Dan's on to it -- If it's noise in the stereo, it might be solved by something like a noise filter.
BTW, on my old Magnatek, some of the fuse positions on the fuse block were set up to be filtered by the battery, whereas others were for noisy appliances like fridge, fan, furnace, etc. You might solve the problem by moving some wires around.
It's not noise at this point. The stereo has been odd from day one. It worked perfectly at first -- until the first time I turned on the Maxx fan. Drove the stereo crackerdog, literally crashed it. After a reset and and a 30 minute wait all was well. After that, turned on the fan first, then the stereo, no worries.
At NOG Byron suggested an in-line filter, but I didn't get to it, and then in the driveway, I accidentally turned on the fan second -- and everything was fine.
I changed out the battery before the last trip, and suddenly the stereo will only work on battery power. Switch to shore power, it makes blinky lights
and funny noises. Switch back to battery, all is well. Reset doesn't affect. Fan, doesn't affect. Very strange. Switch back to the previous battery, still no better.
This elderly magnatek has one fuse, and everything is connected in parallel right out of the back from two wires. At SOG Gene L and Peter H concurred on converter being a probable cause, on the theroy that it was putting out raggedy (my word) dc power.
If nothing else, I'll get a charge circuit for the battery out of it.