I have an HB 798c si mounted on my trolling motor. It had a bad interference problem when you turned on the TM. So HB sent me an interference kit including chokes and a new si transducer. Fixed the problem! Real happy!!! BUT now the interference is back a few weeks later and I haven't changed or added a thing..
ANY IDEAS??

I know there was a thread over on Bass Boat Central talking about this recently. Doug V browses both here and there, so he or one of the guys from HB may be along to help out. I have just installed a 997 on the TM so I am beginning to pay more attention to the problems, hope it gets sorted out quick.

Did you try turning the noise filter up? The Interference kit kills RF/Electrical noise 99% of the time. I would check the ground fuse in the head if this is a MinnKota. BTW what brand Trolling Motor it makes a difference?

Also, in addition to what Doug stated, Turn off other accessories..

Another thought; is your unit current on updates?

yes, at least as of three weeks ago.

I have the other depth finder turned off and no lights or pumps are running. Any other accessories you have in mind? Thanks.

It has I think setting 1, 2, and 3. which one? What is the ground fuse in the head and how do I check it? Sorry for my ignorance.
I PM'd you on the TM. should have just put it here.

I tried to set the noise filter. I had done it once before but i forgot how to do it and the manual says nothing about it. How do you set it? And how do you get instructions that are updated to the new software?

I bet its in your TM, your bushing might be wearing out. And your unit is picking up the noise from it.

Go under SONAR Tab and you will see Noise Filter. Turn it up until your interference goes away. Not sure if you need to be in the advanced mode to get the Noise Filter option to show up or not?

Mike

Electrical Motors create harmonic noise called Radio Frequency or Electrical Noise - similar to older vehicles with alternator static on some radio stations. if the noise is similar to the frequency of the trolling motor then it can become transmitted into the unit. I have seen 20 brand new identical boats and one had RF/Electrical Noise concerns and the other 19 did not and all components where new. First check is the fuse in the head if it's a MinnKota. If not Humminbird has taken a pro-active approach and has Interference kits to Kill RF/Electrical Noise. Give HB Custoemr Service a call 800-633-1468 - new units many of these components are now standard equipment.