apt holman problems

Manual is at https://www.hifiengine.com/manual_library/apt-corporation.shtml
Does hum only come when channel cuts out? That sounds like cable.
If hum in both channels, the turntable to pre-amp separate ground wire is suspect as others wrote. This ground is a separate wire, not the shield of the RCA interconnects. Post on back of pre-amp to ground on turntable.

What is Holman's current company?

Also, even on pre-amps that are encased in solid metal on all six sides, do not have a pre-amp too close to a power amp. Even if the power amp is off, some keep the power supply on standby, generating an electromagnetic field that can cause hum. I had this problem, and ended up moving the pre-amp a foot from any amp, and the problem went away. Especially true if your pre-amp has an MC input, and is the kind where there is just a phono input select, meaning it picks up signal from both the MC and MM inputs all the time! For that, put an all metal shorting plug in to the unused phono inputs. This advice is for all pre-amps.