Depth finder problem

[talltimber]

I just got ahold of an old Humminbird 128 wide. I already had a transducer mounted under the back seat(aluminum boat). The DF that was on it before was a HB 5000. The old one worked fine when I bought the boat, even at speeds of twenty or more. I hooked the 128 to the leads on the 'ducer that I had already, soldering the connections(shield too). The 128 works fine(very rough picture) at slow speeds but the screen sort of blanks out at speeds of 10 mph and more. It has the scale numbers at the margins, and a small line showing, I assume, that it is scrolling. The scrolling indicator, or that's what I assume, is moving right to left across the top of the screen. It shows no features, including the bottom, however. Got any ideas what it could be? Could the sensitivity setting have some effect on the pickup at speed? I see no "speed" adjust on this model.

[Barnacle Bill]

I strongly suspect that the 128 takes a different xducer.