IBM 3270 Maintenance Manual page 558

17. The adjustment for the diskette drive speed is
a variable resistor on the servo board. To
adjust the speed, carefully insert an insulated
screwdriver down past the left end of the
diskette-drive signal cable to the adjusting
screw on top of the variable resistor.
(Rear View)
(Top View)
October 1984 5-601-25