Hum mystery to me

Calling all Hum guru solvers!

A while back, I started getting a hum out of my right speaker when playing a record on one of my 2 turntables. When I played the 2^nd turntable, there was no hum.

On table 1, I am using an MC cartridge which runs into an Audio Valve phono pre-amp and from there into the system preamp.

Table 2 is plugged directly into the phono input on the system preamp.

 Since the tables have different phono pre-amps, I thought it was the Audio Valve phono pre-amp and took it to the shop for repair. After getting it repaired, I hooked it up and now I am getting a hum out of the right channel. So, I took it back to the shop. We hooked it up to a turntable, played and there was NO hum at the shop. I will say that they had a MM cartridge on their test turntable, but do not know if that makes a difference.

This is a mystery to me and if anyone has any suggestion to try, please chime in,

These are the trouble shooting tests I have done so far:

1. I played a side on turntable 2. No hum. So that should rule out the amp or pre-amp.
2. The table 1 phono pre is connected to a DVD input on the system preamp. I changed the input to another line level input on the system preamp. Hum continued to be produced. So, I concluded that the inputs on the system preamp are not the problem.
3. I disconnected the ground wire on the tonearm—hum persisted.
4. I hooked up a cheap NAD PP2 phone pre-amp to turntable one and there was NO hum.
5. I tried a different interconnect from the Audio Valve phono pre-amp to the system pre-amp and the hum persisted. I conclude it is not the interconnect.