HB will send you ferrite chokes that are designed to combat RFI at the frequencies of their own proprietary xducer functions ...
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First you need to determine if the RFI is migrating into the power supply or the xducer cable ...

This can be diagnosed by removing the xducer from the TM completely and testing the xducer in the water away from the TM ...

If the interference goes away ... The RFI is getting into the xducer or its cable ...
If the interference is still present ... the RFI is getting into the power supply ...

Rickie