To me, the death of The KFRC (original Top 40 - not the Oldies station) is the result of RKO General's stupid decision to sell 106.1 in 1977. It took (IIRC) about 7 years for KMEL to switch from AOR to the "All Hit" format which was the final nail in KFRC's coffin, along with the other nails - K-101, KYUU, and Hot Hits KITS to a lesser extent.
I've never been in the radio biz, but it was pretty clear to me by 1977 that FM was the future - most everybody I knew in my age-group (baby boomers), had abandoned AM for music a few years before 77. K-101 in particular, was getting solid ratings in the early 70s...though their format at the time was closer to "chicken-rock" than real Top 40. I realize that there was still some hope at that time for AM stereo, but for the most part, the ship had sailed.
RKO never seemed to know what to do with 106.1. IIRC, they simulcast Dr. Don Rose, but otherwise ran poorly executed automated pop formats - "K-106" being the worst. Even in those days, with FCC rules about simulcasting, the smart thing would have been to put the same Big 610 format on the FM, and simulcast as many hours as were then allowed.
Llew: You're right. RKO should never have sold 106.1.
The rules allowed up to 12 hours of simulcast. KFRC chose 9 p.m. to 9 a.m. Poor choice. They should have gone for 6 a.m. to 6 p.m. or broken it up by simulcasting DDR 6-9 a.m., going to an automated hit format with zero personality from 9 a.m. to 3 p.m., back to a simulcast 3 p.m. to midnight and back to the automation midnight to 6 a.m. That would have given the younger audience for afternoon drive and evenings the ability to listen on FM, and protected DDR. At some point, you might have been able to ease the AM format completely over to FM, and then automate 12 hours of the AM.
And they started the simulcast in 1974, so I would have done the above from the beginning.