> P.S.: one reason I've got it open is that the keyboard S/H suffers > from S/H droop. Is this likely to be just the capacitor, or am I > possibly looking at having to replace one of those FETs? Try cleaning the S/H circuit area with 91% isopropyl alcohol, and dry thoroughly. Dust, smoke etc. with humidity on the PCB is enough to make S/H's droop slowly.