Yup, sounds like a chisel grind.
You only sharpen the side with the bevel. However, you have to remove the burr from the other side. It's a delicate operation if you want to keep it the way it is.
I don't do chisels myself, but here's my suggestion:
Use the finest grit stone you can lay your hands on. Match the angle exactly and do as little work as possible to get the edge back.
Now draw the blade cross grain on a block of hardwood to remove the burr.
Finally strop both sides using leather with a wood backing, but on the non-bevel side lay the blade flat on the leather and don't use much pressure.
That should do it.
Point is be careful on the non-bevel side. If you create a bevel on there you loose the chisel effect, plus you reduce the included angle A LOT! Reducing the edge strength.
Steve