Set a color TV receiver to the station transmitting
color signals containing VITS.
2. The method shown in Fig. 16 is used to obtain Field 2
vertical signal on CH1.
3. Set the oscilloscope and the receiver for operation.
Connect the CH 1 probe (set at 10 : 1) to the test point
of video detector or other desired test point in the
video section of the television receiver.
4. Set the SYNC switch as follows:
A. If the sync and blanking pulses of the observed
video signal are positive, use the T V + switch
position.
B. If the sync and blanking pulses are negative, use
the T V — switch position.
5. Adjust the sweep time V A R I A B L E control so that 2
vertical fields are displayed on the oscilloscope screen.
6. Connect the CH2 probe (set to 10 : 1) to the same test
point as does the CH1 probe.
7. Set the MODE switch to DUAL position.
8. Place the sweep time V A R I A B L E in the CAL position.
9. Set the S W E E P TIME/DIV control to the 0.1ms
position to expand the display. The VITS information
will appear toward the right hand portion of the ex-
panded waveform displays. The waveform informa-
tion on each trace may appear as shown in Fig. 13.
Because there is no provision for synchronizing the os-
cilloscope display to either of the two fields which
comprise a complete vertical frame, it cannot be
predicted which field display will appear on CH1 or
CH2.
10. Pull the <>POSITION control outward to obtain an
additional X 5 magnification. Rotate the control mov-
ing the trace to the left until the expanded VITS infor-
mation appears as shown in Fig. 13. Because of the
low repetition rate and the high sweep speed com-
bination, the brightness level of the signal displays will
be reduced.
11. Once the CH1 and CH2 displays have been identified
16
V E R T I C A L
S Y N C
P U L S E
V E R T I C A L
B L A N K I N G
l
F
A M P
V I D E O
D E T E C T O R
V I D E O
A M P
V I D E O
A M P
S O U N D
I F
C O L O R '
I F
T E L E V I S I O N S E T
