Belt drive or direct drive turntables can be calibrated, and set up to play correctly. Contacts cleaned, new leads installed if necessary, heads, cartridges and stylus replaced if necessary.
Receivers can be cleaned, inside and out. Bulbs replaced, bias tested, and if necessary, a cap job done, or individual components replaced on the circuit board. This can only be done on vintage pieces, before printed circuit boards, basically pre 1980's.
Speakers can have their surrounds replaced and if necessary, the speaker itself replaced. speaker cabinets can also be re-finished or re-covered in veneer, or paper.