Hum Help...

Look closely at Rane's notes on input and output cabling. It's a TRS balanced device.

 

Since it only works when racked, I wonder if you're getting some sort of inductance..... maybe a powerful transformer or noisy power supply inducing a 60Hz hum in an adjoining rack space. Can you reorder them in the rack? As an odd test, any way you can flip one of them upside down.. or backwards, to do a Left/Right switch of it's electronics and see if the hum stops? I wonder if a grounded thin copper sheet between devices would stop or reduce any RF from an adjoining device.

 

So, now that you know that the Rane is TRS and the Mesa is TS, it's time to re-read the rane notes (IIRC it's notes #110) with regard to connecting TRS to TS devices.

For a troubleshooting process, try just the power amp in the rack, nothing plugged into it except speakers. Is it quiet?

Now with the Rane unit out of the rack, but with the ground lift connected (not lifted), connect the PEQ as recommended by Rane. Is it still quiet?

If so, now move it into the rack but not touching any of the metal in the rack. Is it quiet? If not, it's likely that the power transformer in the amp is inducing noise into the PEQ, which could be indicating an issue with the INTERNAL grounding of the PEQ, or the circuitry can not reject the external electromagnetic field from the amp.

If it's quiet, screw the PEQ into the rack rail, if it's quiet, great. If not, it's lileky that there is a chassis ground loop that's at fault. Does the amp still have it's ground pin on the power cord?

This is the general process for identifying the cause of the problem, once you can identify the cause, you are in a better place for finding a solution.

 

Did you try stacking the units outside the rack?? Often the power supply of one unit can induce hum into another unit that is in close proximity. It's like the hum you get if you stand too close to your amp. If the stack does not produce hum then you are likely getting a ground loop through the rack rails. In this case I use some small plastic insulators call "Humfrees". I bolt the power amp right to the rack and insulate all the other units. I never have problems with hum in any of my racks.

Edit: If the hum IS induced try changing the order of the individual units to find the quietest arrangement.

 

Critical piece of information is if it makes noise not screwed in and not touching any other metal, start without anything else to eliminate other possible fault causes.

 

Fine, but you need to undestand what I wrote... you are missing the important details here.

If when putting the amp with PEQ into the rack, with the amp screwed into the rack rails BUT THE PEQ IN THE RACK NOT TOUCHING ANY OF THE METAL, this eliminates the possibility of a ground loop causing the primary problem (there could be an additional problem but you won't know without following this) and points to inductive coupling of the amp's power supply into the PEQ. If it's quiet, but gets noisy when connected to the rack rail, that eliminates the possibility of inductive coupling between the two devices.

I am asking you to do this in a very specific order as I am extremely familiar with both pieces and have spent over 30 years troubleshooting this kind of problem in the field when handling touring pro audiio support.