I've used Deltronics in Chicago on two occasions, and I'm happy with the work they performed. They specialize in repair of electronic keyboard instruments (especially vintage units). In one case, they fixed a broken key on an Oberheim controller for me. They're at 2911 N. Halsted St in Chicago.
The only downside is their minimum charge for anything, no matter how trivial, including opening up your unit to evaluate, is $75. Of course, that charge can be applied toward the total cost of the repair, as one would expect.