Are you sure its not just a duff cable?
Can you swap the cable for a known good one? Or even, if its one of those cables with 2 plugs at each end, change both plugs over ta both ends and see if the problem moves to the other channel: problems moves - duff cable.
Its possible you have cracked a solder joint where one of the connectors joins to the circuit board, whilst struggling with a tight plug. That should be a simple fix BUT it will means dismantling the unit to get to the board. Do you have a techie friend with a soldering iron and the ability to us eit?
XXX
November 2016
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