I have one that served me well for fifteen years.  Many of those years I didn't use it at all, but it worked fine for the last several.  Until I tried to upgrade the memory.  There is a slot for memory expansion, and the manual said it would work with any generic chip.  It didn't work with one a friend loaned me, so I purchased one for $100 from Calcomp.  That didn't work either, and I damaged something so that the control buttons didn't work.  Then it stopped responding to plot commands at all.  I gave up and purchased an HP111, which is a different topic.