help me trouble shoot this ground issue!

[bjornson]

So...
I got a gig in a strange venue with one of my larger systems.
DDA arena at FOH, with large meyer rig, soundcraft series 5 for mons, and my 3x ATI pres into PT HD for the record rig. All connected with my 3 way w4 iso split with Jenson trannies.
All was good and quiet.....until
Slowly.... and I mean over the course of 3 hrs, 60hz and its harmonics appeared in all three subsystems..... Gradually increasing to troublesome levels.
Record world was 150 ft away, and although all ac was from my distro, I started changing sources of ac on the subsystems. No change, until I shut the whole rig off.
Upon startup, the same ground issue repeated.... slowly...
I got through the gig, but I'll be back in this venue soon.
I'll take my oscope and look at the ac next time, But I've never encountered a ground loop that transformers and pin 1 lift cables couldn't solve.
Ideas?

[Remoteness]

Dave,

Have you tested each and every component in your rig when you got back to your shop?

Also, as a test while at the venue and the power was still on, did you remove the AC ground to hear for any changes?
If so, did the noise get louder, softer, completely silent or no change at all?

Did your recording rig also have the same problem?

If everything checks out okay at your shop you should consider going back to the venue and do further tests -- why wait for the next gig?

Let us know how you made out.

[tmcconnell]

If the building has a resistive ground it might build up like that. Because its unique to the facility one might suspect building power changing its character leading to a ground loop.

also: Heat builds up like that. shutting down the rig would cause caps to bleed of, and components to cool down. A cap in a power supply or rectifier circuit or a resistor might be changing its characteristic spec with heat. That might be facility specific too - lighting, heat etc.