Newly developed ground loop hum in a Stax SR-007A's right channel

[plaurids]

Hi y'all,

I have a Stax SR-007A e-stat (with the blu-tac port mod) energized by a Stax SRM-T1S with the CCS mod running a pair of NOS Tung-Sol 6SN7GTB tubes through angled socket adapters. The signal from my DAC (RME ADI-2) is being fed through custom-made balanced XLR cables and the energizer is powered by an Audioquest NRG-Z3 power cord. The system has been running fine for at least a year.

Yesterday I removed the earpads for a routine cleaning of the driver (dusting off the dust covers, etc.), but after that the 007A developed what seems to be a ground loop in the right channel. The right driver started having this 60 Hz hum that remains even if the energizer is turned off. I've performed the following diagnosis steps:

• If you remove just the signal interconnects or the power cord the hum remains, it goes away only if all cables are removed.
• The hum disappears if I touch the ground pole in the back of the energizer with my hand, regardless of whether the energizer is on or off.
• I also have a Stax Lambda SR-407. It has no hum at all, regardless of whether I plug it alone to the energizer or together with the 007A (the latter's right driver still hums, of course).
• I've checked the soldering points at the right driver by measuring the resistance between each soldering point and the corresponding pin in the plug (no broken wiring) and also between each pair of soldering points (no cable or driver shorts). Everything seems fine in that regard.
• I've replaced the blu-tac in the port mod just in case there was something conductive mixed into the old blu-tac. That didn't change the problem at all.

I insist, I've been using this rig for more than a year with no problems, the hum only started yesterday and the only unusual thing I did was the aforementioned cleaning of the drivers. There are no other background noises (squeaking, buzzing, etc.), just the 60 Hz hum in the right channel. The left channel is fine.

What could be causing this? It's clearly an earspeaker issue, but I have no other clue besides that. Any help is deeply appreciated!

P.S.: in due time - my building has no grounded power outlets. Not much I can do about it, but, as I said above, that has never been a problem before with this system.

Edited Tuesday at 05:39 AM by plaurids