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There will always be some residual hum if it's run off the power line. If the residual hum with the control shorted is acceptable, then you need to discover its source, which obviously is before the volume control.
It's starting to look like a power supply issue. Else there may be a bad ground in the detector circuit. You can disable the detector and see if the hum is gone. More than this, I'd need a diagram.
There may be a poor ground. Trace the 'cold' end of the volume control and make sure it's got a clean connection to chassis.
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