AM uses an internal fixed rod antenna on the tuner board so that may be why it's not effected. Since this seems to have happened suddenly, the first thing I would check is the antenna mast support tab screws inside for the antennas on the back cover. They transfer the signal from the antennas to the wire going to the tuner board. Also, if care is not taken when the back is removed from previous servicing, the harness connection to the tuner board can get pulled and create a cracked solder joint with intermittent connection. So while it's apart, try wiggling the FM antenna connector on the tuner board to see if the signal increases/decreases with movement.
Once you get out of this area of troubleshooting then cleaning switches and checking tuner alignment is next. But proper alignment takes special equipment. You certainly do not want to "tweak" any adjustments as it is next to impossible without test equipment. Usually when a tuner board starts to get weak and drift, they do so slowly. So a sudden change would likely be a lose connection somewhere (cold solder joint, dirty selector switch, loose antenna wiring, etc.)