The other day I had the opportunity to repair an HP10c calculator. I had not seen such model before! Yes, HP11c and HP15c are better, but the HP10c is the rarest of them all. It was a well used, worn unit, that did not work. I think that the problem had to do with dirt and corrosion, because once cleaned internally with anti-rust fluids it worked just after assembly.
I was able to make it work, though:
When opening it, it was of a more advanced design that I am used to, with a fully covered back that showed a large circuit, instead of the several smallr found in HP15c or old HP12. It also had a flexible circuit I was not used to.: