Kent,

if you go this route, spend the extra money and get the interface cable 
to connect between the tuner and the 6 pin molex jack on the rear of the 
590.    With this installed, the radio's internal tuner is deactivated. 
   To tune, you hit the tune button on the radio, the radio drops to a 
low power cw carrier and the  MFJ tuner tunes.  When a match is found, 
the radio reverts to it's original mode and power level.

There are other ways to accomplish this without the interface cable, but 
   the extra expense is well worth it.

73, Steve KW3A