Sound like a short circuit in the path, could it be possibly an interconnect or your phono stage.
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If the motor wasn't grounded to the preamp, or phono preamp, you'd probably get a hum, but you'd still get a signal on the left channel. I still think you have an open signal path. Disconnect both RCA's from your preamp and with the phono selected, slowly turn up the volume moderately. You should get that sound on both channels.