Greg has not been able to visit the forum in a while due to his training responsibilities at the Eufaula plant ... (We all miss his insight) ...
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It sounds like you're experiencing crosstalk interference from running both units at the same time ...
Unfortunately, it's not possible to set HB units to different 2d frequencies ... HB units ping 200kHz all the time for digital depth ... Even when the Beam Frequency is set to 83kHz the 200kHz is pinging in the background ...
To eliminate crosstalk interference 1 unit must have Sonar turned off... Or have both units use the same xducer thru Ethernet (you didn't say if your 798 is the HD version capable of Ethernet) ...
Rickie