IBM 3270 Maintenance Manual page 312

1.
Set the Power switches for all work station
devices to Off.
2. Remove the expansion unit signal cable from
both units.
3.
Check the signal cable continuity.
IS THE CONTINUITY GOOD?
NO Replace the signal cable.
YES Replace the receiver card in the expansion
unit. See Section 5,
"Removal/Replacement and Adjustments"
October 1984 3-1700-25