  During the first 20 years of operation, the Fermilab Tevatron developed
strong coupling between the two transverse degrees of freedom. The required
strength of the circuit of skew quadrupole magnets used to correct for
coupling, though capable, had increased by an order of magnitude since 1983.
Changes made to the Tevatron for colliding beams operation had altered the skew
quadrupole corrector distribution and strong coupling became evident, often
encumbering routine operation during the colliding beam physics run. It was
discovered that the superconducting coils of the main dipole magnets had become
vertically displaced relative to their yokes since their construction. The
ensuing systematic skew quadrupole field introduced by this displacement
accounts for the required corrector settings and observed beam behavior. The
magnets were adjusted in situ over the course of several long maintenance
periods and the Tevatron performance improved as a result.
