I've had another problem with my Raymarine AIS500.
notice incc=deed states to install it well away from other VHF
transceivers or SSB.
Mine is quite close.
I have developped a receiving problme in the AIS - at first intermittent, later permanently where it starts receviving then after a few minutes it stops receiving but the Raym plotter still sees it as "AIS ON".
My suspicions were the SSB wire antenna quite close (3 meters) to the dedicated AIS antenna hence overloading the AIS'receiving unit and blowing some semiconductor?
- I received a new AIS unit under warranty.
I now use the masthead VHF antenna through the antenna splitter on the AIS unit (I kno...not optimal...) and every time I use the SSB I switch off the AIS AND switch the AIS antenna in to a 50 Ohm dummyload.
...just in case.....
though the issue was never proven....
If I were you I would use some clip-on ferrites on the DC power lead to your AIS and also on the antenna IN at the AIS.