IBM 3270 Maintenance Manual page 402

Check the continuity of the top card connector.
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WAS THE CONTINUITY OF THE TOP CARD
CONNECTOR GOOD?
NO
Replace the top card connector(s).
YES Replace the display adapter. See Section
5, "Removal/Replacement and
Adjustments." If the problem persists,
replace the all points addressable adapter.
October 1984 3-3200-53