Plotter printing wrong colors

I am using AutoCAD 2014. I have just purchased a HP1050c plotter and had it worked over by a reputable service company. It prints all test pages beautifully. The problem I am having is that grays print yellow, some greens print yellow also. Basic colors seem to print OK. I am not sure if it is an AutoCAD setting, plotter setting, or printhead issue. The service guy doesn't know much about AutoCAD so he wasn't much help. I have tried different drawings and settings and it always prints any shade of gray in yellow. I am not finding any related topics on the internet. The grays in this drawing are the architectural background for a piping isometric drawing. Nothing is XREF. Can someone please give me some pointers. Thanks in advance to all who reply.

Every plotter has the ability via Printer Properties to print a test page. Have you used this feature? Do all colors print as expected?

Are you experiencing similar issues with color prints when using other Windows programs?

You did not mention AutoCAD pen settings. Have you checked them?

Does AutoCAD still ship with the Color Wheel drawing? That should give the printer a good workout.

We solved this problem. The answer was the driver. I had trouble finding a driver for this old plotter in Windows 7 version. It prints beautifully now. Thanks to everyone for the reply's. I'd be lost with out all of you.