Furuno FCV 620 / Airmar P66 50hz Issue



Have the furuno 620 fcv paired with airmar p66. Sounding with 200hz seems to work just fine I can sound fish and hook them up. Now the 50hz is where the problem lies, I can get awesome readings while stationary or up to about 3/4knots once I travel over that speed I get solid interference which seems to work its way from the bottom to the top.
Interference can be filtered out with gain but it has to be to such a point that no readings for any fish will show up.
Things I have tried. Rewiring, Using on a isolated battery, many extra holes in my transom from repositioning transducer. I think I can rule out cavitation problems as it doesn't actually lose contact with the bottom.
Have revved the engine out of gear which has no effect on the signal so it only presents it self when in motion and engine in gear.
Here is a youtube video showing what happens 50hz is on the left side and the adjustment to the throttle is literally about half a centermeter.
Any help and I will be eternally grateful...
Death adder,
I have a friend with the same set up FCV-620/P-66 that showed similar noise on 50Khz at certain RPM's. He was running a Merc IO on his 29ft Trophy. We traced it to his alternator kicking on. He replaced the P-66 with a TM-258 he found on EBAY but I don't know if the problem went away. He is out fishing right now but I will ask him when he returns.



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Yeah that's the first pic ive seen that looks exactly like mine. Does the alternator kick in when the engine isn't in gear? Because I have tested reving the engine out of gear and don't get the interference. I have also tried it on a isolate battery from the rest of the boat with the same issue. Although the battery that the engine charges is sitting behind the transducer maybe this could be a issue?
Death Adder,
If I remember correctly I don't think his set up allowed you to rev the engine without being in gear. In his case we were able to disconnect the field wire going to the alternator which disabled it for a test.

Dear snips,
I have approximately the same problem. I have a Furuno fcv 585 with a tm260or270wide.
While trolling 200khz works fine where as the 50khz gets full of noise( red clutters) like the picture you posted. If I reduce the gain to 1 which is very low it gets better but I can’t detect fish cause it’s too low and I noticed something too ( gain back to normal) is that when I raise the trim engine switch to 3-4 the noise reduces in the first 20 meters column then I get the clutters back under 20 meters. TVG is set to medium. Any help or ideas?
Thank you in advance