You may want to post this question on the Tuner Information Center Forum.
He'll probably already know this but he'll need a muliplex generator and a tube tester. Replacement of the filter capacitors is a very good idea.
Some of the older mulitplex circuits could make a whine or whistle if they were not properly aligned. I don't recall if the Scotts did this; my recollection is that they were pretty solid performers. So I'm thinking this might be a tube issue. At any rate a complete alignment is a good idea as with most vintage tuners its probably not been done in decades.