Hi Nenad,
One more perhaps superfluous question. In your first post you say
I connected the same signal (10MHz <1Vpp) to both heads inputs just for
test (let's just ignore this blasphemy for now since both inputs are 50Ohm,
The heads have different connectors, SMA and BNC, and your signal generator perhaps one of these. You need a power divider or a splitter or a T and at least an adapter to from one type connector to another. All these things could introduce faults. Are you 100% sure that the sine wave signal arrives "inside" the S-4?
Albert
