Hi and welcome Roby,

Wow, tough problem.

Just a few thoughts....

I had a very strange mains fault in a house I lived in. I only found the fault when I was putting a picture up on the wall!

I had one of those metal stud/mains detectors for when you drill in the wall. There was a light switch on the wall, so I knew there was a cable nearby. Switched on the tester and swept it over the wall. It went berserk! Wherever I placed it on the wall it bleeped the "mains warning" bleep! Just to test it wasn't the tester gone duff, I tried another room downstairs and successfully traced a cable to a socket.

So it appeared that the wall was "live". I investigated further and took out each socket circuit, but the problem remained. Then, when I took out a lighting circuit, it disappeared. After switching out the circuit I opened up the switch on the wall. The earth cable was connected to the steel wall box from the incoming line, but a shoddily cut live cable had too much insulation exposed and had been pressing against the wall box! It hadn't fused because on further investigation the other end of the earth cable had come adrift further down the line!

Fortunately the switchplate was plastic, and the screws were recessed, but it had caused the whole of one wall to radiate a mains field! I didn't have any audio gear in the room, but I'm sure that an elec. guitar pickup would have picked it up, as the coil in the stud-detector definitely felt it!

Could be something similar?

Hope you fix it.

Mark

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"Oscillators don't, amplifiers do....."
Anon.

A live wall, now that's scary! Yikes! Glad you found it though.

I continued my testing yesterday and that included a call to the power company for further investigation.

On a whim I decided to test the audio cabling and it was fine. I have a balanced system in back that keeps everything clean. Just for fun though, I plugged the guitar cable into a preamp (with compressor) and cranked the gain. Waving the wand around the room I was able to crudely test for hum. Waving it around power cables was an eye opener, but revealed just how well shielded modern equipment casings are.

I have the mains for the building coming in near me and my first inclination was maybe that was the culprit, but upon waving the wand over that wall area I received no signal at all. On the inner wall adjacent to that one (common with the apt next to me) I found an aweful hum. Louder than anywhere else. It leads up to an AC outlet. I can trace it through most of the inner wall of the building.

Even though that should say enough in itself, further testing revealed that it wasn't proximity to that field causing the problem. After moving the gear to the other side of the apt, shutting down the mains to the rest of the apt and plugging in on the remaining circuit I was able to replicate the hum exactly. With that in mind I'm believing it's an AC quality problem.

A power company rep was here early this morning but has to turn it over to another tech for testing. He has seen this before though and believes it's probably something plugged into a neighbor's outlet causing the problem, possibly on the wall that's humming in my studio :)

I thought I'd take the opportunity to check connections in my rack while I'm down. Back to the grind!

I had a similar problem when I first moved into my studio in an industrial park. I also asked PG&E to come out and they did some testing and told me I needed to replace all the wall receptacles, switches and breakers. Once I had this done the problem disappeared. Seems the hum was caused by arcing in old corroded breakers, switches and receptacles... Kurt

Aha! Well, that does make me think. When we moved in I was amazed that the walls, outlets and switches all looked new for such an old place. Trying to insert a power cord into a receptacle required the might of Cicuit-Man! They were tough, and new. I can't guarantee that the wiring is new, but I can guarantee that the circuit breaker and box are old. The one obvious link in the chain left untouched.

At our last place if you inserted a power cord into the wall it literally fell out. Talk about overused!

As a sidebar, I'm planning on shielding my guitar. Of course I need to fix the problem so when other people come in they don't have this trouble. Stew-Mac has a nice little copper-foil shielding kit that I think I'll try. I've been looking for an excuse to order more tools from them anyway :)