[Yaesu] unusual 80m problem

[John Geiger]

I have a 2F serial number (so it has SWR mod) FT100D I picked up in a trade, and it is showing an unusual 80m problem.  It is putting out full power on the bands I tried it on-160, 40, 30, 20, 6, 2, and 70cm.  However, on 80m it will put out full power into a dummy load, but when I switch to my antenna, it is only putting out 50 watts.  The antenna (using an antenna tuner) is showing a 1:1 SWR based on both the tuners meter and the meter in the FT100D, so the rig is seeing a 1:1 SWR according to its own meter.  Can't figure out why it is putting out only 50 watts into a matched SWR. Anyone else have this problem before or have any ideas as to what is going on with the radio?

73s JOhn AA5JG