Marine Electronics Questions? Ask a Technician for Answers ASAP
The mushroom antenna does not easily come apart, and there isn't anything inside it that you can 'fix'.
This problem is usually due to either a bad connection or a bad antenna.
First, check that the antenna cable is securely connected, and not damaged in any way.
If the cable and connections appear ok then try fitting a replacement antenna - after 20 years, the antenna has likely failed and needs to be replaced.