The 2270 FM signal strength meter is driven off the P551 board (H553), so it is possible that you simply have no power to the P300 FM decoder board.
Since the AM works, the 13V power supply (J801) should be OK. I would look for a broken wire at J303 or one of the connecting points. The wire probably is daisy chained at several points on it's way from J801 to the P300 board.
Tom