There's a shop on Huancheng Nan Lu, just east of Beijing Lu, on the north end of the street. They have a team of people in the back of the small shop that specialise in repairing and cleaning cameras.
There used to be 2 camera shops on that little strip. The first one (coming from Beijing Lu) sells mostly new camera equipment. Go to the second one. It doesn't look like much, but they know what they're doing.
I don't think they speak English.