Technics SL1500C hum through external phono??

[Entrigo]

Hello wise forum-dwellers!

Going mad and would really really appreciate some expert tech advice!

Posted a week or so ago about this issue, thinking the problem was either with my amp or the phono stage but now I think it might be the turntable.

The lowdown:
Over the last couple of months, I ran my Technic TT through different phono options. All of this bar using the Technics own preamp produce a hum. This at full volume is roughly 40dB. As in, measuring around 75dB in total, with a background noise level of 35dB - measured in front of the speakers with a meter app on my phone (it's not scientific, I am a heathen etc so please bear with me).

Combinations tried follows:

• TT through Rega Fono MM into line in of my Marantz PM7200 amp - hum
• TT through Simaudio Moon 110LP into line in of Marantz - hum
• TT through Marantz phono in - hum (though quieter - but then the Marantz sounds MUCH quieter than any of the two external phono - whatever the reason, at the same volume knob position/distance/record playing, it reads nearly 30dB quieter than the Moon)
• TT through Moon into line in of a brand new Fosi mini amp - hum
• TT using own preamp into line in of Marantz - SILENT!

I also tried:
switching off all electrics, double grounding (extra metal wire from ground of Moon to ground pin of next electrical socket), moving away from amp, sitting on foam isolation pads, plugging Moon into extension from next door's socket, swapping RCAs around. Nothing. Also no hum from any of the other channels of the amp.

So:

1. Could it be the turntable phono out signal path or connections that is the problem?
2. Or some other issue in the connection between the TT and the external phonos?
3. Has anyone with a Technics SL1500C which has a similar issue?

I appreciate a proper answer is hard just on paper, but would really appreciate some insights before I take my 'baby' to my local shop - as they have a 4 weeks lead time - and I would rather avoid being without for 4 weeks and them saying "nothing wrong with this"...

Thanks!

 

[Gray]

Could it be the turntable phono out signal path or connections that is the problem?

Without doubt, that's the most likely explanation.
Based on the tests detailed in your 5 bullet points above, it can't be anything else.

You can be very specific to your local shop - the hum is only present from the phono out and not the line out RCA socket pair.
It will take them minutes to confirm that into a phono preamp of their own