Yeah my tip will probably not be the most helpful if you are looking for a place in the Slope, but I go to a place on Flatbush ave. Caddy corner to the Duane Reade.
It's usually pretty cheap. The place across the street next door to the Pool Hall is a scam. I took some earrings there to replace the hinges, cause they were too heavy and the guy totally ripped me off.
But the place I am talking about is fairly cheap. I don't know how nice you're jewelry is and I only mention that because they seem to charge random prices based on whoever thinks has a lot of money. Before they only charged me like $5 to fix some costume jewelry.
I think the cross street is Sterling. It's a shoe repair / jewelry shop.