For RCA stuff I have found Clarence Kane of Enak Mic Repair to be tops. His details:
420 Carew Avenue
Pitman, NJ 08071
(856) 589-6186
Fax: (856) 589-7791
Contact:Clarence Kane. Website:
http://enakmic.com/
Stephen Sank is also supposed to do good work, but I have never used him so cannot vouch for it. A little web searching will, however, find a ton of people who vouch for him:
Stephen Sanks website