All those models have the exact same transducer.
There is no such model as a 1198 HD, just the 1198c SI Combo.
The difference between the 998c SI Combo and the 998c HD SI Combo is the backlighting and that is the ONLY difference.
You may want to change the transducer to a newer one if you have been using it for a few years. They do wear out with use.
Humminbird Helix 12 SI, 1198c SI, 1198c SI, ONIX 10 SI NT, 360, RC-1