Since the title is" first HF set up" that might indicate a ham operation.
Also saying you Magnum works on the antenna indicates you do not have an antenna problem..(good trouble shooting technique<< seperate and substitute>>)
but the rest is pretty vague.
any change on the output of receive if you just touch the center pin on the coax connector .. some change n the background noise.?
work only on the receive problem.. one bug at a time.. and they may be the same bug.
under stand that the question you asked is like you asking
"""my car wont start,,, whats wrong?
you need to collect alot of data to ask the right quiestins.
what is the voltage at the input to the radio.
what does the S meter show .(tune strong signal on the magnum, then transfer antenna to icom.. tune same place )
take covers off and tap with a pencil eraser all around the boards.. any changes.
wiggle components. tighten mounting screws. push on center off boards to test for cracks.
check knobs.. say pushed in from falling on its face.
eyeball it from end to end. look for machanical problems..
And if you really plan on using it on CB then just start hammering on it with a 24 ounce ball peen.