HP 7475A plotter problem

Here is hoping there is someone here familiar with the HP7475A 6 pen plotter. I have one that runs the demo self test plot with no errors. When plotting a 18k file (the drill in KeyCad) it inserts two extraneous lines at the same place every time in the plot(one on the c in kcf drawing title and one on the drill body). I know KeyCad uses HP7470 drivers but they work when plotting the drill.kcf on three other 7475A's. TIA.
AL

Once again I asked before looking. The plotter uses 4 sections of a DIP switch to select parity.
One section was intermittent. A few times setting it to on/off cleared up the problem. I think the
section that was not making contact was allowing that input to float.
AL

Nice fault, you did well to find it!

Might be a cracked solder joint, if it recurrs try fixing it by flicking but if it continues to fail doing a quick touchup on the solder joints with fresh solder probably wouldn't hurt. Defiantly agree with Nigel, good catch =)