IBM 3270 Maintenance Manual page 394

Check the continuity of the top card connectors.
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WAS THE CONTINUITY OF THE TOP CARD
CONNECTORS 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
programmed symbols adapter.
October 1984 3-3200-45