Feder is reporting on some changes at CBS Radio’s JACK-FM (WJMK 104.3):

John Sebastian, a widely traveled radio programmer and consultant, abruptly resigned Wednesday after 14 months as program director of WJMK-FM (104.3), the CBS Radio classic hits station known as “Jack FM.”

Peter Bowen, vice president and general manager of WJMK and Top 40 WBBM-FM (96.3), called the parting amicable. “The station is in a good position but needs to get better,” he said.

While a search is under way for a replacement, “Jack FM” will be overseen by Todd Cavanah, vice president of programming at B96. Cavanah also ran programming at both stations before Sebastian was hired in January 2006.

The latest shakeup is likely to see “Jack FM” go back to being an ’80s rock-based station, as it was first conceived. Under Sebastian, the playlist grew to encompass music from the ’60s to the ’90s.

Since it replaced the traditional oldies format at WJMK (and eliminated all on-air personalities) in June 2005, “Jack FM” has been waging an uphill struggle in the ratings.

In the latest Arbitron quarterly survey, WJMK ranked 21st overall with a 1.9 percent share. Among the target audience of listeners between the ages of 25 and 54, it tied for 13th with a 2.7 share.