Then the preamp started producing a random popping sound and a motor sound like there was poor electric motor plugged into the same outlet. I thought it was a tube, but swapping out tubes didn't change it. I took it to my local service department. They cleaned and resoldered some of the connections. Now the popping sound is gone, but the constant motor sound persists. The motor sound goes away if I disconnect my turntable.I would not assume that the phono stage necessarily has anything to do with the problem. You didn't mention what turntable you have, but a possibility that occurs to me is that a capacitor associated with the AC wiring within the turntable has become leaky or otherwise defective. And it could be that the cleaning and resoldering that was done on the phono stage had nothing to do with the elimination of the popping sounds. Perhaps they just went away, probably temporarily if my theory is correct, as a result of the turntable being unpowered for some period of time, allowing the capacitor to temporarily recover to some degree.
Just a guess, but a possibility to consider. Good luck. Regards,
-- Al