SC-540 vertical lines problem
Hello. If after cleaning the encoder strip and the encoder sensor won't fix your problem, then the problem is one of the FFC or Flat Cables between the PrintHead Board and the Mainboard. One of them is getting a disconnection every time the Carriage gets to that printing area, so causes little short circuits in the boards and one of the Print Heads (this case the black) is printing Vertical lines.
If I remember correctly, there are 3 FFC long cables between two boards, and the one that carries the signal and data for Black Print Head is the top one.
The best test you can do to make sure it is a problem with the FFC cables, is to turn off the printer (both Sub Power and Main Power buttons) and then take out the covers and disconnect two of the FFC cables in the Print Head Board and also in the Main Board, then reconnect them but in the other position (let say the top cable in the middle cable and the middle cable in the top cable) then turn on the printer and run a print. If the problem moves to another color then you know is a problem with the cable, if not, probably the Print Head is damage printing at the resolution you are using. If that is the case, try to send another resolution and check if the problem appears again
Regards