[Arran]

Mike;
  RCA Nipperettes, along with their G.E and Westinghouse clones are like that, two big IF cans under the chassis, attached to the inside of the rear apron to be exact. If you want to get at the bottoms of two or three tube sockets you have to remove them. I haven't run into that under the chassis IF problem on any of my Sparton sets yet, but then again Sparton of Canada did their own thing much like other Canadian branches of U.S companies. Are they mounted, cans and all, under the chassis, or just the coils and trimmers? Maybe the trimmers are open circuit in one of the IFs?
Regards
Arran