Testing
and
Troubleshooting
3-13
How
It
Works
A color CRT tube differs from a monochrome tube
in
that
it
has three separate cathodes,
one
for
each primary color. Otherwise, the tubes are similar. The
98760A tube has three screen grids, but
they are connected
in
parallel, with
one
adjustment affecting
all
three.
In
order to obtain a certain spot size
on
the CRT face, the lowest cut -off cathode
is
set to
OV
and
the
screen grid voltage adjusted to achieve cut-off.
Then the cathode bias
is
used to set the threshold of visibility for the low cut-off gun. The gain
pot
is
then adjusted to obtain the proper step 15 intensity, since step 15 equals threshold plus gain. This
procedure
is
then repeated for the cathode with the middle cut-off
and
once again for the cathode
with the high cut
-off.
The Intensity Adjustment Procedure
Now that you know when to perform the intensity adjustment
and
how
it
works, you are ready to
start the actual procedure.
Using the System Functional Test discs,
first
load the test operating system (Test O.S.). Then load
the mainframe test disc
and
select AGRAPHICS, INTERACTIVE. Step through the procedures as
directed. The video board contains
all
clamp pots, gain pots,
and
test points (Figure 3-2). The
sweep board contains the screen grid adjustment (Figure 3-3), as
well
as several other adjustments.
TP301, GREEN
GAIN
POT,
CLAMP
POT,
GREEN GREEN
Figure 3-2. Video Board Pots and Test Points
09020-90037, rev:
1185
CLAMP
POT,
BLUE
GAIN POT,
BLUE
TP201,
BLUE
GAIN
POT,
RED
CLAMP POT
RED
TP101,
RED
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