As is often the case with vintage audio,

oxidation on contacts is frequently the cause of open circuits on tuning caps, trimmer caps , speaker relays and potentiometers. If the original trimmer caps continue to be tap sensitive after adjustment,  replace them. Normally we would just replace all the trimmer caps in the Adcom tuners, and tuner preamps as intermittents in the trimmers would disrupt proper operation in all of the front end stages. ( and we did not want the customer to return the unit for another trimmer cap replacement )  If you can receive weak stations at 90mhz and 106 mhz ( or there abouts ) chances are good that the front end alignment is fine. 

HRW