Fluke PM5414V - Handbuch (Seite 28)

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3 - 1 4 OPERATING INSTRUCTIONS PM 5414 V
Demodulator check:
This pattern can also be used to pick upfaults in another important part of the color TV, the demo­
dulators. The subcarrier frequency should be applied to the R-Y and B-Y demodulators in the
correct phase; otherwise, all four squares will contain color.
When the phase of the subcarrier fed to a demodulator is correct, the R-Y demodulator will only
demodulate R-Y information, and the B-Y unit will only demodulate B-Y information.
When the subcarrier frequency has a phase difference, this results in R-Y information passing
the B-Y demodulator in squares three and four. Similarly the R-Y demodulator could receive B-Y
information. Then this will be seen as a color information in square one and two. So a phase
error in the subcarrier at both demoduiators (that is, a general phase fault) causes both to pass
incorrect information. The fault shows up a color in all four squares.
A phase fault confined to the 90° phase-shifted subcarrier will only cause one demodulator to
pass incorrect information. This fault will cause color either in the first two or last two squares,
depending on the type of receiver.
12. Purity with a choice of the three primary colors clearly indicated by LEDs. The red pattern is used
for checking color purity, in a correctly adjusted receiver, each electron beam will strike only one
set of color dots or stripes on the screen. Only this color should be visible; the presence of any
other color is an indication that coior purity needs adjustment.
The green pattern provides a purity check for three in-line tubes. Blue is aiso available to check
color performance. The three complementary colors, magenta, yellow, and cyan can also be
displayed by combining the respective primary colors.
These patterns can also be used to ensure that there is no interference between the sound and
chroma carrier. And because the pattern has a 75 % saturation setting, it can be used with VCRs
to align the writing current of the chrominance signal.
In addition to the primary and complementary colors, 100 % white can be selected as well as
black which contains no video information to check, for example, the front and back porch of
the synchronization pulses.