Sounds like it may be the low pass filter board. I would check relay 11 and 12 for 12 volts when above 21.5 Mhz ( P-03 pin 7 ), also you could check from the low pass filter board input to the normally open contact at relay 13 ( or the proper side of TC-01 or C-38 to see if you have continuity through the filter / or from the input of the low pass filter board to ground and see if a cap shorted.
You could also test for output right at J-02 of the 100 watt PA board.
This link may help. http://www.radiomanual.eu/schemi/FT747_sch.pdf
Let us know what you find.