Hi Ichigo,
Before you start fiddling around with the pulleys and string and tuning pointer, why don't you get a list of FM stations in your area with their broadcast frequencies and test the Marantz on other stations. If indeed, the pointer and string have become dislodges or are slipping, it isn't "easy" to fix. It's kind of tricky and you have to be able to get at the pulley system and pointer.
Yes, I love the look of those old vintage tuners. It appears your Marantz is similar to a huge old Pioneer unit I had (it was a stereo receiver) that used a manual tuning condenser (top left corner of your picture) with pulleys and string. If memory serves, mine had a Phase-Lock-Loop tuning circuit that would optimally lock in the station frequency. Your Marantz at least is solid-state, so you don't have to worry about wonky tube vagaries causing the erratic performance.
Alan
