Phaser 7760 print issue

I have two old Phaser 7760 machines. One is for parts the other I print with. I swapped in the accumulator belt assembly from one to the other to resolve a print defect caused by a hole in the transfer sheet. When I put it back together and ran a print it has short vertical magenta lines in one quarter of the printed area and no magenta anywhere else. I put back in the old accumulator belt and it is still doing it..

I've never seen this before. Anyone know what is going on?