Can you hear CW? If CW/SSB both do not work, but both AM/FM do then it lends itself to a BFO path problem (oscillators, amp, or mixer). You can check FM station by switching to GEN mode and listening into a maritime mobile or broadcast segment.
There is a good oscillator block diagram on page 49 of the FT-980 Technical Supplement ... but I usually use the big block diagram as the oscillator one over-simplifies things.
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-Mat Breton, N8TW
Mat Breton N8TW