I've had an Icom 802 SSB with AT140 tuner installed on the boat for
over 4 years now. It uses the more or less standard 23 ft whip
antenna and has the same ground system that the old SSB had which is
alleged to be a copper strap running down to the bonding system. It
has worked fairly well up until recently but now seems to be
transmitting weakly and the control panel is indicating a high SWR.
Nothing has changed as far as I can determine and all connections seem
to be tight and corrosion free. I'm suspecting that the tuner is not
working properly but other than buying and installing a new AT140, I
can't think of any way to trouble shoot this issue.

Any suggestions?