I have a Heathkit GR78 here, which is quite lively on most bands.
But
the top band 18-30 mHz is near deaf and almost impossible to align.

O have one with the same symptoms. Doing a google search I found
someone had poasted a webpage with numerous Heathkit fixes, including
this one. Sorry I can't remember it to post it here, just do the
google and you'll probably find it.

Gary WA7KKP