T EVIS i - AmericanRadioHistory.Com

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MAC glanced along the service
bench to where Barney, his helper,
was scowling down at a lone
nickel in the palm of his hand.
"What's eating you, Little Sunshine?"
Mac asked. "If that is supposed
to be your last nickel, and if
you are putting on this production in
the hope of getting a raise, well -"
"No, not that," Barney interrupted.
"I was just looking at the profile on
this nickel and thinking that Lo the
Poor Indian is rapidly vanishing from
the American scene for the second
time."
"How do you figure ?"
"Well, the Indian -head test pattern,
or any test pattern for that matter, is
rapidly becoming extinct. Soon you
will stand more chance of seeing a
flying saucer than of glimpsing a test
pattern. Stations put them on only for
a few minutes in the morning hours.
Worse yet, they often employ this
period to make adjustments on the
transmitter. The net result is that
you can never be sure nonlinearity
seen in the receiver pattern, for example,
is not actually being transmitted;
and if you start to correct the
pattern at your end, they may be doing
the same thing at their end. While
you're 'zigging' they may be 'zagging'
'
"True, but why get all worked up
about it
"'Why get all worked up about it ?'
the man says," Barney mimicked. "I'll
tell you why: a whole TV service
procedure has been built up around
the test pattern. Reams and reams of
paper have been devoted to showing
the technician how to detect everything
from shorted yoke turns to a
fly standing on top of the horizontal
68
MAC'S RADIO
SERVICE SHOP
By JOHN T. FRYE
"VANISHING AMERICAN"
output tube plate cap just by staring
fixedly at a test pattern. 'Study the
test pattern,' preached the manufacturers;
'Study the test pattern,' advised
the service magazines; 'Study
the test pattern,' commanded the TV
pundits in their service lectures and
books; and now -no test pattern!"
Mac chuckled at Barney's melodramatic
vehemence. "There's a lot
of truth in what you say, Red." he
drawled. "No one can deny TV stations
make with the test patterns like
a circus con man manipulating the pea
in the old shell game: 'Now you see
it; now you don't.' Still and all, I
think we can salvage some of the effort
we put into studying those patterns
for symptoms.
"Especially can we do so if, in our
studying. we took the trouble to look
behind the test pattern symptom to
grasp exactly why a particular circuit
defect resulted in precisely that kind
of pattern distortion. If we did that,
we'd have little trouble in finding the
same symptom in a picture that we
taught ourselves to discover in a test
pattern. Remember that after all a
pattern simply presents a collection of
typical picture elements all in one
'scene' so that it can be quickly determined
if the set is capable of reproducing
every sort of picture. If you
have a quick eye and can concentrate
on picture elements rather than on
the picture as a whole, if you have
made a thorough study of how receiver
troubles affect the deflection and
modulation of that flying spot of light
as it traces out any design on the face
of the tube, you can learn almost as
much from studying an actual picture
as you can from looking at a
test pattern."
"What do you mean by 'concentrating
on picture elements rather than on
the picture itself' ?" Barney wanted to
know.
"Well, suppose you are looking at
Marilyn Monroe in a swim suit standing
in front of a picket fence. The
idea is to concentrate on the fence,
noticing if the individual pickets stand
out clearly and sharply even as the
camera dollys out from the scene. If
they do, the horizontal resolution is
good, and -"
"Say no more!" Barney interrupted.
"There is a limit to what you can expect
of human flesh and blood."
"It will not always be that hard,"
Mac said with a grin. "For example,
you can learn a lot about vertical and
horizontal resolution by watching the
stripes on a football referee's shirt as
he moves about the field. If you can
still distinguish these vertical stripes
when he is standing upright a long
ways from the camera, the horizontal
resolution is good. On the other hand,
if the shirt looks a sort of tattle -tale
gray until he stoops over to pick up
the ball and then the stripes stand out
clearly, you will know that the vertical
resolution is superior to the horizontal
resolution -as is quite often the
case.
"Other tests are the picket fence I
mentioned, checked material, and
brick walls. As the camera moves
away from checked material, the
speed with which the individual
squares start running together is an
indication of the resolution of the set.
Some plaids will look like striped material
at a little distance, and the way
in which the stripes run will give an
idea as to which kind of resolution is
best. Mortar cracks in a brick wall
also furnish reliable information.
When the horizontal resolution is
good, the short vertical lines between
bricks will stand out as clearly and
whitely as the long horizontal lines;
but when this resolution is poor. the
vertical lines will be dimmer and more
of a gray color."
"How about linearity? I never had
much luck lining that up without a
test pattern."
"A bar generator is the logical instrument
for this purpose, and since
it is comparatively inexpensive, there
is little excuse for the TV technician
not having one. There may be a few
times, though, when he will be called
upon to correct the linearity circuits
when neither a test pattern nor bar
generator is right at hand. With a
little patience he can still do it. Vertical
linearity can be easily and quickly
checked by adjusting the vertical
hold control so that the picture drifts
slowly downward and watching the
vertical blanking bar as it moves down
across the face of the screen. This bar
should stay the same width. If it is
wider when at the top than at the
bottom, you know the picture is
stretched at the top, and you correct
this with the vertical linearity control.
If the bar grows wider as it
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