How are you changing antenna bands?? What SWR does each band see?? Do you have an ATU up there ?
If the same issue is seen feeding a dummy load then my money is on
1 BS170 failed (probably short, probably due to voltage or SWR) )
which took out the side of IC5 that was feeding it
and/or probably the other BS170 on that side.?
So you are left with half a signal which is being tidied up by the LPF and giving you half power . .? ? ?If you have a scope check the signals on IC5 3 and 8 . .? ?Failing that compare the DC voltages on 3 vs 8 with a multimeter
Bruce Akhurst