Lenco/Bogen B-75 Turntable Hum Problem

I recently inherited a vintage Lenco/Bogen B-75 turntable. It’s in great shape and still spins nicely except that I can’t seem to get rid of a nasty hum problem. I’m pretty sure it’s not the phono pre-amp (made by ClearAudio) or interconnect wires that run from the pre-amp to the Rotel RSP-1069 processor because when I run my Rotel turntable with the same exact hook up it runs dead-silent.

It’s been completely refurbished with (among other things):

• A brand new Stanton 681EEE cartridge (pins on head-shell were cleaned and polished).
  
• New interconnect cables with gold RCA plugs that’s been silver soldered.
  
• A new ground wire with gold spade that’s been silver soldered and shrink wrapped.
  

Anyone know what could be causing the hum problem and how to eliminate it?