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I suspect that the printhead may be defective. Or perhaps, need cleaning.
How old is the calculator?
I presume it has used black printing for most of its usage lifetime.
If so, removing the ribbon, turning the calculator Off, and removing the paper roll also, then taking a small bit of good fine rag cloth moistened with alcohol, and rubbing the printhead off with the rag cloth, might clean the black gunk off the printhead so that black printing (once the roll of paper, and the black/red ribbon have been put back on and secured) may be restored to good quality.
Let me know how it turns out. Thanks.