Sounds like U501 and or U502. Those are the N type MOSFETs on the AC/DC board. They are $2.88 each at Mouser.com. Then probably a couple resistors on the famous daughterboard, all on the same AC/DC board. Resistors are roughly 25 cents each. You'd need to check all caps, resistors and diodes on the AC/DC board. Go to the Thompdale.com site for all the schematics and drawings.
cityjim