Sharp EL7050 Colour Plotter Calculator Pen Replacement

This calculator has an ALPS plotter mechanism. The original pens aren't available any more, but you can buy some expensive new ones of a slightly different design. Or you can attempt to make a pen out of a holder and a chopped down ballpoint refill.


From some brass rod I machined a holder which is the same shape as the pen, with a hole at the front which will hold a ballpoint refill.

It fits in the plotter:

The refill used turns out to be critical to the functioning of the pen replacement. If the tip is too wide then it simply won't fit through the hole in the sheet spring in front of the pen holder. If it's too small then it has no spring action. The first refills were too big, the second set (0.5mm I think) were a bit small but worked.

The holder jams now and again, but does write successfully.