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Does the turntable produce enough volume on other selections than on AUX?
- if 'yes', then the AUX circuitry, or the AUX plug, has a problem, but not the rest.
- if 'no', then the entire audio amplifier circuitry in the turntable, and/or the cartridge in it, has a defect.
If the volume is low and invariable and perhaps slightly distorted, then you have a common amplifier problem, where the amplifier is partly defective, which diminishes its amplification to a uniform low level.
But this might be various things... tell me more about it, please.