Martin,Is the headphones jack a standard 2.5 or 3.5 stereo jack?.
These normally have a ‘switch’ via a metal link that opens or closes the circuit. Can you see and verify this is actually working and is contacting either or?.
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Martin
Is it buzzing from both headphones and speakers at the same time when headphones are plugged in?.I can just hear a busing from the earplugs and the speakers