Major frame static points exist
around
the
CRT area
of
the
machine. Poor
or
no
contact
at
these points
may cause
intermittent
or erratic display
operation
such as:
• No display (blank screen)
• Erratic raster
on
the
lower
or
the upper half
of
the
screen
• A bright miniature raster
in
the
center
of
the
screen
• The high-voltage power supply cycling on and
off
• A single line on
the
top
of
the
screen
If
the
initial
MAP
procedures
do
not
isolate
the
erratic
display problem,
check
the following static ground
points before retrying
the
MAPs.
•
CRT harness (see Figure 4-5)
• Yoke shield and CRT
connector
(see Figure 5-1)
• Yoke/centering rings (see Figure 5-3)
• Contrast/intensity control (see Figure 4-5)
• Yoke grounding clips (see Figure 5-3)
The
brightness-contrast assembly
is
grounded through
a spring clip
located behind
the
brightness assembly.
Verify this
connection
by
removing
the
brightness
knob and checking for
continuity
between
the
frame
and
the
brightness
potentiometer
shaft. (See Figure
4-5.)
Failure
to
maintain good grounding connections
at
all ground locations may cause erratic
PQR
reset
operation.
1.1.5
Equipment
Lightning Protection
The
3278
is
designed
to
provide a low-impedance
energy path from
the
external coax shield
to
frame
(ac) ground for
potentials above
36
V. Additional
information
on
lightning
protection
for
the
customer
coax (customer responsibility)
is
provided
in
the
IBM
3270
Information
Display System: Installation
Manual-Physical Planning,
GA27 -2787.
SY27-2510-6
(1/81) 1-5
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