If your cable isn't too long, you can temporarily make a "poor man's shielding"...
- get some aluminum foil
- and some Scotch tape
- cut the aluminum foil in long strips of the same width as the circumference of your serial cable
- put some Scotch tape on the table upside down (securing the ends)
- put the aluminum foil strip on top of the tape so that a 1/4" of the tape isn't covered
- wrap the aluminum foil + tape around the serial cable, aluminum side toward the cable
- stick the tape so that it stays
- wrap it again with tape so that none of the aluminum can short to something else (insulating it)
Then put it back into service. A real version of a shielded cable then connects the ground wire to the shielding but this version is good enough. If you find that this poor man's version allows you to connect to your printer, then go buy an expensive one.