I am assuming you've already followed basic trouble-shooting routines such
as checking grounding of the equipment..other equipment operating nearby
(although not necessarity in sight), ballasts from flourescent fixtures
that might have gone bad etc.
then try switching out the components one by one...pre-amp as i suggested
has done that to me before.
Joseph Danto, Ph.D.
Professor & Program Director
Speech Pathology Audiology
212 340 7806
201 567 8408
201 567 7551 (facs)