My boat's HF rig works quite well, but I do get some pulsating noise when the Seatalk instruments are on. I'd guess the pulse frequency is 3-4 per second and is present whether the autopilot is on standby or auto. Instruments are ST60, autopilot is 400G (I think). The noise is most noticeable in the 2-5 mhz range on my transceiver. I have tried snapping on ferrites here and there without any change. Any suggestions for the best way to locate the source? The autopilot box is a few feet from the antenna tuner and the copper groundplane foil. It is my prime suspect, but the Seatalk cables run all over the boat. Disconnecting the wind, depth, and speed displays from SeaTalk seems to have no effect.

I just got an idea! My 2 meter handheld transceivers can receive on virtually the entire spectrum though AM and FM, not SSB. Perhaps I can use one to lead me to the offending circuit or device.

Other ideas?