Equipment Lightning Protection - IBM 3278 Series Maintenance Information

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
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